Motor Unit Stimulation

When a neuron of a motor unit fires an action potential, the muscle fibers undergo a twitch contraction.

The process begins with excitation-contraction coupling, which relays the action potential to the muscle fibers. This phase is known as the latent period.

After the action potential sweeps across the sarcolemma, the fibers enter the contraction phase, and the tension peaks rapidly.

Finally, the tension in the muscle fibers slowly drops to the resting levels in the relaxation phase.

But such single twitches do not produce significant contractions. Instead, an active muscle undergoes a series of smooth contractions of varying strength called graded muscle contractions.

The strength of muscle contractions can be adjusted in two ways — changing the stimulation frequency and recruiting variable number of motor units.

While a neuron firing at a low frequency produces a weak contraction, an increase in this stimulation frequency leads to a stronger contraction.

Alternately, when the voltage strength of the stimulus is increased, it results in the recruitment of more motor units, producing stronger contractions.