20.9: Motor Units
A motor unit consists of two main components: a single efferent motor neuron (i.e., a neuron that carries impulses away from the central nervous system) and all of the muscle fibers it innervates. The motor neuron may innervate multiple muscle fibers, which are single cells, but only one motor neuron innervates a single muscle fiber.
Lower motor neurons are efferent neurons that control skeletal muscle, the most abundant type of muscle in the body. The cell bodies of lower motor neurons are located in the spinal cord or brain stem. Those in the brainstem transmit nerve signals through the cranial nerve, and primarily control muscles in the head and neck. Lower motor neurons originating in the spinal cord send signals along the spinal nerve, and primarily control muscles in the limbs and body trunk.
A lower motor neuron fires an action potential that, at once, contract all skeletal muscle cells that the neuron innervates. Thus, motor units are functional units of skeletal muscle.
The size of a motor unit, or the number of muscle fibers the lower motor neuron innervates, varies depending on the size of the muscle and the speed and precision the movement requires. Muscles in the eyes and fingers, which require rapid, precise control, are generally controlled by small motor units. In these units, motor neurons connect to a small number of muscle fibers. By contrast, motor units for limb and trunk movements are generally large and consist of many muscle fibers.