ANATOMY AND PHYSIOLOGY: Structure and function of the nervous system I. Structures A. The neurologic system consists of two main divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The autonomic nervous system (ANS) is composed of both central and peripheral elements. 1. The CNS is composed of the brain and spinal cord. 2. The PNS is composed of the 12 pairs of the cranial nerves and the 31 pairs of the spinal nerves. 3. The ANS is comprised of visceral efferent (motor) and the visceral afferent (sensory) nuclei in the brain and spinal cord. Its peripheral division is made up of visceral efferent and afferent nerve fibers as well as autonomic and sensory ganglia. B. The brain is covered by three membranes. 1. The dura matter is a fibrous, connective tissue structure containing several blood vessels. 2. The arachnoid membrane is a delicate serous membrane. 3. The pia matter is a vascular membrane. C. The spinal cord extends from the medulla oblongata to the lower border of the first lumbar vertebrae. It contains millions of nerve fibers, and it consists of 31 nerves – 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral. D. Cerebrospinal fluid (CSF) forms in the lateral ventricles in the choroid plexus of the pia matter. It flows through the foramen of Monro into to the third ventricle, then through the aqueduct of Sylvius to the fourth ventricle. CSF exits the fourth ventricle by the foramen of Magendie and the two foramens of Luska. It then flows into the cistema magna, and finally it circulates to the subarachnoid space of the spinal cord, bathing both the brain and the spinal cord. Fluid is absorbed by the arachnoid membrane. II.
Function
A. CNS
1. Brain a. The cerebrum is the center for consciousness,
thought, memory, sensory input, and motor activity; it consists of two hemispheres (left and right) and four lobes, each with specific functions. i The frontal lobe controls voluntary muscle movements and contains motor areas, including the area for speech; it also contains the centers for personality, behavioral, autonomic and intellectual functions and those for emotional and cardiac responses. ii The temporal lobe is the center for taste, hearing and smell, and in the brain’s dominant hemisphere, the center for interpreting spoken language.
iii The parietal lobe coordinates and interprets information from the opposite side of the body. iv The occipital lobe interprets visual stimuli. b.
sensory
The thalamus further organizes cerebral function by transmitting impulses to and from the cerebrum. It also is responsible for primitive emotional responses, such as fear, and for distinguishing between pleasant and unpleasant stimuli.
c. Lying beneath the thalamus, the hypothalamus is an automatic center that regulates blood pressure, temperature, libido, appetite, breathing, sleeping patterns, and peripheral nerve discharges associated with certain behavior and emotional expression. It also helps control pituitary secretion and stress reactions. d. The cerebellum or hindbrain, controls smooth muscle movements, coordinates sensory impulses with muscle activity, and maintains muscle tone and equilibrium.
e. The brain stem, which includes the mesencephalon, pons, and medulla oblongata, relays nerve impulses between the brain and spinal cord. 2. The spinal cord forms a two-way conductor pathway between the brain stem and the PNS. It is also the reflex center for motor activities that do not involve brain control. B. The PNS connects the CNS to remote body regions and conducts signals to and from these areas and the spinal cord. C. The ANS regulates body functions such as digestion, respiration, and cardiovascular function. Supervised chiefly by the hypothalamus, the ANS contains two divisions. 1. The sympathetic
nervous system serves as an emergency preparedness system, the “flight-for-fight” response. Sympathetic impulses increase greatly when the body is under physical or emotional stress causing bronchiole dilation, dilation of the heart and voluntary muscle blood vessels, stronger and faster heart contractions, peripheral blood vessel constriction, decreased peristalsis, and increased perspiration. Sympathetic stimuli are mediated by norepinephrine.
2. The parasympathetic nervous system is the dominant controller for most visceral effectors for most of the time. Parasympathetic impulses are mediated by acetylcholine. III. Differences in nervous system response. The nervous system is one of the first systems to form in utero, but one of the last systems to develop during childhood. A. Accuracy and completeness of the neurologic assessment is limited by the child’s development.
B. The child’s brain constantly undergoes organization in function and myelinization. Therefore, the full impact of insult may not be immediately apparent and may take years to manifest. C. The peripheral nerves are not fully myelinated at birth. As myelinization progresses, so does the child’s fine motor control and coordination. D. Early signs of increased intracranial pressure (ICP) may not be apparent in infants because open sutures and fontanelles compensate to a limited extent. E. The development of handedness before 1 year of age may signify a neurologic lesion. F. Several primitive reflexes are present at birth, disappearing by 1 year of age. Absence, persistence, or asymmetry of reflexes may indicate pathology. G. The spinal cord ends at 13 in the neonate, instead of L1-L2 where it terminates in the adult. This affects the site of lumbar puncture. H. Children have 65 to 140 ml of CSF compared to 90 to 150 ml in the adult.
