Electrophysiology Of The Sciatic Nerve – II.

Strength-Duration Relationship and Refractory Period

Overview Of The Simulation

This simulation, based on data obtained from experiments with the grass frog (Rana pipiens), allows you to perform experiments that investigate the basic electrophysiology of the sciatic nerve. In particular, you will determine the sciatic nerve's threshold and its refractory period, and you will investigate how the strength and duration of a stimulus interact to determine whether the threshold is actually reached. In so doing, you will learn much about how all excitable cells function.

Although you can calculate conduction at least one velocity from the data you’ll be recording, because of the relatively short length of the frog’s sciatic nerve, errors are apt to be large, and in any event, you’d not be able to separate the action potential peaks corresponding to the different categories of neurons (Type A, Type B, Type C, etc.), as you did in the preceding simulation in this series (the Compound Action Potential simulation. The emphasis here is on investigating strength-duration relationships and the refractory period of a compound nerve.

What does the Simulation Allow You To Do?

In this simulation, you will be gathering data that you will analyze to determine the sciatic nerve's threshold, conduction velocity, and refractory periods (both absolute and relative). Accordingly, controls are provided that allow you to:

1. Record the sciatic nerve's compound action potential at different points of varying distance from the site of initiation;

2. Independently vary the strength ( in Volts) and duration (in milliseconds) of the stimulus that you apply to the sciatic nerve;

3. Stimulate the sciatic nerve with a single pulse or with two pulses separated in time by a delay that you specify.

What Is Displayed?

When you initially load the simulation, depending on the size of your computer’s monitor and the browser you’re using to run the simulation, you will see a display similar to this:

Near the top of the display is a diagram of a nerve chamber, which is basically a plastic trough containing two stimulating electrodes (depicted as red lines running across the interior of the nerve chamber) and five recording electrodes (depicted with blue lines). A freshly dissected sciatic nerve (represented by the white wavy line) has been placed across the stimulating and recording electrodes.

A stimulator is connected via a black wire to one of the stimulating electrodes, while the other stimulating electrode is connected via a thinner wire to ground. The blue wire that will connect the recorder to a recording electrode is not connected to any of the recording electrodes; this connection will be made when you select a recording electrode by clicking on one of the checkboxes at the bottom of the display.

On the stimulator is a white display area in which the power value corresponding to the product of the Strength and Duration settings will be displayed when you click the “Go” button and deliver a pulse to the stimulating electrode in the nerve chamber.

In addition, you will see a blank set of 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.

The right side of the display contains a number of control features, some of which (e.g., the “Go”, “Clear”, and “Reset” buttons) you are familiar with from previous exercises in this series.

Immediately above the “Go” button are three sliders, each with a label and a text field that will display the value of the slider. Note that the text fields for the “Strength” and “Duration” sliders depicts the proportion of the slider's range of 0 to 1, while the text field for the “Delay” slider presents a numerical value that ranges between 0 and 20 (units are milliseconds, ms).

At the top right of display are two sets of “Range Settings” that allow you to vary the range within which the “Duration” and “Strength” sliders function. These checkboxes act as multipliers of the sliders’ inherent range from 0 to 1. For example, with the Duration slider set to a nominal value of 0.66 and the

Finally, immediately above the sliders' labels are two, labeled “Pulse Control”. If you select the “Single” button, the neuron will be stimulated just once each time you click on the “Go” button, and you will record just one compound action potential (assuming the combination of Strength and Duration of the stimulating pulse exceeds the threshold of the nerve). If you select the “Dual” button, the stimulator will deliver one pulse followed ^__ after a delay you specify using the “Delay” slider ^__ by a second pulse identical to the first. This feature lets you investigate electrical behavior of the sciatic nerve during its absolute and relative refractory periods.

Across the bottom center of the display is a set of five checkboxes with which you will select a recording electrode to use for recording action potentials generated by the sciatic nerve.