Nerve Cell

Neuron From Wikipedia, the free encyclopedia

"Brain cell" redirects here. For other uses, see Glial see Glial cell.

Drawing by Santiago by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. (A) Denotes Purkinje Denotes  Purkinje cells, an cells, an example of a multipolar neuron. (B) Denotesgranule Denotesgranule cells, which cells, which are also multipolar.

 A neuron (also neuron (also known as a neurone or neurone or nerve cell) cell) is an electrically excitable cell excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, via synapses, specialized  specialized connections with other cells. Neurons connect to each other to form neural form neural networks. Neurons networks.  Neurons are the core components of the  nervous system, which system, which includes the brain, the brain, spinal  spinal cord, and cord,  and peripheral ganglia. peripheral  ganglia. A  A number of specialized types of neurons exist: sensory exist:  sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory the sensory organs that then send signals to the spinal cord and brain.  Motor neurons receive signals from the brain and spinal cord, cause muscle contractions, and contractions,  and affect glands. affect  glands. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord.  A typical neuron neuron possesses possesses a cell cell body (often (often called called the  soma),  soma), dendrites, and  dendrites,  and an axon. an  axon. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree". An axon is a special cellular extension that arises from the cell body at a site called the  axon hillock and travels for a distance, as far as 1 m in humans or even more in other species. The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, 1

many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.  All neurons neurons are electrically electrically excitable, excitable, maintaining maintaining  voltage gradients across their  membranes their  membranes by means of metabolically driven ion driven  ion pumps, which pumps, which combine with ion with  ion channels embedded in the membrane to generate intracellular-versus-extracellular concentration differences of  ions of  ions such as sodium, as sodium, potassium,  potassium, chloride,  chloride, and  and calcium.  calcium. Changes  Changes in the cross-membrane voltage can alter the function of  voltage-dependent of  voltage-dependent ion channels.  If the voltage changes by a large enough amount, an all-or-none electrochemical pulse called an action potential is generated, which travels rapidly along the cell's axon, and activates synaptic connections with other cells when it arrives. With the exception of  neural of  neural stem cells and a few other types of neurons, neurons do not undergo cell undergo  cell division. In division.  In most cases, neurons are generated by special types of  stem of  stem cells. Astrocytes, cells. Astrocytes, a  a type of  glial  glial cell, have cell, have also been observed to turn into neurons by virtue of the stem cell characteristic pluripotency. characteristic  pluripotency. In  In humans, neurogenesis humans,  neurogenesis largely ceases during adulthood— adulthood —only for two brain areas, the  hippocampus and olfactory and olfactory bulb, is bulb,  is there strong evidence for generation of substantial numbers of new neurons.

Overview  A neuron is a special type type of cell found in the the bodies bodies of most  animals (all members of the group Eumetazoa). Eumetazoa) . Only sponges and a few other simpler animals have no neurons. The features that define a neuron are electrical excitability and the presence of synapses, which are complex membrane junctions that transmit signals to other cells. The body's neurons, plus the glial cells that give them structural and metabolic support, together constitute the nervous system. In vertebrates, the majority of neurons belong to the  central nervous system, but system,  but some reside in peripheral ganglia, peripheral  ganglia, and  and many sensory neurons are situated in sensory organs such as the retina the retina and cochlea. and cochlea.  Although neurons are are very diverse diverse and there are exceptions exceptions to nearly every rule, itit is convenient convenient to begin with a schematic description of the structure and function of a "typical" neuron. A typical neuron is divided into three parts: the soma or cell body, dendrites, and axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometres from the soma. The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances, giving rise to hundreds of branches. Unlike dendrites, an axon usually maintains the same diameter as it extends. The soma may give rise to numerous dendrites, but never to more than one axon. Synaptic signals from other neurons 2

are received by the soma and dendrites; signals to other neurons are transmitted by the axon. A typical synapse, then, is a contact between the axon of one neuron and a dendrite or soma of another. Synaptic signals may be excitatory or inhibitory. If the net excitation received by a neuron over a short period of time is large enough, the neuron generates a brief pulse called an action potential, which originates at the soma and propagates rapidly along the axon, activating synapses onto other neurons as it goes. Many neurons fit the foregoing schema in every respect, but there are also exceptions to most parts of it. There are no neurons that lack a soma, but there are neurons that lack dendrites, and others that lack an axon. Furthermore, in addition to the typical axodendritic and axosomatic synapses, there are axoaxonic (axon-to-axon) and dendrodendritic (dendrite-todendrite) synapses. The key to neural function is the synaptic signaling process, which is partly electrical and partly chemical. The electrical aspect depends on properties of the neuron's membrane. Like all animal cells, every neuron is surrounded by a  plasma membrane, a membrane,  a bilayer of  lipid of  lipid molecules with many types of protein structures embedded in it. A lipid bilayer is a powerful electrical insulator, electrical insulator, but  but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane, and ion pumps that actively transport ions from one side of the membrane to the other. Most ion channels are permeable only to specific types of ions. Some ion channels are voltage are  voltage gated, meaning gated,  meaning that they can be switched between open and closed states by altering the voltage difference across the membrane. Others are chemically gated, meaning that they can be switched between open and closed states by interactions with chemicals that diffuse through the extracellular fluid. The interactions between ion channels and ion pumps produce a voltage difference across the membrane, typically a bit less than 1/10 of a volt at baseline. This voltage has two functions: first, it provides a power source for an assortment of voltage-dependent protein machinery that is embedded in the membrane; second, it provides a basis for electrical signal transmission between different parts of the membrane. Neurons communicate by chemical by  chemical and electrical and electrical synapses in a process known as  synaptic transmission. The transmission. The fundamental process that triggers synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the  electrically excitable membrane of the neuron. This is also known as a wave of depolarization.

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Anatomy and histology

Diagram of a typical myelinated vertebrate motoneuron

Neurons are highly specialized for the processing and transmission of cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous system, there is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter. 

The soma is the central part of the neuron. It contains the  nucleus of the cell, and therefore is where most protein most  protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter.

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The dendrites The dendrites of a neuron are cellular extensions with many branches, and metaphorically this overall shape and structure is referred to as a dendritic tree. This is where the majority of input to the neuron occurs.

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The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries  nerve signalsaway signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may —and usually will— will —undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of  voltage-dependent of   voltage-dependent sodium channels.  This makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in electrophysiological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons. 

The axon The axon terminal contains synapses, specialized structures where neurotransmitter neurotransmitter chemicals are released to communicate with target neurons.

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 Although the canonical canonical view of the neuron neuron attributes attributes dedicated dedicated functions functions to its various anatomical components, dendrites and axons often act in ways contrary to their so-called main function.  Axons and and dendrites in the central central nervous nervous system are are typically only about about one micrometer micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10 –25  –25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human  motoneuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes to the  dorsal columns, over 1.5 meters in adults.  Giraffes have single axons several meters in length running along the entire length of their necks. Much of what is known about axonal function comes from studying the squid the squid giant axon, an axon,  an ideal experimental preparation because of its relatively immense size (0.5 –1  –1 millimeters thick, several centimeters long). Fully differentiated neurons are permanently  postmitotic; however,  postmitotic; however, recent research shows that additional neurons throughout the brain can originate from neural  stem cells found throughout the brain but in particularly high concentrations in the  subventricular zone and subgranular and subgranular zone through the process of  neurogenesis. of  neurogenesis.

Histology and internal structure

Golgi-stained neurons in human hippocampal tissue

Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of  Nissl of  Nissl substance (named after German psychiatrist and neuropathologist  Franz Nissl, 1860 Nissl, 1860 –1919),  –1919), which consists of rough endoplasmic rough  endoplasmic reticulum and associated ribosomal associated ribosomal RNA. The RNA.  The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of  protein of  protein synthesis. The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, called neurofilaments, which  which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of  catecholamines) and   catecholamines)  and lipofuscin  lipofuscin (yellowish-brown pigment that accumulates with age).

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There are different internal structural characteristics between axons and dendrites. Typical axons almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body.

Classes

Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons.

SMI32-stained pyramidal neurons in cerebral cortex

Neurons exist in a number of different shapes and sizes and can be classified by their morphology and function. The anatomist  Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further divided by where the cell body or soma is located. The basic morphology of type I neurons, represented by spinal  motor neurons, consists of a cell body called the soma and a long thin axon covered by the  myelin sheath. Around the cell body is a branching dendritic tree that receives signals from other neurons. The end of the axon has branching terminals (axon terminal) that release neurotransmitters into a gap called the  synaptic cleft between the terminals and the dendrites of the next neuron.

Structural classification Polarity Most neurons can be anatomically characterized as: 

Unipolar or   pseudounipolar: dendrite and axon emerging from same process.

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Bipolar: axon and single dendrite on opposite ends of the soma.

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Multipolar: more than two dendrites: 

Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.

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Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.

Other Furthermore, some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are: 

Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and  cerebellum.

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Betz cells, large motor neurons.

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Medium spiny neurons, most neurons in the corpus striatum.

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Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron.

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Pyramidal cells, neurons with triangular soma, a type of Golgi I.

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Renshaw cells, neurons with both ends linked to  alpha motor neurons.

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Granule cells, a type of Golgi II neuron.

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 Anterior horn cells, motoneurons located in the spinal cord.

Functional classification Direction 

 Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called  sensory neurons.

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Efferent neurons transmit signals from the central nervous system to the  effector cells and are sometimes called motor neurons.

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Interneurons connect neurons within specific regions of the central nervous system.

 Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain region. Action on other neurons  A neuron affects other neurons by releasing a neurotransmitter that binds to  chemical receptors. The effect upon the target neuron is determined not by the source neuron or by the neurotransmitter, but by the type of receptor that is activated. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same type of key can here be used to open many 7

different types of locks. Receptors can be classified broadly as excitatory  (causing an increase in firing rate), inhibitory  (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate). The two most common neurotransmitters in the brain,  glutamate and GABA, have actions that are largely consistent. Glutamate acts on several different types of receptors, but most of them have effects that are excitatory. Similarly GABA acts on several different types of receptors, but all of them have effects (in adult animals, at least) that are inhibitory. Because of this consistency, it is common for neuroscientists to simplify the terminology by referring to cells that release glutamate as "excitatory neurons," and cells that release GABA as "inhibitory neurons." Since over 90% of the neurons in the brain release either glutamate or GABA, these labels encompass the great majority of neurons. There are also other types of neurons that have consistent effects on their targets, for example "excitatory" motor neurons in the spinal cord that release acetylcholine, and "inhibitory" spinal neurons that release glycine. The distinction between excitatory and inhibitory neurotransmitters is not absolute, however. Rather, it depends on the class of chemical receptors present on the target neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF  bipolar cells are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited  by glutamate, because they lack the typical  ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors. When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them. Discharge patterns Neurons can be classified according to their   electrophysiological characteristics: 

Tonic or regular spiking. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.

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Phasic or bursting. Neurons that fire in bursts are called phasic.

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Fast spiking. Some neurons are notable for their high firing rates, for example some types of cortical inhibitory interneurons, cells in  globus pallidus, retinal ganglion cells.

