Motor Units

The motor neurons that innervate skeletal muscle fibers are called alpha motor neurons. As the alpha motor neuron enters a muscle, it divides into several branches, each innervating a muscle fiber (note this in the image above). One alpha motor neuron, along with all of the muscle fibers it innervates, is a motor unit. The size of the motor unit correlates with the function of the muscle. In muscles involved with fine, coordinated control, the motor units are very small with 3–5 muscle fibers per motor neuron. Muscles that control eye movement and muscles in our hands have relatively small motor units. On the other hand, in muscles involved with more powerful but less coordinated actions, like the muscles of the legs and back, the motor units are large with thousands of muscle fibers per motor neuron.

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