Recruitment of Motor Units – Experimental Protocols
Tips & hints
1. Although the slider affords the ability to change the stimulus
voltage in 1 or 10 mV increments, you will probably not want (or need) to make
such small changes – except when you’re trying to determine the threshold of
the nerve and the muscle. Most of the
time you can make 100 mV changes between runs.
This will save considerable time while still providing you with the data
you need to complete the experiment.
2. The timer for the x-axis starts when the “Go” button is clicked and the action potential is initiated at the point of stimulation. Consequently, there will be delays between the time you click the Go button and the appearance of action potentials or tension in the muscle, and these delays will be reflected in the position along the x-axis of the action potential and tension graphs. It is important for you to note and quantify these delays, as they provide important information about how the neuromuscular system functions.
3. Note that the action potential scale starts at 0, rather than at some negative value, say –70 mV, that you’re probably used to seeing. What is really being plotted here is the membrane potential relative to the resting potential of the membrane. This is standard procedure in experiments of this sort because it makes it easier to monitor the total amplitude (in mV) of the action potential..
Exercises
1. With all parameters set to their default values (i.e.,
stimulating electrode positioned on the sciatic nerve, Stimulus Intensity
slider set to 1000 mV, Intensity x10 checkbox deselected), click the Go
button. You should observe the
stimulating electrode flash briefly to indicate that a depolarizing stimulus
has been applied to the sciatic nerve.
This should be followed shortly by the appearance of three tracings on
the x-y axes. The first tracing to
appear represents the compound action potential generated by the sciatic nerve,
while the second represents the electrical activity of the gastrocnemius muscle. The last tracing to appear represents the
tension imposed on the myograph by the contracting muscle. All three tracings should be of large
amplitude and easily observed.
2. Without clearing the display or changing the default settings
(click the Reset button if you’ve made any changes to the Stimulus Intensity
setting), reposition the stimulating electrode from the nerve to the muscle by
selecting the Muscle check box. Click
the Go button and observe the new tracing (the lines should be red). Compare the new tracing with the original
one (it should still be visible on the display as black lines). How many differences between the two sets of
tracings can you detect?
3. Reposition the stimulating electrode on the sciatic nerve and set
the Stimulus Intensity to 0 mV. Do a
run. Do you observe any electrical
activity on the sciatic nerve? Do
several run in which you increase the Stimulus Intensity by 10 mV between each
run. Once you observe an action
potential, decrease the Stimulus Intensity by 10 mV and use 1 mV increments in
Stimulus Intensity to more precisely define the Stimulus Intensity that just
gives you an action potential on the sciatic nerve. Use a copy of the included data sheet to record the data.
4. Once you’ve determined the sciatic nerve’s threshold, do a series
of runs in which you increase Stimulus Intensity in ‘jumps’ of 50 – 100 mV
between each run. Your goal is to
obtain data that you can use to construct each of the six graphs specified in
the table under Question #10, below.
Use a copy of the included data sheet to record the data.
5. Reposition the stimulating electrode to the muscle by selecting
the Muscle checkbox. Repeat #2 to
determine the gastrocnemius muscle’s threshold. Use a copy of the included data sheet to record the data.
Questions:
1. What was the time lag between the stimulus and the recording of the sciatic nerve’s compound action potential? How does that value compare with the ones you observed when you performed the Compound Action Potential simulations? Can you account for any difference?
2. Why is the compound action potential not resolved into its components (alpha, beta, etc.) as it was in the simulation dealing with compound action potentials on the sciatic nerve that you worked with earlier?
3. Use the data you obtained in Exercise #3 above to answer the following questionos:
a. Which gave you a more sensitive indicator of electrical activity
on the sciatic nerve, the graphical display or the table? Can you explain the reason for the difference?
b. Was electrical activity detected in the muscle whenever Stimulus
Intensity exceeded the sciatic nerve’s threshold?
c. Did the muscle always respond with a tension-generating contraction when it exhibited electrical activity? If not, can you propose an explanation.
4. What where the threshold potentials for the sciatic nerve and the gastrocnemius muscle? Are they the same voltage? If they’re different, propose an explanation for the difference. (Hint: remember, you’re not dealing with individual cells – either neurons or muscle fibers – in this exercise.)