Classification by neurotransmitter production Neurons differ in the type of neurotransmitter they manufacture. Some examples are: 8

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Cholinergic neurons—acetylcholine. Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as a ligand for both ligand-gated ion channels and metabotropic (GPCRs) muscarinic receptors. Nicotinic receptors, are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind  nicotine. Ligand binding opens the channel causing influx of Na + depolarization and increases the probability of presynaptic neurotransmitter release.

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GABAergic neurons—gamma aminobutyric acid. GABA is one of two neuroinhibitors in the CNS, the other being Glycine. GABA has a homologous function to ACh, gating anion channels that allow Cl- ions to enter the post synaptic neuron. Cl- causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (recall that for an action potential to fire, a positive voltage threshold must be reached).

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Glutamatergic neurons—glutamate. Glutamate is one of two primary excitatory amino acids, the other being Aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor (often referred to as GPCR). 1. AMPA and Kainate receptors both function as cation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission 2. NMDA receptors are another cation channel that is more permeable to Ca 2+. The function of NMDA receptors is dependant on Glycine receptor binding as a coagonist within the channel pore. NMDA receptors do not function without both ligands present. 3. Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability. Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons causing NMDA and AMPA receptor activation moreso than would normally be the case outside of stress conditions, leading to elevated Ca

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Na+entering the post synaptic neuron and cell damage. 

Dopaminergic neurons—dopamine. Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and 9

PKA. Dopamine is connected to mood and behavior, and modulates both pre and post synaptic neurotransmission. Loss of dopamine neurons in the  substantia nigra has been linked to Parkinson's disease. 

Serotonergic neurons—serotonin. Serotonin,(5-Hydroxytryptamine, 5-HT), can act as excitatory or inhibitory. Of the four 5-HT receptor classes, 3 are GPCR and 1 is ligand gated cation channel. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft.

Connectivity Main articles: Synapse and  Chemical synapse Neurons communicate with one another via synapses, where the axon terminal or en  passant  boutons (terminals located along the length of the axon) of one cell impinges upon another neuron's dendrite, soma or, less commonly, axon. Neurons such as Purkinje cells in the cerebellum can have over 1000 dendritic branches, making connections with tens of thousands of other cells; other neurons, such as the magnocellular neurons of the supraoptic nucleus, have only one or two dendrites, each of which receives thousands of synapses. Synapses can be  excitatory or   inhibitory and either increase or decrease activity in the target neuron. Some neurons also communicate via electrical synapses, which are direct, electrically-conductive junctions between cells. In a chemical synapse, the process of synaptic transmission is as follows: when an action potential reaches the axon terminal, it opens  voltage-gated calcium channels,  allowing calcium ions to enter the terminal. Calcium causes  synaptic vesicles filled with neurotransmitter molecules to fuse with the membrane, releasing their contents into the synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate receptors on the postsynaptic neuron. The human brain has a huge number of synapses. Each of the 10 11 (one hundred billion) neurons has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 10

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 synapses (1 quadrillion).

This number declines with age, stabilizing by adulthood. Estimates vary for an adult, ranging from 1014 to 5 x 10 14 synapses (100 to 500 trillion).

Mechanisms for propagating action potentials

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 A signal propagating down an axon to the cell body and dendrites of the next cell

In 1937, John Zachary Young suggested that the squid giant axon could be used to study neuronal electrical properties. Being larger than but similar in nature to human neurons, squid cells were easier to study. By inserting electrodes into the giant squid axons, accurate measurements were made of the membrane potential. The cell membrane of the axon and soma contain voltage-gated ion channels that allow the neuron to generate and propagate an electrical signal (an action potential). These signals are generated and propagated by charge-carrying  ions including sodium (Na +), potassium (K+), chloride (Cl-), and calcium (Ca 2+). There are several stimuli that can activate a neuron leading to electrical activity, including pressure, stretch, chemical transmitters, and changes of the electric potential across the cell membrane. Stimuli cause specific ion-channels within the cell membrane to open, leading to a flow of ions through the cell membrane, changing the membrane potential. Thin neurons and axons require less  metabolic expense to produce and carry action potentials, but thicker axons convey impulses more rapidly. To minimize metabolic expense while maintaining rapid conduction, many neurons have insulating sheaths of  myelin around their axons. The sheaths are formed by  glial cells: oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The sheath enables action potentials to travel faster than in unmyelinated axons of the same diameter, whilst using less energy. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed  nodes of Ranvier, which contain a high density of voltage-gated ion channels.  Multiple sclerosis is a neurological disorder that results from demyelination of axons in the central nervous system.

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Some neurons do not generate action potentials, but instead generate a graded electrical signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons tend to be sensory neurons or interneurons, because they cannot carry signals long distances.

Neural coding Neural coding is concerned with how sensory and other information is represented in the brain by neurons. The main goal of studying neural coding is to characterize the relationship between the stimulus and the individual or  ensemble neuronal responses, and the relationships amongst the electrical activities of the neurons within the ensemble. It is thought that neurons can encode both  digital andanalog information.

All-or-none principle The conduction of nerve impulses is an example of an  all-or-none response. In other words, if a neuron responds at all, then it must respond completely. Greater intensity of stimulation does not produce a stronger signal but can produce more impulses per second. There are different types of receptor response to stimulus, slowly adapting or tonic receptors respond to steady stimulus and produce a steady rate of firing. These tonic receptors most often respond to increased intensity of stimulus by increasing their firing frequency, usually as a power function of stimulus plotted against impulses per second. This can be likened to an intrinsic property of light where to get greater intensity of a specific frequency (color) there have to be more photons, as the photons can't become "stronger" for a specific frequency. There are a number of other receptor types that are called quickly-adapting or phasic receptors, where firing decreases or stops with steady stimulus; examples include: skin when touched by an object causes the neurons to fire, but if the object maintains even pressure against the skin, the neurons stop firing. The neurons of the skin and muscles that are responsive to pressure and vibration have filtering accessory structures that aid their function. The pacinian corpuscle is one such structure. It has concentric layers like an onion, which form around the axon terminal. When pressure is applied and the corpuscle is deformed, mechanical stimulus is transferred to the axon, which fires. If the pressure is steady, there is no more stimulus; thus, typically these neurons respond with a transient depolarization during the initial deformation and again when the pressure is removed, which causes the corpuscle to change shape again. Other types of adaptation are important in extending the function of a number of other neurons.

History 12

Drawing by Camillo Golgi of ahippocampus stained with the silver nitrate method

Drawing of a Purkinje cell in the cerebellum cortex done by Santiago Ramón y Cajal, demonstrating the ability of Golgi's staining method to reveal fine detail

The term neuron was coined by the German anatomist  Heinrich Wilhelm Waldeyer. The neuron's place as the primary functional unit of the nervous system was first recognized in the early 20th century through the work of the Spanish anatomist  Santiago Ramón y Cajal. Ramón y Cajal proposed that neurons were discrete cells that communicated with each other via specialized junctions, or spaces, between cells. This became known as the  neuron doctrine, one of the central tenets of modern  neuroscience. To observe the structure of individual neurons, Ramón y Cajal improved a silver staining process known as  Golgi's method, which had been developed by his rival,  Camillo Golgi. Cajal's improvement, which involved a technique he called "double impregnation", is still in use. The silver impregnation stains are an extremely useful method for  neuroanatomical investigations because, for reasons unknown, it stains a very small percentage of cells in a tissue, so one is able to see the complete micro structure of individual neurons without much overlap from other cells in the densely packed brain.

The neuron doctrine The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units. 13

Later discoveries yielded a few refinements to the simplest form of the doctrine. For example, glial cells, which are not considered neurons, play an essential role in information processing. Also, electrical synapses are more common than previously thought, meaning that there are direct, cytoplasmic connections between neurons. In fact, there are examples of neurons forming even tighter coupling: the squid giant axon arises from the fusion of multiple axons. Ramón y Cajal also postulated the Law of Dynamic Polarization, which states that a neuron receives signals at its dendrites and cell body and transmits them, as action potentials, along the axon in one direction: away from the cell body. The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons and axons can receive synaptic inputs.

Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. One estimate puts the human brain at about 100 billion ( 1011) neurons and 100 trillion (1014) synapses. Another estimate is 86 billion neurons, of which 16.3 billion are in the cerebral cortex, and 69 billion in the cerebellum. By contrast, the  nematode worm Caenorhabditis elegans has just 302 neurons making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. The fruit fly Drosophila melanogaster , a common subject in biological experiments, has around 100,000 neurons and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems. Neurological disorders Main article: Neurology Charcot-Marie-Tooth disease (CMT), also known as Hereditary Motor and Sensory Neuropathy (HMSN), Hereditary Sensorimotor Neuropathy (HMSN), or  Peroneal Muscular  Atrophy, is a heterogeneous inherited disorder of nerves (neuropathy)  that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms in the advanced stages of disease. Presently incurable, this disease is one of the most common inherited neurological disorders, with 37 in 100,000 affected. Alzheimer's disease (AD), also known simply as Alzheimer's, is a neurodegenerative disease characterized by progressive cognitive deterioration together with declining 14

activities of daily living and  neuropsychiatric symptoms or behavioral changes. The most striking early symptom is loss of short-term memory (amnesia), which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression, with relative preservation of older memories. As the disorder progresses, cognitive (intellectual) impairment extends to the domains of language (aphasia), skilled movements (apraxia), and recognition (agnosia), and functions such as decision-making and planning become impaired. Parkinson's disease (PD), also known as Parkinson disease, is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills and speech. Parkinson's disease belongs to a group of conditions called  movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia), and in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the  motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level  cognitive dysfunction and subtle language problems. PD is both chronic and progressive. Myasthenia Gravis is a neuromuscular disease leading to fluctuating  muscle weakness and fatigability during simple activities. Weakness is typically caused by circulatingantibodies that block acetylcholine receptors at the post-synaptic neuromuscular  junction, inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia is treated with immunosuppressants, cholinesterase inhibitors and, in selected cases, thymectomy. Demyelination Demyelination is the act of demyelinating, or the loss of the myelin sheath insulating the nerves. When myelin degrades, conduction of signals along the nerve can be impaired or lost, and the nerve eventually withers. This leads to certain neurodegenerative disorders like multiple sclerosis and chronic inflammatory demyelinating polyneuropathy. Axonal degeneration  Although most injury responses include a calcium influx signaling to promote resealing of severed parts, axonal injuries initially lead to acute axonal degeneration (AAD), which is rapid separation of the proximal and distal ends within 30 minutes of injury. Degeneration follows with swelling of the axolemma, and eventually leads to bead like formation. Granular disintegration of the axonal  cytoskeleton and inner organelles occurs after axolemma degradation. Early changes include accumulation of  mitochondria in the paranodal regions 15

at the site of injury. Endoplasmic reticulum degrades and mitochondria swell up and eventually disintegrate. The disintegration is dependent on Ubiquitin and Calpainproteases (caused by influx of calcium ion), suggesting that axonal degeneration is an active process. Thus the axon undergoes complete fragmentation. The process takes about roughly 24 hrs in the PNS, and longer in the CNS. The signaling pathways leading to axolemma degeneration are currently unknown.

