The term motor unit refers to one lower motor neuron and all the skeletal muscle cells that it innervates. Usually, each time a lower motor neuron fires an axon potential, all the skeletal muscle cells in its motor unit contract. The lower motor neurons are in the brainstem or the spinal cord, and their axons travel in cranial or spinal nerves, and in their subsequent branches, until they reach and synapse on skeletal muscle cells.
This type of synapse is called a neuromuscular junction. Small muscles that need rapid, fine control, like those that move the eyes and fingers, have small motor units, meaning that each lower motor neuron synapses on a small number of skeletal muscle cells. Large muscles that do not need rapid, fine control, like those those of the torso and proximal limbs, have large motor units with each lower motor neuron synapsing on a large number of skeletal muscle cells.
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