Recruitment of Motor Units -- Introduction

Introduction And Background

This exercise builds on the experience you gained with the first exercise in this series, the Compound Action Potential simulation. In that simulation, you studied the propagation of compound action potentials along the sciatic nerve and the electrical behavior of its component neurons. Among other things, you were able to identify, through their high conduction velocities, a group of neurons collectively referred to as Type A neurons. You learned that many of these neurons were motor neurons innervating the muscle fibers of some of the hind limb muscles. These neurons will be the focus of your efforts when you conduct the exercises contained in the present simulation.

T set of exercises will extend your knowledge about the sciatic nerve’s electrical behavior to a study of how the motor neurons interact with their target muscle (in this case the gastrocnemius muscle) and how the muscle responds to input from its motor neurons. You will also develop a better understanding of the interaction of electricity with biological tissues.

As in the case of the Compound Action Potential simulations you worked with earlier, the data on which this simulation is based came from experiments with cats. What you will observe, however, is applicable to a wide range of taxa, including many invertebrates.

What Does The Simulation Allow You To Do?

This simulation provides controls that allow you to vary the strength of a stimulus that is applied to the sciatic nerve or the gastrocnemius muscle and quantify (i) the effect of different stimulus intensities on the electrical behavior of both the nerve and the muscle, and (ii) the mechanical characteristics of the resulting contraction of the gastrocnemius. You are also able to stimulate the muscle itself directly, which will give you some important insights into the electrophysiology of the neuromuscular system.

What Does The Simulation’s Interface Look Like?

The display you will observe when you first start the simulation will look similar to

As you can see, this display is similar to that of the Compound Action Potential simulation exercise. The major differences are:

1. The stimulating electrode (indicated by a yellow arrow) is close to the gastrocnemius muscle, as it typical for this experiment.

2. The simulation has a feature that lets you switch the position of the stimulating electrode from the sciatic nerve to the gastrocnemius muscle.

3. The numerical values for the maximum amplitude (relative to resting) of the 2 action potentials and the maximum tension generated by the gastrocnemius muscle are displayed in a color-coded Legend. This feature will facilitate your ability to deal with the rather large amount of data you’ll be gathering during this experiment. Note: depending on the size of your computer’s monitor screen, the Legend ‘has room’ for 10 – 19 entries; you can conduct more runs that this without clearing the display, but it will get a bit untidy.

4. There are two y-axes instead of the usual single y-axis. Both are located in the left-hand side of the display. The left axis (mV) is for plotting the action potentials generated by the Sciatic nerve and the Gastrocnemius muscle itself. The other y-axis (Newtons) is for the plot of the tension generated by the muscle when it contracts.

If you need a little review the concept of the compound action potential, now is a good time to access the Compound Action Potential simulation.

The slider/text field combination at the bottom center of the display allows you to vary the strength of the stimulus being sent to the stimulating electrode. As in the other simulations, you can change the Stimulus Intensity 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). Note that the units for Stimulus Intensity are mV, and the default range is 0 to 1500 mV ( = 1.5 V).

Immediately below the Stimulus Intensity slider is a checkbox, labeled “Intensity x10”, that has the effect of multiplying by 10 the Stimulus Intensity’s displayed value, which means that the available range of Stimulus Intensities is 0 – 15,000 mV (15 V) when this checkbox is selected. This feature is equivalent to the Range control that one finds on most stimulators used for this sort of experiment, and is necessary because of the difference in response of the sciatic nerve and the gastrocnemius muscle to a given strength of stimulus.

Across the top of the display is a cartoon representation of the spinal cord, the sciatic nerve and the gastrocnemius muscle. In addition, two small black recording electrodes are displayed, one in contact with the sciatic nerve, the other in contact with the external surface of the gastrocnemius muscle.

Two radio buttons near the top right of the display allow you to position the stimulus electrode either on the sciatic nerve (the default location) or on the gastrocnemius muscle itself.

Clicking the “Go” button applies a depolarizing stimulus of the specified strength to the sciatic nerve. The resulting compound action potential progresses to the gastrocnemius muscle and stimulates it to generate its own action potential. The muscle responds by contracting and the resulting tension is displayed.