Nerve regeneration Main article: Nerve regeneration It has been demonstrated that  neurogenesis can sometimes occur in the adult vertebrate brain, a finding that led to controversy in 1999. However, more recent studies of the age of human neurons suggest that this process occurs only for a minority of cells, and the overwhelming majority of neurons comprising the  neocortex were formed before birth and persist without replacement. It is often possible for peripheral axons to regrow if they are severed. A report in Nature suggested that researchers had found a way to transform human skin cells into working nerve cells using a process called   transdifferentiation in which "cells are forced to adopt new identities."

Understanding the Structure of the Nerve Cell The nerve cell structure of neurons are highly specialized and designed to specifically receive, process and relay electrochemical signals from one cell to another. When these cells are placed in a series, each part is perfectly positioned to function in unison for the nerve system. Soma The nerve cell structure is based around the soma, the part of the cell which contains the nucleus. The soma acts as the basic body of the nerve cell structure. All secondary parts of the neuron are based around the soma, although unlike other cell types, the nucleus is not located at the center of the cell. The RNA of the nerve cell is located within the nucleus. The messenger RNA exists around the nucleus,  producing proteins that make the cells and nervous system in general function. Soma range in size from four micrometers all the way up to a single millimeter in larger invertebrates. Dendrites Extending from the soma are branches of biological material known as dendrites. These branches appear similar to the limbs of a tree. The main purpose of the dendrites in the nerve 16

cell structure is to receive electrochemical stimulation from other neurons. Stimulation from one cell to another travel across the synapses, areas between each cell. The electrical stimulation makes contact with the dendrites and the signal is sent to the soma for processing. Axon The soma also is connected to another projection known as the axon. This resembles a long cable that extends in diameter much further than the soma itself. This distance can be anywhere from ten times the length of the soma, all the way to tens of thousands. Between the axon itself and the soma is a portion of the neuron known as the axon hillock, the most sensitive portion of the nerve cell due to the high volume of sodium channels. The axon itself is divided into three specific components. Covering the entire axon is a material known as the myelin sheath, an insulating layer which prevents the electrical signals from being lost as they flow through the axon. Essentially, the myelin sheath acts as the rubber that covers and electrical cord. The actual conduction of the signal is handled by a part of the axon known as the Schwann cell. In addition to sending signals, these are also responsible for regeneration and overall development of the nerve. In between each Schwann cell is a gap connecting the axon together, known as nodes of Ranvier. These nodes are not insulated with the myelin sheath, meaning additional electrical signals can be generated at these points. Axon Terminal The final component of the nerve cell structure is the axon terminal. These are similar to the dendrites, however, they have the opposite purpose. The axon terminal receives the signal from the rest of the neuron and sends it along the long branches across the synapse and to the dendrites of another cell. This is how nerve cells connect to each other and communicate as the overall nervous system. References

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"Nerve Cells" Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/H BASE/Biology/nervecell.html

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"The Nerve Cell in General" Fortune City: http://www.fortunecity.com/greenfield/buzzard/387/nervecellgen.htm

THE NERVE CELL IN GENERAL The basic unit of the nervous system is a highly specialized cell, also known as a neuron. Its main purpose is to transport messages from one part of the body to another in the form of nerve impulses. Because of their highly specialized function, neurons have certain special characteristics. First they are extremely long lived. They can live and function for over 100 years. This is very important since neurons are also amitotic. This means that they do not replicate or reproduce. In fact most 1 year old humans have approximately the maximum number of neurons that they will ever have in their lives, about 1011-12. From this point on 17

humans tend to lose about 200,000 neurons a day. Neurons are very active and so have a high metabolic rate, requiring large amounts of oxygen and glucose. . NEURON STRUCTURE The neuron can be generally divided into three main functional parts, the cell body, the axon, and the dendrites. The cell body is the biosynthetic center of the cell. This is where cellular metabolism occurs, as well as the production of proteins and membrane. Some people believe that this production machinery, consisting of free ribosomes and rough endoplasmic reticulum (rER), is probably the most active and best developed of any cell in the body. The ribosomes and rER are the cellular organelles responsible for protein production and packaging. The dendrites are responsible for receiving signals and conducting them up the cell to the cell body and on to the axon. The axon is the portion of the neuron that is responsible for the passing of the cellular message from the, neuron to either other neurons, or neural receptors. Bundles of these  processes, axons and dendrites so called because they extend outward from the cell body, are called tracts in the CNS(this stands for central nervous system and consists of the brain and spinal column), and nerves in the PNS (this stands for peripheral nervous system and it represents all other nerves outside of the CNS). The cell body contains all of the basic cellular organelles of most other cells but is conspicuously lacking centrioles. Centrioles are organelles (the cellular equivalent of human organs) that are responsible for the formation of the mitotic spindle, which is important for cellular replication. The rER is very well developed and often referred to as Nissl bodies or Chromaticphilic substance because it stains darkly with basic dyes. The cell body contains intermediate filaments called neurofibrils, which are important in intracellular transport and maintain cell shape and integrity. Bundles of these neurofibrils are called neurofilaments.  Aging neurons are abundant with a pigment called lipofuscin. This pigment accumulation is a harmless by-product of certain cellular functions. Most cell bodies are clustered within the CNS, in groups called nuclei. Fewer groupings of cell bodies are found outside of the protection of the CNS in the PNS. These clusters are called ganglia. Dendrites are the part of the cell that receive signals and conduct those signals to the rest of the neuron. These electrical signals are not nerve impulses but are short distance signals called graded potentials, or receptor potentials. Motor neurons can have hundreds of dendrites. This is a large amount of surface area dedicated to signal and impulse reception,

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hinting that a substantial stimulation is required before an action potential can be generated. Dendrites contain most organelles present in the cell body. Neurons can have no more than a single axon (certain neurons in the CNS don't have any), responsible for the passing of messages from the neuron to other nerve cells or their effectors, targets of nerve impulses such as muscles. This where the actual nerve impulse, or action potential, occurs, on the axon. Axons can be very long, or very short in length. Some axons are 3-4 feet in length, extending from the middle of the spine to the bottom of the feet. Long axons are sometimes called nerve fibers. Just as axonal length may vary so too does the diameter of the axon. The larger the diameter, the thicker the axon, the faster it delivers nerve impulses. The region of the axon that attaches to the cell body is called the axon hillock. It is at this junction that action potentials are generated. The axon hillock is where the numerous graded potentials are summed, and thus the site for determination of whether threshold has been met. From here the axon extends a distance until it terminates into what is known as the secretory component of the neuron. The secretory component is the actual part of the neuron that transmits the nerve impulse on to the next neuron, and is the place in which synaptic potentials occur. This will be dealt with in a later section. The conducting component of the neuron lies between the axon hillock and the secretory component, consisting entirely of a length of axon. It is responsible for the propagation of nerve impulses away from the cell body. The conducting component sometimes branches into what are called axon collaterals. Though some axons may remain un branched along their length, nearing their end, they usually branch into many (sometimes in excess of 10,000) smaller end branches called telodendria. The telodendria endings are variously called axonal terminals, synaptic knobs, or boutons, and are all part of the secretory component of the cell. Axons contain the same organelles as the cell body, except they lack Nissl bodies, so are dependent on the cell body to supply them with proteins and membrane components. Because of this transport mechanisms in the neuron are extremely important for transporting needed materials along the axon, and waste products from the axon. One of the most effective transporter mechanisms involves the use of kinesin, an ATP driven "carrier" protein that transports materials from one part of a cell to another. CLASSIFYING NEURONS  As with most things there are multiple ways to classify neurons. The two most obvious ways are by structure and function. It is important to note that with neurons, as well as most biological entities, function follows form (structure). This means that the structure of a cell is the way it is because after millions of years of evolution that particular structure facilitates a 19

specific function. This will become more evident as knowledge increases, and it is important to understand the connection between a neuron's form and it's particular function. Structurally neurons can be classified as multipolar, bipolar or unipolar. Multipolar neurons have three or more processes and are the most common type in the CNS. Usually this means that there are numerous dendritic branches and one axonal process, but some neurons completely lack the axon and have only dendrites. Bipolar neurons have just two processes, an axon and a dendrite. These neurons are rare in adults, but when found are usually acting as receptor cells in some of the sense organs such as the eyes or nose. Unipolar neurons have a single process, which is very short, and almost always immediately divides into proximal and distal fibers, which head in different directions. The distal fiber is often associated with a sensory receptor and is sometimes referred to as the peripheral process. The proximal fiber, or central process, is generally associated with the CNS and is considered an axon as it has an action potential and is the "sender" of a neural message. The peripheral process is somewhat of an enigma however. It acts like an axon in that it has an action potential, is normally myelinated when large, has a uniform diameter, and is indistinguishable from an axon microscopically. However it also acts as a dendrite in that it conducts message a toward the cell body of the neuron. We choose to think of it as an axon, and the actual receptor ends of it as miniature dendrites that skip the cell body and go straight on to the axon. Neurons can also be functionally classified as either sensory (or afferent) neurons, motor (or efferent) neurons, or association (or inter) neurons. This classification scheme is largely based on the direction of the nerve impulse. Sensory neurons conduct messages from sensory receptors toward the CNS and are usually always unipolar. Motor neurons are almost always multipolar and conduct impulses away from the CNS. All motor neurons form  junctions with their effector cells. Between the sensory and motor neurons lie the interneurons. These cells are typically multipolar and confined to the CNS.

Chapter 21 Nerve Cells Neonatal rat cortical brain cells, cultured for 25 days  in vitro, stained with a fluorescent antibody to the cytoskeletal intermediate filament  protein GFAP (Glial Fibrillary  Acidic Protein, green) and with the dye DAPI that causes DNA to fluoresce blue. Two distinct types of astrocytes (green cells) are present in this culture, along with other types of cells (non-green) that appear as isolated blue nuclei. [Photograph courtesy of Nancy Kedersha.]

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The nervous system regulates all aspects of bodily function and is staggering in its complexity. The human brain — the control center that stores, computes, integrates, and transmits information — contains about 10 12 neurons (nerve cells), each forming as many as a thousand connections with other neurons. Millions of specialized neurons sense features of both the external and internal environments and transmit this information to the brain for processing and storage. Millions of other neurons regulate the contraction of muscles and the secretion of   hormones. The nervous system also contains glial (neuroglial) cells that occupy the spaces between neurons and modulate their functions (see chapter opening figure). The structure and function of individual nerve cells is understood in great detail, perhaps in more detail than for any other type of cell. The function of a neuron is to communicate information, which it does by two methods. Electric signals process and conduct information within a cell, while chemical signals transmit information between cells, utilizing processes similar to those employed by other types of cells to signal each other (Chapter 20). Sensory neurons have specialized receptors that convert diverse types of stimuli from the environment (e.g., light, touch, sound, odorants) into electric signals. These electric signals are then converted into chemical signals that are passed on to other cells called interneurons,  which convert the information back into electric signals. Ultimately the information is transmitted to muscle-stimulating motor neurons or to other neurons that stimulate other types of cells, such as glands. The output of a nervous system is the result of its circuit properties, that is, the wiring, or interconnections, between neurons, and the strength of these interconnections. Complex aspects of the nervous system, such as vision and consciousness, cannot be understood at the single-cell level, but only at the level of networks of nerve cells that can be studied by techniques of systems analysis. The nervous system is constantly changing; alterations in the number and nature of the interconnections between individual neurons occur, for example, in the development of new memories. In this chapter we focus on how individual neurons function and how small groups of cells function together. A great deal of information has been gleaned from simple nervous systems. Squids and sea slugs have large neurons that are relatively easy to identify and manipulate experimentally. Moreover, in these species, only a few identifiable neurons may be involved in a specific task; thus their function can be studied in some detail. Analyses of humans, mice, nematodes, and flies with  mutations that affect specific functions of the nervous system have provided important insights, as have molecular cloning of key 21

neuron proteins, such as ion channels and receptors. Genetic and molecular studies on the development of the nervous system, detailed in Chapter 23, have elucidated how neurons form and maintain specific connections with other neurons and other types of cells. Because the principles studied are basic, all of these findings are applicable to complex nervous systems, including that of humans.

