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Lab 13 - Recording Muscle Contraction
with the MacLab A/D Converters and Computers


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The process of contracting takes about 50 msec. Relaxation of the fiber takes another 50-100 msec. Because the refractory period is so much shorter than the time needed for contraction and relaxation, the fiber can be maintained in the contracted state so long as it is stimulated frequently enough (e.g., 50 stimuli per second). Such sustained contraction is called tetanus

The purpose of this lab was to record twitch and tetanus muscle contractions using the MacLab A/D converter and computer.

Dr. Allen first performed this experiment on himself to demonstrate how it is done. We then had one volunteer from the lab, Misty, in which we collected data on.

transducer.jpg (26200 bytes) An electrode was attached to her upper arm. This electrode along with a mobile electrode was connected to a physiological stimulator. One of her fingers was connected with a string to the strain gauge force transducer.

hands2.gif (24524 bytes) Misty herself then used the mobile electrode and probed the inner part of her forearm looking for stimulation, which indicates a motor point. A low voltage stimulus at 3/sec was applied while she did this. A motor point represents an area where the nerve supplying one of the finger flexor muscles dives into the muscle. Typically it takes a lower stimulating voltage to produce action potentials and contractions at these points.

hands.gif (1305 bytes) Now comes the fun! Starting with a low stimulating voltage, she found a motor point where stimulation causes involuntary muscle contraction (barely perceptible recording). Then Dr Allen increased the voltage in small increments. The sizes of the single twitch contractions increased as more motor units are recruited. For a voltage (maximal stimulus) that gets every motor unit contracting you max out.

hands2.gif (24524 bytes) Two maximal stimuli in short sequence were then applied. If you apply them closely together in time you should be able to see summation.

hands.gif (1305 bytes) Next the stimulus frequency was increased to 10/sec, forming incomplete tetanus.

Then up to 25 simuli/sec, complete tetanus. There were larger contractile responses than there were with the single twitch responses.


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Here are the results from the MacLab computer screen after performing this experiment on Misty.

When shocks were given at 3/sec, the muscle responded with a single twitch. At 5/sec and 10/sec, the individual twitches begin to fuse together, a phenomenon called incomplete tetanus. At 25 shocks per second, the muscle goes into the smooth, sustained contraction of tetanus.

Incomplete tetanus and tetanus are possible because the refractory period is much briefer than the time needed to complete a cycle of contraction and relaxation. Note that the amount of contraction is greater in incomplete tetanus and tetanus than it is in a single twitch.


wpe4.jpg (3755 bytes) Mac computer and A/D converter
wpe5.jpg (3030 bytes) Strain gauge force transducer

A/D converters (e.g., our ) sample the level of some signal (contractile response of muscle) at some rate (e.g., 50/sec) and assign a digital value to it. In this case, the program chart will collect some sampled points and connect them together with lines to make it look like a continuous recording. Here is a diagram that illustrates the process.

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