The Muscle Twitch and the Latent Period (Virtual Lab)

Activity 1 of the Skeletal Muscle Physiology virtual lab. Stimulate the muscle, then measure the delay before force starts to rise. No response at 0 V. Above…

Background

In the body, a motor neuron's action potential releases acetylcholine at the neuromuscular junction, depolarizing the sarcolemma and triggering contraction. Here an electrical stimulus takes the place of acetylcholine.

A single stimulus gives a twitch with three phases. The latent period runs from the stimulus to the first rise in force: the action potential spreads down the T tubules and Ca²⁺ leaves the sarcoplasmic reticulum, but no force is measurable yet. The contraction phase lasts until force peaks, and the relaxation phase until force returns to baseline as Ca²⁺ is pumped back.

Steps

1. With the voltage at 0.0 V, click Stimulate, then Record Data.
2. Raise the voltage to 4.0 V, click Stimulate, then Record Data.
3. Turn on Measure and drag the line (or use the slider) to the point where the trace first leaves zero. Read the time: that is the latent period. Record Data again to save it.
4. Increase the voltage in 2 V steps up to 10 V, stimulating and recording each time. Measure the latent period at each voltage.

What you should find

No response at 0 V. Above threshold, force grows with voltage, but the latent period stays the same, about 2.8 ms, because it is the time excitation–contraction coupling takes, not something the stimulus strength changes.

Check your understanding

1. The latent period is the time between the stimulus and:
2. As the voltage was raised from 4 V to 10 V, the latent period:
3. What is happening inside the fibers during the latent period?