What Is a Nerve Cell? Nerve cells—also known as neurons—are the primary building blocks of the nervous system in humans and animals. On a fundamental level, a nerve cell functions by transmitting and receiving electrochemical messages. These messages can serve several purposes, including the transmission of sensory information to the central nervous system and the regulation and control of organs in the body. The function of a single nerve cell could be described as relatively straightforward, but when bundled together in groups, nerve cells can allow for complex processes like brain cognition. Like most other cells in an organism, a nerve cell generally has both a  nucleus and a cell body.  Around the cell body, there are extensions called  dendrites,  which are specialized in receiving different kinds of stimuli depending on the location and purpose of the nerve cell. Once the dendrites detect some form of stimuli, the cell body generates an electrical impulse called an action potential, which travels down a wire-like structure called an axon to its destination. The three basic types of nerve cells are motor neurons, sensory neurons and interneurons. A motor  neuron is a nerve cell that transmits a signal to a muscle or gland. Sensory neurons receive information from sensory organs and transmit that information back to the central nervous system. Interneurons, which do most of the work in the brain and   spinal cord,  relay information between sensory and motor neurons. The speed of the electrical impulse that’s transmitted through a nerve cell can vary depending on a number of factors, but the average is about 200 mph ( 321.8688 kph), which is slower than electricity travels over a wire. The average human brain has about 100 billion neurons and about 10 times as many glial support cells, which perform several vital functions that help the neurons work properly. One difference between neurons and other cells in the body is their lifespan. While most cells die and are replaced in relatively short cycles, research has shown that many neurons in the body aren’t replaced, and some will last for a person’s entire life. Over the course of a long lifespan, some neurons will gradually die off, but there are generally more than enough surviving neurons to compensate for any normal losses. Scientists have discovered that one part of the brain called the hippocampus has the capacity to regenerate lost neurons, but this does not appear to be possible anywhere else in the body.

Nervous system - Nerve Cells and Nerves Function: To transmit messages from one part of your body to another Neurons: Messenger cells in your nervous system Nerve impulses: Electrical signals carrying messages

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Neurotransmitters: Chemicals released by one neuron to excite a neighboring one Millions of messengers Your nervous system contains millions of nerve cells, called neurons. Neurons are highly specialized to transmit messages from one part of your body to another.  All neurons have a cell body and one or more fibers. These fibers vary in length from microscopic to over 1 meter. There are two different kinds of nerve fibers: fibers that carry information towards the cell body, called dendrites, and fibers that carry information away from it, called axons. Nerves are tight bundles of nerve fibers. Teamwork Your neurons can be divided into three types: •Sensory neurons, which pass information about stimuli such as light, heat or chemicals from both inside and outside your body to your central nervous system •Motor neurons, which pass instructions from your central nervous system to other parts of your body, such as muscles or glands •Association neurons, which connect your sensory and motor neurons Electrical and chemical signals Your neurons carry messages in the form of electrical signals called nerve impulses. To create a nerve impulse, your neurons have to be excited. Stimuli such as light, sound or pressure all excite your neurons, but in most cases, chemicals released by other neurons will trigger a nerve impulse.  Although you have millions of neurons that are densely packed within your nervous system, they never actually touch. So when a nerve impulse reaches the end of one neuron, a neurotransmitter chemical is released. It diffuses from this neuron across a junction and excites the next neuron. Protecting cells Over half of all the nerve cells in your nervous system do not transmit any impulses. These supporting nerve cells are located between and around your neurons to insulate, protect and nourish them.

What Is a Neuron? Question: What Is a Neuron? Answer:  A neuron is a nerve cell that is the basic building block of the nervous system. Neurons are similar to other cells in the human body in a number of ways, but there is one key difference

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between neurons and other cells. Neurons are specialized to transmit information throughout the body. These highly specialized nerve cells are responsible for communicating information in both chemical and electrical forms. There are also several different types of neurons responsible for different tasks in the human body. Sensory neurons carry information from the sensory receptor cells throughout the body to the brain. Motor neurons transmit information from the brain to the muscles of the body. Interneurons are responsible for communicating information between different neurons in the body. Neurons vs. Other Cells Similarities with other cells:   

Neurons and other body cells both contain a nucleus that holds genetic information. Neurons and other body cells are surrounded by a membrane that protects the cell. The cell bodies of both cell types contain organelles that support the life of the cell, including mitochondria, Golgi bodies, and cytoplasm.

Differences that make neurons unique: 

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Unlike other body cells, neurons stop reproducing shortly after birth. Because of this, some parts of the brain have more neurons at birth than later in life because neurons die but are not replaced. While neurons do not reproduce, research has shown that new connections between neurons form throughout life. Neurons have a membrane that is designed to sends information to other cells. The axon and dendrites are specialized structures designed to transmit and receive information. The connections between cells are known as a synapses. Neurons release chemicals known as neurotransmitters into these synapses to communicate with other neurons.

The Structure of a Neuron There are three basic parts of a neuron: the dendrites, the cell body and the axon. However, all neurons vary somewhat in size, shape, and characteristics depending on the function and role of the neuron. Some neurons have few dendritic branches, while others are highly branched in order to receive a great deal of information. Some neurons have short axons, while others can be quite long. The longest axon in the human body extends from the bottom of the spine to the big toe and averages a length of approximately three feet! Action Potentials How do neurons transmit and receive information? In order for neurons to communicate, they need to transmit information both within the neuron and from one neuron to the next. This process utilizes both electrical signals as well as chemical messengers.

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The dendrites of neurons receive information from sensory receptors or other neurons. This information is then passed down to the cell body and on to the axon. Once the information as arrived at the axon, it travels down the length of the axon in the form of an electrical signal known as an action potential. Communication between Synapses Once an electrical impulse has reached the end of an axon, the information must be transmitted across the synaptic gap to the dendrites of the adjoining neuron. In some cases, the electrical signal can almost instantaneously bridge the gap between the neurons and continue along its path. In other cases, neurotransmitters are needed to send the information from one neuron to the next. Neurotransmitters are chemical messengers that are released from the axon terminals to cross the synaptic gap and reach the receptor sites of other neurons. In a process known as reuptake, these neurotransmitters attach to the receptor site and are reabsorbed by the neuron to be reused. Neurotransmitters Neurotransmitters are an essential part of our everyday functioning. While it is not known exactly how many neurotransmitters exist, scientists have identified more than 100 of these chemical messengers. What effects do each of these neurotransmitters have on the body? What happens when disease or drugs interfere with these chemical messengers? The following are just a few of the major neurotransmitters, their known effects, and disorders they are associated with. Acetylcholine:  Associated with memory, muscle contractions, and learning. A lack of acetylcholine in the brain is associated with Alzheimer’s disease. Endorphins: Associated with emotions and pain perception. The body releases endorphins in response to fear or trauma. These chemical messengers are similar to opiate drugs such as morphine, but are significantly stronger. Dopamine: Associated with thought and pleasurable feelings. Parkinson’s disease is one illness associated with deficits in dopamine, while schizophrenia is strongly linked to excessive amounts of this chemical messenger. The brain and spinal cord are made up of many cells, including neurons and glial cells. Neurons are cells that send and receive electro-chemical signals to and from the brain and nervous system. There are about 100 billion neurons in the brain. There are many more glial cells; they provide support functions for the neurons, and are far more numerous than neurons. There are many type of neurons. They vary in size from 4 microns (.004 mm) to 100 microns (.1 mm) in diameter. Their length varies from a fraction of an inch to several feet.

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THE BRAIN: Brain Cells

Neurons are nerve cells that transmit nerve signals to and from the brain at up to 200 mph. The neuron consists of a cell body (or soma) with branching dendrites(signal receivers) and a projection called an axon, which conduct the nerve signal. At the other end of the axon, the axon terminals transmit the electro-chemical signal across a synapse (the gap between the axon terminal and the receiving cell). The word "neuron" was coined by the German scientist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891 (he also coined the term "chromosome"). The axon, a long extension of a nerve cell, and take information away from the cell body. Bundles of axons are known as nerves or, within the CNS (central nervous system), as nerve tracts or pathways. Dendrites bring information to the cell body. Myelin coats and insulates the axon (except for periodic breaks called nodes of Ranvier), increasing transmission speed along the axon. Myelin is manufactured by Schwann's cells, and consists of 70-80% lipids (fat) and 20-30% protein. The cell body (soma) contains the neuron's nucleus (with DNA and typical nuclear organelles). Dendrites branch from the cell body and receive messages.  A typical neuron has about 1,000 to 10,000 synapses (that is, it communicates with 1,00010,000 other neurons, muscle cells, glands, etc.). DIFFERENT TYPES OF NEURONS There are different types of neurons. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are found in different parts of the body. 

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Sensory neurons or Bipolar neurons carry messages from the body's sense receptors (eyes, ears, etc.) to the CNS. These neurons have two processes. Sensory neuron account for 0.9% of all neurons. (Examples are retinal cells, olfactory epithelium cells.) Moto neurons or Multipolar neurons carry signals from the CNS to the muscles and glands. These neurons have many processes originating from the cell body. Moto neurons account for 9% of all neurons. (Examples are spinal motor neurons, pyramidal neurons, Purkinje cells.) Interneurons or Pseudopolare (Spelling) cells form all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord; one with either the skin or muscle. These neurons have two processes. (Examples are dorsal root ganglia cells.)

LIFE SPAN OF NEURONS 26

Unlike most other cells, neurons cannot regrow after damage (except neurons from the hippocampus). Fortunately, there are about 100 billion neurons in the brain.

GLIAL CELLS Glial cells make up 90 percent of the brain's cells. Glial cells are nerve cells that don't carry nerve impulses. The various glial (meaning "glue") cells perform many important functions, including: digestion of parts of dead neurons, manufacturing myelin for neurons, providing physical and nutritional support for neurons, and more. Types of glial cells include Schwann's Cells, Satellite Cells, Microglia, Oligodendroglia, and Astroglia. Neuroglia (meaning ―nerve glue‖) is the type of brain cell. These cells guide neurons during fetal development.

