Motor Neuron

A motor neuron, or motoneuron, is a nerve cell involved in controlling muscle activity. In the somatic motor system, lower motor neurons located in the spinal cord or brainstem send axons to skeletal muscle and provide the final neural pathway that produces muscle contraction. Motor neurons participate in both voluntary movements and automatic motor responses such as reflexes .^1,2

Alpha motor neurons innervate the ordinary force-producing skeletal muscle fibers, called extrafusal fibers. One alpha motor neuron and all the muscle fibers supplied by its axon form a motor unit. When the neuron generates action potentials, acetylcholine is released at neuromuscular junctions, initiating electrical activity and contraction in the muscle fibers it controls .^1,2

Another class, gamma motor neurons, innervates the intrafusal fibers within muscle spindles rather than the main force-producing fibers. By activating contractile regions of these specialized fibers, gamma motor neurons adjust muscle-spindle sensitivity and contribute to proprioceptive and reflex control of muscle activity .^1,2

In broader neuroanatomical terminology, motor pathways are often described in terms of upper and lower motor neurons. Upper motor neurons arise in the brain and influence motor circuits in the brainstem and spinal cord, whereas lower motor neurons directly innervate skeletal muscle. For this reason, lower motor neurons are often described as the final common pathway through which the nervous system controls skeletal muscle .^1,2