13 - Recording Muscle Contraction
|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.
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.
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.
Two maximal stimuli in short sequence were then applied. If you apply them closely together in time you should be able to see summation.
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.
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.