Parts of a Cell Every part of a nerve cell is important. Each oart has a certain function that must be fulfilled for the cell to live. The cell body (soma)  - This is the factory of the neuron. It produces all proteins for the dendrites, and axons. The Nucleus is the architect of the cell. It contains DNA that contains the cell history, and the basic information about the cell. Myelin Sheath - The Myelin Sheath (or layer) is an increase in the speed at which the impulses pass through. Schwann Cells provide myelination to axons in the peripheral nervous system. Dendrites receive signals from other cells. Node of Ranvier  - a gap that occurs at regular intervals between pieces of myelin sheath along the nerve axon Axon - the appendage of the neuron that transmits impulses away from the cell body. Axon Terminals  - The somewhat enlarged, often club-shaped endings by which axons make synaptic contacts with other nerve cells or with effector cells. Also called end-feet, neuropodia, terminal boutons.

Parts of the Nerve Cell and Their Functions Silvia Helena Cardoso, PhD

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1. Cell body

The cell body (soma) is the factory of the neuron. It produces all the proteins for the dendrites, axons and synaptic terminals and contains specialized organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and polysomes to provide energy and make the parts, as well as a production line to assemble the parts into completed products. Cytosol - Is the watery and salty fluid with a potassium-rich solution inside the cell containing enzymes responsible for the metabolism of the cell. 1. Nucleus - Derived from the Latin word for "nux", nut, the nucleus is the archivist and the architect of the cell. As archivist it contains the genes, consisting of DNA which contains the cell history, the basic information to manufacture all the proteins characteristic of that cell. As architect, it synthesizes RNA from DNA and ships it through its pores to the cytoplasm for use in protein synthesis. The. N u c l e o l u s is an organelle within the nucleus which is involved actively in ribosome synthesis and in the transfer of RNA to the cytosol. 2. Golgi Apparatus - membrane-bound structure that plays a role in packaging peptides and proteins (including neurotransmitters) into vesicles. 3. Polyribosomes - there are several free ribosomes attached by a thread. The thread is a single strand of mRNA (messenger RNA, a molecule involved in the synthesis of proteins outside the nucleus). The associated ribosomes work on it to make multiple copies of the same protein. 4. Neuronal membrane (see next box) 5. Mitochondrion - this is the part of the cell responsible for the supply of energy in the form of ATP (adenosine triphosphate). Neurons need an enormous amount of energy. The brain is one of the most metabolically active tissues in the body. In man, for example, the brain uses 40 ml of oxygen per minute. Mitochondria use oxygen and glucose to produce most of the cell's energy. The brain consumes large amounts of ATP. The chemical energy stored in  ATP is used to fuel most of the biochemical reactions of the neuron. For example, special proteins in the neuronal membrane use the energy released 28

by the breakdown of ATP into ADP to pump certain substances across the membrane to establish concentration differences between the inside of the neuron and the outside. 6. Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum (7) - A system of tubes for the transportation of materials within the cytoplasm. It may have ribosomes (rough ER) or no ribosomes (smooth ER). With ribosomes, the ER is important for protein synthesis. Nissl Bodies - Groups of ribosomes used for protein synthesis.

2. Neuronal Membrane

The neuronal membrane serves as a barrier to enclose the cytoplasm inside the neuron, and to exclude certain substances that float in the fluid that bathes the neuron. The membrane with its mosaic of proteins is responsible for many important functions: 

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keeping certain ions and small molecules out of the cell and letting others in, accumulating nutrients, and rejecting harmful substances, catalyzing enzymatic reactions, establishing an electrical potential inside the cell, conducting an impulse being sensitive to particular neurotransmitters and modulators .

The membrane is made of lipids and proteins - fats and chains of amino acids. The basic structure of this membrane is a bilayer or sandwich of phospholipids, organized in such a way that the polar (charged) regions face outward and the non-polar regions face inward. The external face of the membrane contains the receptors, small specialized molecular regions which provide a kind of "attachment port" for other external molecules, in a scheme analogous to a key and a keyhole. For each external molecule there is a corresponding receptor. Whenever receptors become attached to a molecule, some alterations of the membrane and in the interior of the cell ensue, such as the modification of permeability to some ions.

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3. Dendrites

These structures branch out in treelike fashion and serve as the main apparatus for receiving signals from other nerve cells. They function as an "antennae" of the neuron and are covered by thousands of synapses. The dendritic membrane under the synapse (the post-synaptic membrane) has many specialized protein molecules called receptors that detect the neurotransmitters in the synaptic cleft. A nerve cell can have many dendrites which branch many times, their surface is irregular and covered in dendritic spines which are where the synaptic input connections are made.

4. Axon

Usually a long process which often projects to distant regions of the nervous system. The axon is the main conducting unit of the neuron, capable of conveying electrical signals a long distances that range from as short as 0.1 mm to as long as 2 m. Many axon split into several branches, thereby conveying information to different targets. Many neurons do not have axons. In these so-called amacrine neurons, all the neuronal processes are dendrites. Neurons with very short axons are also found.

Axon The axons of many neurons are wrapped in a myelin sheath, which is composed of the membranes of intersticial cells and is wrapped around the axons to form several concentric layers. The myelin sheath is broken at various points by the nodes of Ranvier, so that in cross section it looks like a string of sausages. The myelin protects the axon, and prevents interference between axons as they pass along in bundles, sometimes thousands at time. The cells that wrap around peripheral nerve fibers - that is, nerve fibers outside of the brain and spinal cord - are called Schwann cells (because they were first described by Theodor Schwann). The cells that wrap around axons within the central nervous system (brain and spinal cord) are called oligodendrocytes. The axon, with its surrounded sheath, is called a nerve fiber. Between each pair of successive Schwann cells is a gap of a node of Ranvier .

The Axon Hillock The axon hillock is where the axon is joined to the cell. It is from here that 30

the electrical firing known as an action potential usually occurs.

5. Nerve Ending (Presynaptic Terminals)

Synapses are the junctions formed with other nerve cells where the presynaptic terminal of one cell comes into ' contact' with the postsynaptic membrane of another. It is at these junctions that neurons are excited, inhibited, or modulated. There are two types of synapse, electrical and chemical. Electrical synapses occur where the presynaptic terminal is in electrical continuity with the postsynaptic. Ions and small molecules passing through, thus connecting channels from one cell to the next, so that electrical changes in one cell are transmitted almost instantaneously to the next. Ions can generally flow both ways at these junctions i.e. they tend to be bi-directional, although there are electrical junctions where the ions can only flow one way, these are known as rectifying junctions. Rectifying junctions are used to synchronize the firing of nerve cells. Chemical synaptic junction is more complicated. The gap between the postand presynaptic terminals is larger, and the mode of transmission is not electrical, but carried by neurotransmitters, neuroactive substances released at the presynaptic side of the junction. There are two types of chemical junctions. Type I is an excitatory synapse, generally found on dendrites, type II is an inhibitory synapse, generally found on cell bodies. Different substances are released at these two types of synapse. The direction of flow of information is usually one way at these junctions. Each terminal button is connected to other neurons across a small gap called a synapse. The physical and neurochemical characteristics of each synapse determines the strength and polarity of the new input signal. This is where the brain is the most flexible, and the most vulnerable. Changing the constitution of various neurotransmitter chemicals can increase or decrease the amount of stimulation that the firing axon imparts on the neighboring dendrite. Altering the neurotransmitters can also change whether the stimulation is excitatory or inhibitory.

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Nerve Cell

Many nerve cells are of the basic type illustrated above. Some kind of stimulus triggers an electric discharge of the cell which is analo gous to the discharge of a capacitor. This produces an electrical pulse on the order of 50-70 millivolts called an action potential. The electrical impulse  propagates down the fiber-like extension of the nerve cell (the axon). The speed of transmission depends upon the size of the fiber, but is on the order of tens of meters per second - not the speed of light transmission that occurs with electrical signals on wires. Once the signal reaches the axon terminal bundle, it may be transmitted to a neighboring nerve cell with the action of a chemical neurotransmitter. The dendrites serve as the stimulus receptors for the neuron, bu t they respond to a number of different types of stimuli. The neurons in the optic nerve respond to electrical stimuli sent by the cells of the retina. Other types of receptors respond to chemical neurotransmitters. The cell body contains the necessary structures for keeping the neuron functional. That includes the nucleus, mitochondria, and other organelles. Extending from the opposite side of the cell  body is the long tubular extension called the axon. Surrounding the axon is the myelin sheath, which plays an important role in the rate of electrical transmission. At the terminal end of the axon is a branched structure with ends called synaptic knobs. From this structure chemical signals can be sent to neighboring neurons.

Transmission of a nerve impulse along an axon

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A nerve cell is like a receiver, transmitter and transmission line with the task of passing a signal along from its dendrites to the axon terminal bundle. The stimulus triggers an action potential in the cell membrane of the nerve cell, and that action  potential provides the stimulus for a neighboring segment of the cell membrane. When the  propagating action potential reaches the axon, it proceeds down that "transmission line" by successive excitation of segments of the axon membrane. Just the successive stimulation of action potentials would result in slow signal transmission down the axon. The propagation speed is considerably increased by the action of the myelin sheath.

The myelin sheath around the axon prevents the gates on that part of the axon from opening and exchanging their ions with the outside environment. There are gaps between the myelin sheath cells known as the Nodes of Ranvier. At those uncovered areas of the axon membrane, the ion exchange necessary for the production of an action potential can take place. The action potential at one node is sufficient to excite a response at the next node, so the nerve signal can propagate faster by these discrete jumps than by the continuous propagation of depolarization/repolarization along the membrane. This enhanced signal transmission is called saltatory conduction (from the Latin saltare, to jump or hop). Tuzynski and Dixon offer some quantification of t he sizes involved in these nerve cells. The axon is made up of connected segments of length about 2 mm and diameter typically 20 m. This diameter compares to about 100 m for the diameter of a human hair. Axon diameters may vary from 0.1 m to 20 m and may be up to a meter long. The much-studied squid has a giant axon of about a millimeter in diameter. The myelin sheaths are about 1mm in length. The action  potential travels along the axon at speeds from 1 to 100 m/s.

THE NERVOUS SYSTEM The Neuron  Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.

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 Nerve Cells(yellow)and Astrocyte(green) (SEM x2,250). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission. The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three  parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body.

Structure of a typical neuron. The above image is from http://eleceng.ukc.ac.uk/~sd5/pics/research/big/neuron.gif. Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the system. Motor hav e a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.

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Structure of a neuron and the direction of nerve message transmission. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission. Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwan n cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second.

Cross section of myelin sheaths(grenn)  that surround axons (yellow) (TEM x191,175). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Structure of a nerve bundle. Image from Purves et al., Life: The Science of Biology, 4th Edition,  by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with  permission. The Nerve Message

The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a  positive charge, inside has a negative charge. This charge difference is a resting potential and is measured in millivolts. Passage of ions across the cell membrane passes the electrical charge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (resting potential). Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium  pump. The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients. 35

Transmission of an action potential. The above image is from http://eleceng.ukc.ac.uk/~sd5/pics/research/big/actpot.gif. Changed polarity of the membrane, the action potential, results in propagation of the nerve impulse along the membrane. An action potential is a temporary reversal of the electrical  potential along the membrane for a few milliseconds. Sodium gates and potassium gates open in the membrane to allow their respective ions to c ross. Sodium and potassium ions reverse  positions by passing through membrane protein channel gates that can be opened or closed to control ion passage. Sodium crosses first. At the height of the membrane potential reversal,  potassium channels open to allow potassium ions to pass to the outside of the membrane. Potassium crosses second, resulting in changed ionic d istributions, which must be reset by the continuously running sodium-potassium pump. Eventually enough potassium ions pass to the outside to restore the membrane charges to those of the original resting potential. The cell begins then to pump the ions back to their original sides of the membrane. The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period, during which the membrane cann ot  be stimulated. This prevents the message from being transmitted backward along the membrane. Steps in an Action Potential

1. At rest the outside of the membrane is more positive than the inside. 2. Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside. 3. Potassium ions flow out of the cell, restoring the resting potential net ch arges. 4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.