5. Compare the tension generated by the muscle in response to a stimulus applied to the sciatic nerve with the tension generated when the same stimulus is applied directly to the muscle. Was the tension the same or different? If different, which produced the greater tension – stimulating the sciatic nerve, or the muscle? What do you suppose accounts for the difference? Three hints for you:
a. Think about the phenomenon in terms of (i) the relative amount of membrane area that must be depolarized ( = a major contributor to what neurophysiologists term capacitance losses) when depolarizing the nerve versus depolarizing the muscle, and (ii) the relative distances from the stimulating electrode to the membranes that must be depolarized to threshold ( = a major contributor to what are termed resistance losses).
b. Would differences in the respective chronaxie and/or rheobase values for the nerve and the muscle contribute to the observed difference in Stimulus Intensity required to produce a given twitch tension? If you don’t recall the meaning of chronaxie and rheobase, see the instructions for the Compound Action Potential Simulation I
c. Would the nerve’s anatomy (with branches extending to, and synapsing with, all of the individual muscle fibers in the gastrocnemius make the nerve more efficient at eliciting action potentials and contraction of the muscle fibers than a stimulus applied directly to the muscle?
6. With respect to 3b above, suppose you wanted to test the hypothesis that the chronaxie and/or rheobase phenomena played a role in the differential response of sciatic nerve and gastrocnemius muscle to the same stimulus intensity. Design a experiment or experiments that would enable you to conclusively test that hypothesis, and discuss how the resulting data would enable you to conduct the test.
7. If action potentials are supposed to obey the all-or-none law, how is it possible to produce action potentials on the sciatic nerve and in the muscle that vary in amplitude? (Hint: recall your earlier work with the compound action potential simulation.)
8. Is the graph of Tension vs. Time symmetrical? What can you infer about muscle contraction/relaxation from the shape of this graph?
9. Does the twitch contraction commence immediately following the appearance of an action potential in the muscle? If not, what do you think accounts for the delay?
10. Using the following table as a guide, construct the specified graphs:
|
Graph # |
Independent
Variable |
Dependent Variable |
Significant
Feature(s) Of the Graph |
|
1 |
SI |
SNAP |
|
|
2 |
SI |
MAP |
|
|
3 |
SI |
Tension |
|
|
4 |
SNAP |
MAP |
|
|
5 |
SNAP |
Tension |
|
|
6 |
MAP |
Tension |
|
Note: SI = Stimulus Intensity, in mV
SNAP = Sciatic nerve action potential amplitude, in mV
MAP = Muscle action potential amplitude, in mV
Tension = Maximal tension developed by Gastrocnemius, in Newtons
By careful study and interpretation of these graphs, you can reach some important conclusions about the function of the neuromuscular system. Just to get you started, consider:
a. What is the nature of the dependence of SNAP, MAP, Tension and SI? Is it linear, exponential, sigmoid, or some other relationship? Do SNAP, MAP, and Tension show the same dependence on SI?
b. What is the relationship between Tension and MAP?
The answers to these and other questions you might come up with as a result of careful study of the graphs will lead you to some important insights about the function of the neuromuscular system.
11. Did you observe an action potential on the sciatic nerve when you stimulated the muscle directly? Explain your observation.
12. Note that the recording electrode placed on the sciatic nerve was positioned as close to the gastrocnemius muscle as possible. Why do you think this was done?
13. Describe in detail the mechanism by which the sciatic nerve’s compound action potential produces electrical activity in the gastrocnemius muscle. Based on that description, describe in precise terms what the tracing of the muscle’s “electrical activity” actually represents.
14. Early investigators working with the sciatic nerve – gastrocnemius muscle preparation didn’t have the luxury of working with the sophisticated equipment we have available to us today. The stimulators they used didn’t allow them to control voltages too precisely, and their recording equipment was very crude in comparison to modern instruments. The result was typically a graph of data that looked something like this:
From data such as these, it was inferred (i) that skeletal muscle fibers are grouped into motor units, (ii) that there are a relatively small number of motor units in the gastrocnemius muscle, and that there are large differences in the threshold potentials for the motor neurons controlling their contractile activity.
Do those inferences seem
reasonable given the data available to the early researchers? What do your data suggest might be a
more realistic picture?
Stimulus Intensity ( mV ) Sciatic Nerve Action Potential Amplitude ( mV ) Muscle Action Potential Amplitude ( mV ) Twitch Tension ( N ) Notes Data Sheet for Recruitment of Motor Units Simulation