Electrophysiology of the Sciatic Nerve
I. The Compound Action Potential

Experimental Protocols

  1. Click on the “Go” button to do a run using the default settings for the location of the recording electrode (10 mm from the stimulating electrode), x-axis range (10 ms), and y-axis range (150 mV). When the simulation run is completed, note the general appearance of the compound action potential that you've recorded. Use the attached data sheet to record:

    • the time required for the action potential to reach the recording electrode's location on the sciatic nerve (see Tip & Hints #4, below),
    • the amplitude of the action potential, and
    • the peak width at half height (see Tip & Hints #5, below).

Note: in contrast with earlier simulations, the numerical data you need to complete the exercise will not be displayed for you on the computer screen. As would be the case if you were running this experiment 'for real' (i.e., using living experimental animals and an oscilloscope to record the action potentials), you'll have to measure and/or visually estimate times and amplitudes (I'd recommend doing the measurements).

  • After recording the relevant data in the data sheet, move the recording electrode to a position further away from the stimulating electrode and do another run. Repeat this procedure at successively greater distances from the stimulating electrode, until you feel you have enough data to enable you develop a clear picture of the nature of a compound action potential and what it tells you about the sciatic nerve.
    Important Note: the timer for the x-axis starts when the “Go” button is clicked and the compound action potential is initiated at the point of stimulation. Thus, the x-axis represents the time since the compound action potential was initiated on the sciatic nerve. Consequently, you will not see anything displayed on the graph until the compound action potential reaches the recording electrodes.
    Tips & Hints
    1. You may place the recording electrode wherever you wish along the sciatic nerve (within limits, of course). However, I'd recommend you start close to the sciatic nerve's origin near the spinal cord and progressively move it outward. I think you'll find it easier to figure out what's going on with this approach.
    2. Although there's no reason to change the position of the recording electrode by only 1 or 2 mm at a time (in experiments using real animals, you'd never do this), you should still be careful about moving the recording electrode position too far between runs; you may lose data unless you change the range setting for the x-axis. Until you get some experience with the effect on the tracings of changing the recording electrode's position, you probably should not change its position by more than 10-20 mm between runs.
    3. Feel free to change the scale for the x- and/or y-axis if you need to do so in order to 'see things' better. In particular, for large separations between the stimulus and recording electrodes (> 90 mm or so), you'll probably want to switch the x-axis range to 20 ms to be sure that you don't lose data. Note that whenever either of the axis scales is changed, you'll need to clear the screen & start over. Otherwise, you'll have multiple scales plotted on the same axis…..very confusing.
    4. To complete this exercise (see Question #6, below), you'll need to decide which arrival time you're going to use in your calculations: the time that you can first detect an upward deflection of the tracing from baseline (used by many investigators) or the time that the peak reaches its maximum amplitude (used by many other investigators). Perhaps do it both ways, or have one-half of the class do it one way and the other half of the class do it the other way, and see which approach seems to give more reasonable results.
    5. Not sure what "peak width at half height" is or how to measure it? Click here to access a short set of instructions.

    Questions:

    1. Compare and contrast the appearance of the compound action potential with that of an action potential recorded from the axon of a single neuron.
    2. How does the general appearance of the tracing change as you move the position of the recording electrode further from the stimulating electrode? Does the height change? Does W½ change? Are these changes uniform with distance from the spinal cord? What is your explanation for the changes in the compound action potential's appearance?
    3. Construct a graph of peak amplitude versus recording electrode distance for as many of the peaks as you can. How does the amplitude of the compound action potential's peaks change as they progresses along the sciatic nerve? What do you think is responsible for this change?
    4. Construct a graph of W1/2 versus recording electrode distance for as many of the peaks as you can. How does W1/2 change as the action potentials progress along the sciatic nerve? What do you think is responsible for this change?
    5. What does the shape of each peak in the compound action potential suggest about the distribution of conduction velocities within each class of neuron? In turn, what does that suggest about the nature of the axons that make up each class (hint: remember what determines the conduction velocity of action potentials)?
    6. Calculate the conduction velocities ( = distance/time) for every peak you record at each position of the recording electrode. Do this two ways: first, using the distance from the stimulating electrode to each recording electrode and the time required for each peak to arrive at the recording electrode's location, then by using the data for the time required for the action potential to progress from one recording electrode to another. Which technique seems to give more reasonable results?
    7. How many conduction-velocity classes of neurons were you able to detect in the sciatic nerve? Calculate the conduction velocity for each of the classes of neurons that you were able to detect. (Note: researchers generally use the time that the tracing first deviates from baseline in calculating conduction velocity. Does this seem reasonable? How would your estimates of conduction velocity change if you used the time of the maximum amplitude of each component instead? Does this seem like a better approach?)
    8. Can you suggest any adaptive benefit(s) for the different conduction velocities of the various class of neurons that make up the sciatic nerve?

    Data Sheet For Conduction Velocity Calculations

    Distance to
    Recording Electrode
    (mm)
    		Peak #      Amplitude (mV)
    Width at Half-height
    (W½, ms)
    Arrival Time
    (ms)
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