The Effect of Stimulus Voltage (Virtual Lab)

Activity 2 of the Skeletal Muscle Physiology virtual lab. Find the threshold and the maximal stimulus: motor unit recruitment. Force is zero until the thresho…

Background

A whole muscle contains many motor units whose fibers have different thresholds. The threshold stimulus is the smallest voltage that produces a detectable contraction. As the voltage rises, more motor units are recruited and force increases.

Once every fiber has been recruited, the muscle has reached maximal contraction and a stronger stimulus adds nothing. The smallest voltage that achieves this is the maximal stimulus. A single fiber obeys the all-or-none law; the graded response here comes from recruitment.

Steps

1. With the voltage at 0.0 V, click Stimulate, then Record Data.
2. Raise the voltage to 0.2 V, stimulate and record.
3. Keep raising the voltage in 0.1 V steps, stimulating and recording each time, until a force trace appears. That voltage is the threshold.
4. Click Clear Tracings. Continue raising the voltage (0.5 V steps are enough) up to 10 V, stimulating and recording each time.
5. Click Plot Data to see active force against voltage.

What you should find

Force is zero until the threshold (about 0.8 V), rises steeply, then levels off at the maximal stimulus (about 8.5 V, 1.82 g). 10 V gives no more force than 8.5 V, which is why later activities use 8.5 V.

Check your understanding

1. Why does active force increase between 0.8 V and 8.5 V?
2. Raising the stimulus from 8.5 V to 10 V:
3. The threshold stimulus is: