Each skeletal muscle fiber is stimulated to contract by chemicals released from a somatic motor neuron. Most of these elongated nerve cells originate in the gray matter of the spinal cord. Their thread-like axon processes carry electro-chemical impulses to a target muscle bundled inside a nerve.
A motor neuron axon branches many times after entering a target muscle, and each branch make its way to a different muscle fiber. As it approches the midpoint of a muscle fiber, the axon splits again, forming a small cluster of terminal branches. The tips of the terminal branches expand into a small synpatic bulbs, which fit into grooves along the surface of the muscle fiber. Together, the expanded axon tips and the nearby muscle fiber membrane make up a neuromuscular junction.
A motor unit is the term applied to a single motor neuron and all of the muscle fibers that it stimulates. When a motor neuron fires, all the muscle fibers in the motor unit contract at once. The size of a motor unit varies from just a few fibers in the eye muscles (precise movements) to over a thousand fibers in the large leg muscles (powerful movements). .
