EXPERIMENT 5
NEUROPHYSIOLOGY OF NERVE IMPULSES (ACTIVITY 6-9) Y. A. Azucena, J. A. Bacud, S. C. Baquiran, T. J. Bautista 4BIO5 Department of Biological Sciences, College of Science, University of Santo Tomas, España Avenue, Manila
Keywords:
Summary
Introduction The nervous system functions as the signaling center of a whole organism. It is composed of neurons that fire up electrical signals, known as action potentials, to respond to the stimulus from the environment. Action potentials are produced by the depolarization of neurons due to the opening of voltage-gated channels on the membrane. Voltage-gated sodium channels are integral membrane proteins that allow the passage of sodium ions inside the cell causing the depolarization. Potassium ions, on the other hand, are released out of the cell through voltage-gated potassium channels in order
to reestablish the neuron to its nonconducting, polarized state. The fluctuation of the amount of ions inside the cell affects the membrane potential, which is measured in millivolts (mV). The amplitude of an action potential is constant, regardless of how strong the stimuli. The conduction or propagation of an electrical signal through the axon is considered as an all-or-none event. The signal is regenerated as it passes along the axon to ensure undiminished amplitude. The intensity of the stimulus is defined through the frequency of action potentials 1 | Page
per second. The time needed to recover from the previous stimulation is also affected by stimulus intensity and these are called refractory periods. The relative refractory period is the time after the first action potential when a second action potential can already be conducted if there is an increase of stimulus intensity. Conversely, an absolute refractory period is time after the first action potential is conducted when a second action potential cannot be conducted even with increased stimulus intensity. An action potential traverses through an axon at different velocities depending on the size and presence of glial cells or myelination. Glial cells or neuroglia are nonneuronal cells that support conduction of electrical signals. Myelination is the wrapping of certain glial cells around the axon. The glial cells form myelin sheaths around the axon that increase the velocity of the propagation of signals. These sheaths have gaps that separate them. These are called the nodes of Ranvier. Oligodendrocytes and Schwann cells are the types of glial cells that wrap around the central nervous system and peripheral nervous system respectively. The axon propagates the signal until it reaches the axon terminal synapse so it can relay the signal to another neuron’s dendrites then cell body. Sensory neurons communicate with the motor neuron through interneurons. An action potential causes the release of neurotransmitters to the synaptic gap and binds to receptor proteins of neurons to produce the appropriate reaction to a stimulus through a cascade of molecular events. Axon terminals are branches at the end of axons. These terminals are separated by synaptic gaps where neurotransmitters from intracellular
vesicles are released to a region called the chemical synapse. Calcium ions in the axon terminal trigger the exocytosis of the neurotransmitter-containing synaptic vesicles. The neurotransmitters diffuse and bind to membrane receptor proteins, causing a postsynaptic potential. Neurotransmitters can act as paracrine agents that affect local targets, autocrine agents that target other neurons, or endocrine agents that travel long distances through circulation. The objective of the simulation is to determine the effect of stimulus intensity the frequency of action potentials, effect of myelination and axon diameter to the conduction velocity of action potentials, role of calcium ions in the release of neurotransmitters, and response of the functional areas of neurons on varying stimuli.
Methodology Activity 6 - The Action Potential: Coding for Stimulus Intensity An axon was placed in a nerve chamber. The oscilloscope was used to observe if the timing of stimuli is appropriate and changes in the voltage along the axon. A stimulator was used to set the voltage of the stimulus and deliver electric signals for axon depolarization through stimulation wires (S). Voltage changes in the axon were recorded using recording electrodes, which are set 2 cm from the stimulation wires. The Oscilloscope was set at 100 milliseconds per division. Activity 7 – The Action Potential: Conduction Velocity The oscilloscope, stimulator, and stimulator wires were also used in the simulation. Three types of axon were placed 2 | Page
in the nerve chamber in order to test the differences in their conducting velocity (m/s). Two recording electrodes were used (R1 and R2). R1 was still 2 cm from the stimulation wires while R2 was 2 cm from R1. The distance between R1 and R2 was 10 cm (0.1 m).