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Synapses

The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of  neurotransmitters. Neurotransmitters are stored in small synaptic vesicles clustered at the tip of the axon.

A synapse. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Excitatory Synapse from the Central Nervous System (TEM x 27,360). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission. Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft. Released n eurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action pot ential, others are inhibitory.  Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either destroyed  by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell's membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses. Once in the cleft, neurotransmitters

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are active for only a short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled. Diseases that affect the function of signal transmission can have serious consequences. Parkinson's disease has a deficiency of the neu rotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting as a substitute neurotransmitter)  but cannot reverse the progression of the disease. The bacterium Clostridium tetani produces a toxin that prevents the release of GABA. GABA is important in control of skeletal muscles. Without this control chemical, regulation of muscle contraction is lost; it can be fatal when it effects the muscles used in breathing. Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes the  progressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in the synapses: cocaine, LSD, caffeine, and insecticides. Nervous Systems

Multicellular animals must monitor and maintain a constant internal environment as well as monitor and respond to an external environment. In many animals, these two functions are coordinated by two integrated and coordinated organ systems: the nervous system and the endocrine system. Three basic functions are performed by nervous systems: 1. Receive sensory input from internal and external env ironments 2. Integrate the input 3. Respond to stimuli Sensory Input

Receptors are parts of the nervous system that sense changes in the internal or external environments. Sensory input can be in many forms, including pressure, taste, sound, light, blood  pH, or hormone levels that are converted to a signal and sent to the brain or spinal cord. Integration and Output

In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc. Endocrine Systems

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Some animals have a second control system, the endocrine system. The n ervous system coordinates rapid responses to external stimuli. The endocrine system controls slower, longer lasting responses to internal stimuli. Activity of both systems is integrated. Divisions of the Nervous System

The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops. The Central Nervous System (CNS) includes the brain and spinal cord. The Peripheral Nervous System (PNS) conn ects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).  Not all animals have highly specialized nervous systems. Those with simple systems tend to be either small and very mobile or large and immobile. Large, mobile animals have highly developed nervous systems: the evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility. Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net. These creatures have radial symmetry and lack a head. Although lacking a brain or either nervous system (CNS or PNS) nerve nets are capable of some complex behavior.

 Nervous systems in radially symmetrical animals. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission. Bilaterally symmetrical animals have a body plan that includes a defined head and a tail region. Development of bilateral symmetry is associated with cephalization, the development of a head with the accumulation of sensory organs at the front end of the organism. Flatworms have neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrates have a spinal cord in addition to a more developed brain.

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Some nervous systems in bilaterally symmetrical animals. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission. Chordates have a dorsal rather than central nervous system. Several evolutionary trends occur in chordates: spinal cord, continuation of cephalization in the form of larger and more complex  brains, and development of a more elaborate nervous system. The vertebrate nervous system is divided into a number of parts. The central nervous system includes the brain and spinal cord. The peripheral nervous system consists of all body nerves. Motor neu ron pathways are of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands). The autonomic system is subdivided into the sympathetic and parasympathetic systems. Peripheral Nervous System

The Peripheral Nervous System (PNS) contains only nerves and connects the brain and spinal cord (CNS) to the rest of the body. The axons and dendrites are surrounded by a white myelin sheath. Cell bodies are in the central nervous system (CNS) or  ganglia. Ganglia are collections of nerve cell bodies. Cranial nerves in the PNS take impulses to and from the brain (CNS). Spinal nerves take impulses to and away from the spinal cord. There are two major subdivisions of the PNS motor pathways: the somatic and the autonomic. Two main components of the PNS: 1. Sensory (afferent) pathways that provide input from the body into the CNS. 2. Motor (efferent) pathways that carry signals to muscles and glands (effectors). Most sensory input carried in the PNS remains below the level of conscious awareness. Input that does reach the conscious level contributes to perception of our external environment. Somatic Nervous System

The Somatic Nervous System (SNS) includes all nerves controlling the muscular system and external sensory receptors. External sense organs (including skin) are receptors. Muscle fibers 40

and gland cells are effectors. The reflex arc is an automatic, involuntary reaction to a stimulus. When the doctor taps your knee with the rubber hammer, she/he is testing your reflex (or knee jerk). The reaction to the stimulus is involuntary, with the CNS being informed but not consciously controlling the response. Examples of reflex arcs include balance, the blinking reflex, and the stretch reflex. Sensory input from the PNS is processed b y the CNS and responses are sent by the PNS from the CNS to the organs of the body. Motor neurons of the somatic system are distinct from those of the autono mic system. Inhibitory signals cannot be sent through the motor neurons of the somatic system. Autonomic Nervous System

The Autonomic Nervous System is that part of PNS consisting of motor neurons that control internal organs. It has two subsystems. The autonomic system controls muscles in the heart, the smooth muscle in internal organs such as the intestine, bladder, and uterus. The Sympathetic  Nervous System is involved in the fight or flight response. The Parasympathetic Nervous System is involved in relaxation. Each of these subsystems operates in the reverse of the other (antagonism). Both systems innervate the same organs and act in opposition to maintain homeostasis. For example: when you are scared the sympathetic system causes your heart to beat faster; the parasympathetic system reverses this effect. Motor neurons in this system do not reach their targets directly (as do those in the somatic system) but rather connect to a secondary motor neuron which in turn innervates the target organ. Central Nervous System

The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS is surrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain and spinal cord.

Areas of the brain. The above image is from http://www.prs.k12.nj.us/schools/PHS/Science_Dept/APBio/pic/brain.gif. 41

The brain is composed of three parts: the cerebrum (seat of consciousness), the cerebellum, and the medulla oblongata (these latter two are "part of the unconscious b rain"). The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccupping. The hypothalamus regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature, water balan ce, and blood pressure, and links the Nervous System to the Endocrine System. The midbrain and pons are also part of the unconscious brain. The thalamus serves as a central relay point for incoming nervous messages. The cerebellum is the second largest part of th e brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates  balance. The conscious brain includes the cerebral he mispheres, which are separated by the corpus callosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory dat a and motor functions. The cerebrum governs intelligence and reasoning, learning and memory. While the cause of memory is not yet definitely known, studies on slugs indicate learning is accompanied  by a synapse decrease. Within the cell, learning involves change in gene regulation and increased ability to secrete transmitters. 1 INTRODUCTION

In this chapter we consider the structure of nerve and muscle tissue and in particular their membranes, which are excitable. A qualitative description of the activation process follows. Many new terms and concepts are mentioned only briefly in this chapter but in more detail in the next two chapters, where the same material is dealt with from a quantitative rather than a qualitative point of view. The first documented reference to the nervous system is found in ancient Egyptian records. The Edwin Smith Surgical Papyrus, a cop y (dated 1700 B.C.) of a manuscript composed about 3500 B.C., contains the first use of the word "brain", along with a description of the coverings of the brain which was likened to the film and corrugations that are seen on the surface of molten copper as it cooled (Elsberg, 1931; Kandel and Schwartz, 1985). The basic unit of living tissue is the cell. Cells are specialized in their anatomy and  physiology to perform different tasks. All cells exhibit a voltage difference across the cell membrane. Nerve cells and muscle cells are excitable. Their cell membrane can produce electrochemical impulses and conduct them along the membrane. In muscle cells, this electric phenomenon is also associated with the co ntraction of the cell. In other cells, such as gland cells and ciliated cells, it is believed that the membrane voltage is important to the execution of cell function. The origin of the membrane voltage is the same in nerve cells as in muscle cells. In both cell types, the membrane generates an impulse as a consequence of excitation. This impulse propagates in both cell types in the s ame manner. What follows is a short introduction to the anatomy and physiology of nerve cells. The reader can find more 42

detailed information about these questions in other sources such as Berne and Levy (1988), Ganong (1991), Guyton (1992), Patton et al. (1989) and Ruch and Patton (1982). 2.2 NERVE CELL 2.2.1 The Main Parts of the Nerve Cell

The nerve cell may be divided on the basis of its structure and function into three main  parts: (1) the cell body, also called the soma; (2) numerous short processes of the soma, called the dendrites; and, (3) the single long nerve fiber, the axon. These are described in Figure 2.1. The body of a nerve cell (see also (Schadé and Ford, 1973)) is similar to that of all other cells. The cell body generally includes the nucleus, mitochondria, endoplasmic reticulum, ribosomes, and other organelles. Since these are not unique to the nerve cell, they are not discussed further here. Nerve cells are about 70 - 80% water; the dry material is about 80% protein and 20% lipid. The cell volume varies between 600 and 70,000 µm³. (Schadé and Ford, 1973) The short processes of the cell body, the dendrites, receive impulses from other cells and transfer them to the cell body (afferent signals). The effect of these impulses may  be excitatory or  inhibitory. A cortical neuron (shown in Figure 2.2) may receive impulses from tens or even hundreds of thousands of neurons (Nunez, 1981). The long nerve fiber, the axon, transfers the signal from the cell body to another nerve or to a muscle cell. Mammalian axons are usually about 1 - 20 µm in diameter. Some axons in larger animals may be several meters in length. The axon may be covered with an insulating layer called the myelin sheath, which is formed by Schwann cells (named for the German physiologist Theodor Schwann, 1810 -1882, who first observed the myelin sheath in 1838). The myelin sheath is not continuous but divided into sections, separated at regular intervals by the nodes of Ranvier  (named for the French anatomist Louis Antoine Ranvier, 1834-1922, who observed them in 1878).

Fig. 2.1. The major components of a neuron.

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Fig. 2.2. Cortical nerve cell and nerve endings connected to it. 2.2.3 The Synapse

The junction between an axon and the next cell with which it communicates is called the synapse. Information proceeds from the cell body unidirectional over the synapse, first along the axon and then across the synapse to the next nerve or muscle cell. The part of the synapse that is on the side of the axon is called the presynaptic terminal ; that part on the side of the adjacent cell is called the postsynaptic terminal . Between these terminals, there exists a gap, the synaptic cleft, with a thickness of 10 - 50 nm. The fact that the impulse transfers across the synapse only in one direction, from the presynaptic terminal to the postsynaptic terminal, is due to the release of a chemical transmitter by the presynaptic cell. This transmitter, when released, activates the postsynaptic terminal, as shown in Figure 2.5. The synapse between a motor nerve and the muscle it innervates is called the neuromuscular junction. Information transfer in the synapse is discussed in more detail in Chapter 5.

