Answer
A motor unit:
A motor unit is a somatic afferent neuron and all the muscle fibers it innervates.
In a small motor unit, a single motor neuron my serve no more than five muscle fibers.
In medium-sized a somatic motor unit, a neuron innervates about 250 myofibers.
But in a large and powerful motor unit, a single somatic motor neuron may innervate as many as
2000 muscle fibers.
Examples of of muscles with small motor units are muscles of eye and hand
Eye muscles: levator palpebrae superioris and inferior rectus
Hand muscles: flexor pollicis brevis and palmar interossei
Examples of muscles with large motor units are muscles of the thigh and of the hips;
gluteus maximus, vastus lateralis, and biceps femoris
Work Step by Step
Skeletal muscles are innervated by somatic motor neurons. These neurons have their cytons in in the spinal cord (SC) or the brain stem (BS). Axons or nerve fibers of these neurons travel some distance and then divide into several branches that target muscle fibers. Each nerve fiber supplies several muscle fibers , but each muscle fiber is supplied by only one motor neuron.
When a nerve signal reaches the end of an axon it, spreads out over all its terminal branches and stimulate all the muscle fibers that its branches supply. . Therefore , when the nerve stimulus travels down the axon it stimulates all the muscle fibers that its branches supply and all the myofibers contract in unison.
This is the motor unit--a nerve fiber and all the muscle fibers supplied/innervated by it. The fibers of a motor unit are not necessarily close together but may be dispersed throughout a muscle. .
The contraction of a single motor unit may not generate a great deal of force. Usually, effective muscle contraction requires the simultaneous activation of many motor units.
