A motor unit consists of a single motor neuron.
Most mature extrafusal skeletal muscle fibers in mammals are innervated by only a single alpha motor neuron. Since there are more muscle fibers by far than motor neurons, individual motor axons branch within muscles to synapse on many different fibers that are typically distributed over a relatively wide area within the muscle, presumably to ensure that the contractile force of the motor unit is spread evenly .
In addition, this arrangement reduces the chance that damage to one or a few alpha motor neurons will significantly alter a muscle's action. Because an action potential generated by a motor neuron normally brings to threshold all of the muscle fibers it contacts, a single alpha motor neuron and its associated muscle fibers together constitute the smallest unit of force that can be activated to produce movement.
Sherrington was the first to recognize this fundamental relationship between an alpha motor neuron and the muscle fibers it innervates, for which he coined the term motor unit.
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