Fig. 2.5. Simplified illustration of the anatomy of the synapse. A) The synaptic vesicles contain a chemical transmitter. B) When the activation reaches the presynaptic terminal the transmitter is released and it diffuses across the synaptic cleft to activate the postsynaptic membrane. 2.4 BIOELECTRIC FUNCTION OF THE NERVE CELL

The membrane voltage (transmembrane voltage) (V m) of an excitable cell is defined as the potential at the inner surface (Φ i) relative to that at the outer (Φ o) surface of the membrane, i.e. V m= (Φi) - (Φo). This definition is independent of the cause of the  potential, and whether the membrane voltage is constant, periodic, or nonperiodic in  behavior. Fluctuations in the membrane potential may be classified according to their character in many different ways. Figure 2.7 show s the classification for nerve cells developed by Theodore Holmes Bullock (1959). According to Bullock, these transmembrane potentials may be resolved into a resting potential and potential changes due to activity. The latter may be classified into three different types: 44

1. Pacemaker potentials: the intrinsic activity of the cell which occurs without external excitation. 2. Transducer potentials across the membrane, due to external events. These include generator potentials caused by receptors or synaptic potential  changes arising at synapses. Both subtypes can be inhibitory or excitatory. 3. As a consequence of transducer potentials, further response will arise. If the magnitude does not exceed the threshold, the response will be nonpropagating (electrotonic). If the response is great enough, a nerve impulse (action potential impulse) will be produced which obeys the all-or-nothing law (see below) and  proceeds unattenuated along the axon or fiber.

Fig. 2.7. Transmembrane potentials according to Theodore H. Bullock. 2.5 EXCITABILITY OF NERVE CELL

If a nerve cell is stimulated, the transmembrane voltage necessarily change s. The stimulation may be excitatory (i.e., depolarizing ; characterized by a change of the potential inside the cell relative to the outside in the positive direction, and hence by a decrease in the normally negative resting voltage) or inhibitory (i.e., hyperpolarizing , characterized by a change in the potential inside the cell relative to the outside in the negative direction, and hence by an increase in the magnitude of the membrane voltage). After stimulation the membrane voltage returns to its original resting value.

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If the membrane stimulus is insufficient to cause the transmembrane potential to reach the threshold, then the membrane will not activate . The response of the membrane to this kind of stimulus is essentially passive. Notable research on membrane behavior under subthreshold  conditions has been performed by Lorente de Nó (1947) and Davis and Lorente de Nó (1947). If the excitatory stimulus is strong enough, the transmembrane potential rea ches the threshold, and the membrane produces a characteristic electric impulse, the nerve impulse. This potential response follows a characteristic form regardless of the strength of the transthreshold stimulus. It is said that the action impulse of an activated membrane follows an all-or-nothing  law. An inhibitory stimulus increases the amount of concurrent excitatory stimulus necessary for achieving the threshold (see Figure 2.8). (The elect ric recording of the nerve impulse is called the action potential . If the nerve impulse is recorded magnetically, it may be called an action current . The terminology is further explicated in Section 2.8 and in Figure 2.11, below.)

Fig. 2.8. (A) Experimental arrangement for measuring the response of the membrane potential (B) to inhibitory (1) and excitatory (2, 3, 4) stimuli (C). The current stimulus (2), while excitatory is, however, subthreshold, and only a  passive response is seen. For the excitatory level (3), threshold is marginally reached; the membrane is sometimes activated (3b), whereas a t other times only a local response (3a) is seen. For a stimulus (4), which is clearly transthreshold, a nerve impulse is invariably initiated. 2.6 THE GENERATION OF THE ACTIVATION

The mechanism of the activation is discussed in detail in Chapter 4 in connection with the Hodgkin-Huxley membrane model. Here the gene ration of the activation is discussed only in general terms.

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+

The concentration of sodium ions (Na ) is about 10 times higher outside the membrane + than inside, whereas the concentration of the potassium (K  ) ions is about 30 times higher inside as compared to outside. When the membrane is stimulated so that the transmembrane potential rises about 20 mV and reaches the threshold - that is, when the membrane voltage changes from -70 mV to about -50 mV (these are illustrative and common numerical values) - the sodium and potassium ionic permeabilities of the membrane change. The sodium ion permeability increases very rapidly at first, allowing sodium ions to flow from outside to inside, making the inside more positive. The inside reaches a potential of about +20 mV. After that, the more slowly increasing potassium ion permeability allows potassium ions to flow from inside to outside, thus returning the intracellular potential to its resting value. The maximum excursion of the membrane voltage during activation is about 100 mV; the duration of the nerve impulse is around 1 ms, as illustrated in Figure 2.9. While at rest, following activation, the Na-K pump restores the ion concentrations inside and outside the membrane to their original values.

Fig. 2.9. Nerve impulse recorded from a cat motoneuron following a transthreshold stimulus. The stimulus artifact may be seen at t  = 0. 2.7 CONCEPTS ASSOCIATED WITH THE ACTIVATION PROCESS

Some basic concepts associated with the activation p rocess are briefly defined in this section. Whether an excitatory cell is activated depends largely on the strength and duration of the stimulus. The membrane potential may reach the threshold by a short, strong stimulus or a longer, weaker stimulus. The curve illustrating this dependence is called the strength-duration curve; a typical relationship between these variables is illustrated in Figure 2.10. The smallest current adequate to initiate activation is called the rheobasic current  or rheobase. Theoretically, the rheobasic current needs an infinite duration to trigger activation. The time needed to excite the cell with twice rheobase current is called chronaxy.  Accommodation and habituation denote the adaptation of the cell to a continuing or repetitive stimulus. This is characterized by a rise in the excitation threshold. Facilitation denotes an increase in the excitability of the cell; correspondingly, there is a decrease in the threshold. Latency denotes the delay between two events. In the  present context, it refers to the time between application of a stimulus pulse and the  beginning of the activation. Once activation has been initiated, the membrane is insensitive to new stimuli, no matter how large the magnitude. This phase is called the absolute refractory period . Near the end of the activation impulse, the cell may be 47

activated, but only with a stimulus stronger than normal. Th is phase is called the relative refractory period . The activation process encompasses certain specifics such as currents, potentials, conductivities, concentrations, ion flows, and so on. The term action impulse describes the whole process. When activation occurs in a nerve cell, it is called a nerve impulse; correspondingly, in a muscle cell, it is called a muscle impulse. The bioelectric measurements focus on the electric potential  difference across the membrane; thus the electric measurement of the action impulse is called the action  potential  that describes the behavior of the membrane potential during the activation. Consequently, we speak, for instance, of excitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP). In biomagnetic measurements, it is the electric current  that is the source of the magnetic field. Therefore, it is logical to use the term action current  to refer to the source of the biomagnetic signal during the action impulse. These terms are further illustrated in Figure 2.11.

Fig. 2.10. (A) The response of the membrane to various stimuli of changing strength (B), the strength-duration curve. The level of current strength which will  just elicit activation after a very long stimulus is called rheobase. The minimum time required for a stimulus pulse twice the rheobase in strength to trigger

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activation is called chronaxy. (For simplicity, here, threshold is shown to be independent on stimulus duration.)

Fig. 2.11. Clarification of the terminology used in connection with the action impulse: A) The source of the action impulse may be nerve or muscle cell. Correspondingly it is called a nerve impulse or a muscle impulse. B) The electric quantity measured from the action impulse may be potential or current. Correspondingly the recording is called an action potential or an action current. 2.8 CONDUCTION OF THE NERVE IMPULSE IN AN AXON

Ludvig Hermann (1872, 1905) correctly proposed that the activation propagates in an axon as an unattenuated nerve impulse. He suggested that the potential difference  between excited and unexcited regions of an axon would cause small currents, now called local circuit currents, to flow between them in such a direction that they stimulate the unexcited region. Although excitatory inputs may be seen in the dendrites and/or soma, activation originates normally only in the soma. Activation in the form of the nerve impulse (action  potential) is first seen in the root of the axon - the initial segment of the axon, often called the axon hillock . From there it propagates along the axon. If excitation is initiated artificially somewhere along the axon, propagation then takes place in both directions from the stimulus site. The conduction velocity depends o n the electric properties and the geometry of the axon. An important physical property of the membrane is the change in sodium conductance due to activation. The higher the maximum value achieved by the sodium conductance, the higher the maximum value of the sodium ion current and the higher the rate of change in the membrane voltage. The result is a higher gradient of voltage, increased local currents, faster excitation, and increased conduction velocity. The decrease in the threshold potential facilitates the triggering of the activation process. The capacitance of the membrane per unit length determines the amount of charge required to achieve a certain potential and therefore affects the time needed to reach the

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threshold. Large capacitance values, with other parameters remaining the same, mean a slower conduction velocity. The velocity also depends on the resistivity of the medium inside and outside the membrane since these also affect the depolarization time constant. The smaller the resistance, the smaller the time constant and the faster the co nduction velocity. The temperature greatly affects the time constant of the sodium conductanc e; a decrease in temperature decreases the conduction velocity. The above effects are reflected in an expression derived by Muler and Markin (1978) using an idealized nonlinear ionic current function . For the velocity of the propagating nerve impulse in unmyelinated axon, they obtained (2.1) where

v

= velocity of the nerve impulse [m/s]

i Na max = maximum sodium current per unit length [A/m] V th

= threshold voltage [V]

r i

= axial resistance per unit length [Ω/m]

cm

= membrane capacitance per unit length [F/m]

A myelinated axon (surrounded by the myelin sheath) can produce a nerve impulse only at the nodes of Ranvier. In these axons the nerve impulse propagates from one node to another, as illustrated in Figure 2.12. Such a propagation is called saltatory conduction ( saltare, "to dance" in Latin). The membrane capacitance per unit length of a myelinated axon is much smaller than in an unmyelinated axon. Therefore, the myelin sheath increases the conduction velocity. The resistance of the axoplasm per unit length is inversely proportional to the crosssectional area of the axon and thus to the square of the diameter. The membrane capacitance per unit length is directly proportional to the diameter. Because the time constant formed from the product controls the nodal transmembrane potential, it is reasonable to suppose that the velocity would be inversely proportional to the time constant. On this basis the conduction velocity of the myelinated axon should be directly  proportional to the diameter of the axon. This is confirmed in Figure 2.13, which shows the conduction velocity in mammalian myelinated axons as linearly dependent on the diameter. The conduction velocity in myelinated axon has the approximate value shown: v = 6d   where

v = velocity [m/s] d  = axon diameter [µm]

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(2.2)

Fig. 2.12. Conduction of a nerve impulse in a nerve axon. (A) continuous conduction in an unmyelinated axon; (B) saltatory conduction in a myelinated axon.

Fig. 2.13. Experimentally determined conduction velocity of a nerve impulse in a mammalian myelinated axon as a function of the diameter. (Adapted from Ruch and Patton, 1982.)

Neurons: Our Internal Galaxy Silvia Helena Cardoso, PhD Not only have the stars in the Universe fascinated Man with its impressive numbers. In another universe, our own, biological one, a gigantic "galaxy" with billions of small neural cells forms our brain and the rest of the nervous system, and communicates among themselves by means of  flashes of electrochemical pulses. They are responsible for everything: our feelings, thinking, emotions, pain, dreams, movements and sensations, and many other mental and physical  functions. Without them, it would be impossible to achieve our rich internal world and to communicate with the surrounding environment, by means of sound, smell, taste, touch and light; including that of the stars in our Universe.