Activity 9 – The Action Potential: Putting It All Together
A large sensory neuron and a large interneuron were impaled with small microelectrode probes. Hook electrodes were used to record the changes in the extracellular voltage along the axon. The oscciloscope was also utilized to observe voltage changes in the neuron and interneuron. The stimulator was also used to deliver low or high stimulus intensity.
Results & Discussion Activity 6
Figure 1.0 From left to right A) A fiber – large-diameter, heavily myelinated axon B) B fiber – medium-diameter, lightly myelinated axon C) C fiber – thin, unmyelinated axon
Activity 8 – Chemical Synaptic Transmission and Neurotransmitter Release In the simulation, an axon terminal was utilized to test the effect of varying calcium ion levels and magnesium ion on the release of neurotransmitters. A stimulator was used to deliver a low stimulus intensity or high stimulus intensity on the axon terminal. The axon terminal was immersed in solutions that contained normal calcium ion level, low calcium ion level, magnesium ion, and a solution that contained no calcium.
A single stimulus for 0.5 milliseconds at threshold voltage (20 mV) resulted to a single spike or action potential. When the duration of the stimulus was set at 500 milliseconds at the same voltage, multiple action potentials were recorded. The interspike interval (ISI) was used to determine the frequency of the action potential: 1 / ISI (sec). The interspike interval was calculated by subtracting the time in milliseconds of an action potential to a previous action potential. When the stimulus voltage was set 30 mV for 500 milliseconds, an increase in the frequency of action potentials were noted. With a duration of 500 millisceonds and a higher stimulus voltage (45 mV), the action potential frequency increased of the axon increased. An increase in the stimulus intensity also increases the action potential frequency. After one action potential was propagated, the axon has become fully recovered from its absolute refractory period and relative refractory period. The stimulus voltage was still present to produce another potential.
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Activity 7 Conduction velocity, was calculated by dividing the distance the action potential travels along axon by the amount of time it takes to traverse the axon. A sensory Pacinian corpuscle and visceral sensory fiber were examples of A and B fibers respectively. Olfactory sensory neurons or free nerve endings were examples of C fibers. In the simulation, the conduction velocity was calculated by dividing the distance between wires R1 and R2 (0.1 m) by the time it took for an action potential to travel from R1 to R2. The stimulus voltage was set at 30 mV for the three fibers. The oscilloscope time scale for A, B, and C fibers were set at 1, 10, and 50 milliseconds per division respectively. The timescale was adjusted for B and C fibers because the total time displayed would have been too short for an action potential to be seen at R2. The results suggest that the amount of myelination and diameter of the axon or fiber itself affect the conduction velocity of action potentials. A greater diameter and myelination increases the conductance of an action potential along the fiber. This is why A fiber has the fastest conduction velocity while C fiber has the slowest conduction velocity among the three fibers used in the simulation. Table 1.0 Summary of Results for Activity 6 Stimulus Voltage (mV) 20 20 30 45
Stimulus Duration (msec) 0.5 500 500 500
ISI (msec)
Action Potential Frequency (Hz)
10 10 10
100 100 100
Table 2.0 Summary of Results for Activity 7 Axon Type
Myelination
A fiber
Heavy
Stimulus Voltage (mV)
Distance From R1 to R2 (m)
30
0.1
Time Between Aps at R1 and R2 (msec)
(sec)
Conduction Velocity (m/sec)
2
0.002
50 4 | Page
B fiber C fiber
Light
30
0.1
10
0.01
10
None
30
0.1
100
0.1
1
Activity 8
Activity 9
Conclusion
References Neuroglial Cells. (2001). Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10869/
The nervous system. (2008). Retreived from http://lrrpublic.cli.det.nsw.edu.au/lrrSecure / Sites/LRRView/7700/documents/565 7/5657/5657_05.htm
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