Question

Since individual motor unites for "all or none," how do skeletal muscles perform smooth movement? Why can asynchronous recruitment of motor units be beneficial?

Answer 1

A) Skeletal muscles are able to perform smooth muscle movement because of the phenemenon known as motor recruitment, which is a process by which different motor units are activated to produce a given level and type of muscle contraction.

Signals are sent to the brain, which estimates number of motor units required to stimulate and than to contract, the process continues until desired movement is acheived in a smooth fashion. Motor units are recruited in a orderly fashion,  weak inputs onto motor neurons will cause only a few motor units to be active, which results in smaller force, with stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle.

B) Asynchronous recruitment. ,when the nervous system modulates the firing rates of the motor unit so that different motor units take turns maintaining muscle tension, i.e they contract alternatively, this alteration of active motor units allows some of the motor units to rest between contractions and thus prevents fatigue