Electrophysiology of the Sciatic Nerve
I. The Compound Action Potential

Introduction & Background

    One of the truly significant findings to come from early electrophysiological studies of the nervous system was that action potentials recorded from nerves and individual neurons differed in appearance. At about the same time, it was discovered that under the right set of experimental conditions the nerve's action potential, while appearing to be a single entity, could be decomposed into a number of smaller-amplitude action potentials that differed in the speed with which they travelled along the nerve (i.e., they differed in their conduction velocity). The recognition that action potentials recorded from a nerve actually represented the aggregate electrical effect of individual action potentials being generated by a large number of neurons led investigators to apply the term compound action potential to the electrical response of nerves to stimulation. Among other things, use of this term made it clear that the action potential recorded from nerves was not the same as the action potentials generated by individual neurons.
    This simulation lets you replicate one of THE classic experiments in all of physiology. The experimental setup you'll be working with -- and the protocols you'll use -- date from the era when scientists were just beginning to understand how the vertebrate neuromuscular system functions. By performing the exercises in this simulation, you will gain extensive experience with the sciatic nerve experimental preparation.
    Note: the numerical values generated by this simulation are based on data obtained from experiments with cats, but the results you obtain will give you insights into nerve function in most organisms, including many invertebrates.

What Does The Simulation Allow You To Do?

    When you conduct the exercises that are part of this simulation, you will be dissecting the compound action potential into components that differ in the speed with which they propagate along the sciatic nerve. Accordingly, the simulation provides a slider/textfield combination that lets you position a recording electrode at varying distances from the stimulating electrode. In addition, checkboxes are provided that allow you to change the range of the x- and/or y-axis. The latter feature will facilitate data gathering and interpretation.

What Does The Simulation's Display Look Like?

    Activate the simulation by clicking on the “Run The Simulation” link. Depending on your monitor's size, your computer's operating system, and the browser you're using, you'll see a display that looks something like this:

As you can see, the display is dominated by a set of blank x-y axes on which graphs of Voltage (in mV) versus Time (in ms) will be displayed when you stimulate the sciatic nerve by clicking the “Go” button. You will immediately note that the y-axis values range from 0 to 150 mV. How does this compare with the y-axis ranges you employed when you ran the membrane potential simulations earlier in the term?
    Across the top of the display is a cartoon representation of the spinal cord, the sciatic nerve (note the dorsal and ventral roots extending from the spinal cord), and the gastrocnemius muscle. A stimulating electrode is indicated by the thick yellow arrow located close to the sciatic nerve's origin from the spinal cord. Whenever you click the Go button, the stimulating electrode will flicker briefly to indicate that a stimulus has been applied to the sciatic nerve. A recording electrode is represented by a small black disk with a short black line extending downward from the sciatic nerve. Immediately below the recording electrode is a numerical display of the distance (in mm) between the stimulating and recording electrodes.
    At the bottom center of the display is a slider/text field combination that allows you to vary the position of the recording electrode relative to the stimulating electrode. As with previous simulation exercises in this series, you can change the position of the recording electrode either by using the mouse to move the slider or by typing a number in the text field (see the general instructions link on the Neurophysiology Simulations home page if you need a refresher on this). Whichever method you choose, clicking the Clear button will move the recording electrode to the new location along the sciatic nerve.
    In the upper right corner of the display is a set of checkboxes that let you set the scale for the x-axis and the y-axis. The usual simulation-control buttons are located in the lower right corner of the display, along with a text field that displays the current status of the simulation.
    Clicking the “Go” button delivers a maximal stimulus to the sciatic nerve, causing all of its component neurons to generate an action potential. The resulting compound action potential then progresses from left to right along the sciatic nerve (for sake of clarity, the progress of the compound action potential has been slowed considerably, but if you're working with a particularly fast computer, you might not actually be able to see the action potential progressing along the nerve).
    When the compound action potential reaches the location of the recording electrodes,diagram. The compound action potential stops when reaches the recording electrode, and a graph of the temporal changes in potential at that location will be generated on the axes. In this simulation, the compound action potential does not progress beyond the recording electrode because we've blocked its further progress by applying some xylocaine or ether to the nerve just past the recording electrode's position (see Spotlight 6-1, p. 172 and Fig. B on p. 173 in your textbook). As a result, we get a monophasic action potential. We work with monophasic action potentials in this simulation because they make it much easier to garner information about the conduction velocities properties of the various categories of axons that comprise the sciatic nerve.