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All stimuli of our environment causing sensations such as pain and hot, all feelings, thoughts, programming of motor and emotional responses, neural bases of learning and memory, actions of psychoactive drugs, causes of mental disorders, and any other action or sensation of the human being cannot be understanding without the knowledge of the fascinating process of communication between neurons. Neurons are specialized cells. They are made to receive certain specific connections, to perform appropriate functions and pass their decision of a particular event to other neurons which are also concerned with those events. These specializations include a cell membrane, which is specialized to convey nerve signals as electrochemical pulses; the dendrite,  (from the Greek Dendron, or tree) which gets and delivers the signals, the axon (from the Greek axoon, or axis), the conducting cable of electrical signals, and points of synaptic contacts, where information can be passed on from one cell to another (see fig.1).

Fig.1. The structure of the neuron. A typical neuron has four morphologically defined regions: dendrites (1), cell body (2), axon (3), and presynaptic terminals (5).

Neurons receive nerve signals from axons of other neurons. Most signals are delivered to dendrites (1). The signals generated by a neuron are carried away from its cell body (2), which contains the nucleus (2a), the storehouse of genetic information. Axons (3) are the main conducting unit of the neuron. The axon hillock (2b) is the site at which the cell's signs are initiated. Schwann cells (6), which are not a part of a nerve cell, but one of the types of glial cells, perform the important function of insulating axons by wrapping their membranous processes around the axon in a thight spiral, forming a myelin sheath (7), a fatty, white substance which helps axons transmit messages faster than unmyelinated ones. The myelin is broken at various points by the nodes of Ranvier (4), so that in crosssection it looks rather like a string of sausages. Branches of the axon of one neuron (the presynaptic neuron) transmit signals to another neuron (the postsynaptical cell) at a site called the synapse (5). The branches of a 52

single axon may form synapses with as many as 1000 other neurons. What Make Neurons Different from Other Cells? Just like other cells, neurons feed, breath, have the same genes, the same biochemical mechanisms and the same organelles. So, what makes the neuron different? Neurons differ from other cells in one important respect: they process information. They must gather information about the internal state of the organism and his external environment, evaluate this information, and coordinate activities appropriate to the situation and to the person's current needs. The information is processed through an event known as the nerve impulse. A nerve impulse is the transmission of a coded signal from a given stimulus along the membrane of the neuron, from the point that it was stimulated. Two types of phenomena are involved in processing the nerve impulse: electrical and chemical . Electrical events propagate a signal within a neuron, and chemical processes transmit the signal from one neuron to another  or to a muscle cell. The chemical process on interaction between neurons occurs at the end of the axon, called synapse. Touching intimately against the dendrite of other cell (but without material continuity between both cells), the axon releases chemical substances called neurotransmitters,  which attach themselves to chemical receptors in the membrane of the following neuron. The Brain is Grey and White. Why? Maybe you have heard the term "grey matter" for the brain; there is also "white matter". In a section made through the brain, it is easy to see both grey and white areas. The cortex and other nerve centers are grey, the regions in between, and white. The grey coloration is produced by the aggregation of thousands of cell bodies, while the white is the color of myelin. The white color reveals the presence of bundles of axons passing through the brain, rather than areas in which connections are being made. No neuron has direct connection with any other. At the far end of the axon are a number of terminal filaments and these run up close to other neurons? They may be close to the dendrites of the other neuron ( sometimes to special structures called dendritic spines, or close to the cell body itself. Where the first neuron comes close to the second neuron, a synapse is formed, a space acreoss which the first neuron communicate with the second.

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Anatomical Diversity of Neurons

a. Purkinge cell (human).pyramidal cell (rabbit). c. Motoneuron (cat).

g. Visual amacrine cell (mechanosensory interneuron- crayfish). h. Multipolar neuron (fly) i. Visual monopolar neuron (f ly). j. Premotor interneuron (crayfish)..

d. Horizontal cell (cat) e. Horizontal cell (cat).f. Premotor interneuron (locust).

k. Visual interneuron (locust).

The Nervous System The human ability to feel, reproduce, and even see the very text on this page is controled, in computer like calculations, by the magical nervous system. Yes, the ner vous system is quite like magic because you can't see it, but its working through electric impulses through your body. Although it seems magical , you can comprehend it all by keeping your eye s glued to this very web page.

Function One of the world’s most "intricately organized" electron mechanisms is the nervous system. Not even engineers have come close to making circuit boards and computers as delicate and precise as the nervous system. To understand this system, one has to know the tree simple functions that it puts into action: sensory input, integration, motor output.

Sensory input When your eyes see something or your hands or touch a warm surface, the sensory cells, also known as

Neorons, send a message straight to your brain. This action of getting information from your surrounding environment is called sensory input because your putting things in your brain by way of your senses. 54

Integration Integration is best known as the interpretation of things you have felt, tasted, and touched w ith your sensory cells, also known as neurons, into responses that the body recognizes. This process is all accomplished in the brain where many, many neurons work together to understand the environment.

Motor Output Once your brain has interpreted all that you have learned, either by touching, tasting, or using any other sense, then your brain sends a message through neurons to effecter cells, muscle or gland cells, which actually work to perform your requests and act upon your environment. The word motor output is easily remembered if one should think that you’re putting something out into the environment through the use of a motor, like a muscle which does the work for our body.

Nervous System Cells The two kinds of cells in the nervous system are: neurons and support cells.

Neurons We all have the common idea that neurons make up the majority of t he nervous cells. However, this idea is wrong since support cells which re inforce or help the neurons are more in number. Nevertheless, neuron cells are the primal cells of the system because they tr ansmit messages. Neuron cells differ in size and shape depending on where they ar e found in the body, but the structure of neurons stays constant. The three basic structures o f the neuron are: the dendrite, cell body, and axon.

Dendrite Dendrites are short, thick branched extensions which extend like the roots of a tree over other neurons or body cells. The dendrites all branch off dendritic spines, which in turn branch of the cell body. Dendrites are the receptive sites of the neurons. Here, the neurons receive electric messages from other neurons or body cells. The site where one dendrite meets another neuron's impulse is called the

synapse. Usually, neurons have hundreds of dendrite extensions. These extensions are spread over a large area, giving the neuron better reception of signals. Some dendrites are specialized for the accumulation of information. These cells are finer than other dendrites and fo und near the brain.

Cell Body Also called the perikaryon-sound or soma-sound,  the cell body contains a spherical nucleus with a nucleolus and lots of cytoplasm. Like many cells, the neuron cell body of the neuron contains the usual cellular particles or organelles-sound, except centrioles-sound. Centrioles are the basis by which cells are able to divide and form new cells. Because the neurons lack ce ntrioles, they are unable to divide and reproduce themselves. Therefore, if one should damage nerves, then they are not able to be replaced. Nevertheless, neurons do have specialized hard working endoplasmic reticulum-sound (ER), which help transport proteins and molecules at high speeds due to the fact that neurons work at lightning speeds.

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Also, the neurofibrils, bundles of micro filaments and micro tubules, which are important in intracellular transport, are seen through the body. A pigment called lipofuscin, which is yellow-brown, is one of the many pigments believed to be in the neuron. This one particular pigment is believed to be the cause of aging because it is found mostly neurons of elderly individuals.

Axon The axon is a long cylindrical tube, with t he same consistent diameter, which runs through the body for long or short lengths. For example, the axon of your neuron controlling your toe e xtends all the way from the lumbar back area. The axon branches off a cone shaped region of the cell body called the axon

hillock-sound. Axons diameters differ in many parts of the body, but the ruel is the thicker the axon, the more messages it transmits through the neurons. The main purpose of the axon is to se nd impulses away from the cell body to neuron dendrite or other body cells called effecter cells-sound. A nerve impulse travels from a dendrite, to the cell body, and down the axon to thousands of branches called telondria which connect at a synapse to dendrites from other neurons. Once the impulse reaches t he synapse, neurotransmitters, chemicals, which excite or calm effecter or neurons, diffuse into the ex tra cellular space and reach the dendrite, once again turning into an impulse. Protecting and insulating electric fibers from one another is the myelin sheath. It is a whitish, fatty, se gmented sheath which covers the majority of nerve fibers and helps transmit nerve impulses faster. Thought the axon of the neuron, cells which protect the neuron envelope . These cells forms slope like structures with indents in between them called a Node of Ranvier-sound. The myelin sheath is exceedingly important because one can lose control of your muscles due t o the uncoordinated fibers of an axon without myelin sheath.

Impulse movement Neurons communicate or send impulses through an action potential., This takes place from t he dendrite and all the way to the axon ends. An action potential is a change of voltage within the axon. In other words, the negative state off the inner axon turns positive when the impulses comes by. This happens by the use of a sodium and potassium pump. Sodium [Na] surrounds the axon with a positive charge, while the potassium [K] is within the axon. As an impulse enters at the axon hillock, the sodium, potassium pump puts positive sodium into the axon while it puts negative potassium out of the axon. As more sodium enters the potential of the impulse changes from -70 mV to +30 mV, (a difference of 100 mV) this change is called an action potential. The sodium, potassium pump works furiously to pass the impulse through the axon. As the impulse leaves the axon, it is reverted to a normal state which is called the resting potential. At the end of the axon, the impulse or stimulus enters the synapses and is called a post synaptic potential. From here, the impulse is tr ansferred into neurotransmitters, some of which are chemicals called epinephrine and dopamine. These neurotransmitters flow into the fluid filled gap called the synaptic cleft and enter the dendrites. And, again, the process is re peated.

Supporting cells

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Nine times more numerous than neurons, supporting cells assist, segregate and insulate neurons. Each supporting cell has its specific function and location. The majority of the centr al nervous system (CNS) support cells are referred to as glial cells-sound which wrap around the nerves o f the space to protect them. Other specific support cells are also very recognizable. Astrocyles are specific supporting cells which anchor neurons to capillaries for energy and regulate the ions around the nerve. Microgelia are a specific macrophage-hyp immune which help protect the central nervous system by engulfing invading microorganisms and dead nerve tissue. Ependymal cells form cerebral spinal fluid and help circulate t he fluid by the beating of their cilia. Scw huan cells are massive cells which cover the mylein sheath and protect neurons by acting as a phagocyte-hyp immune to clean damaged nerves. Satellite ce lls help control the chemical environment around nerve cells.

Nervous system The human nervous system is diagramed into two separately different system: peripheral nervous system (PS) and central nervous system (CNS).

Central Nervous System The central nervous system is the c entral of the nervous. The two and only organs included in t his system are of utmost importance: the spinal cor d and the brain.

Spinal cord Within the spinal cord one finds the association neuron. This neuron composes the majority of the spinal cord, and serves as an integration center or interpretation cente r, of sensory neurons and motor neurons. A sensory neuron informs the body of its environment, the association neuron interprets the information, and responds to the environment with the motor neuron.

Brain The brain is divided into three segments: the forebrain, midbrain, and hind brain. The chart and diagram below will help you understand the brain.

Peripheral Nervous System The peripheral nervous system is divided into two main groups of neurons: sensory and motor.

Sensory Sensory neurons or afferent-sound neurons provide information from the environment to the body. For example, when you touch a hot surface, a sensory neuron informs your body that the temperature near your skin is raising.

Motor Motor neurons or efferent-sound neurons are the neurons the body uses to react t o the environment. For example, if you touch a hot surface, then your body will make your hand move away from that 57