Motoneuron

Motoneurons are neurons in the central nervous system that project to skeletal muscles and initiate their contraction. They are a key component of the final common pathway by which the nervous system controls movement. The term often refers to alpha motor neurons, which innervate extrafusal muscle fibers, and gamma motor neurons, which innervate intrafusal fibers within muscle spindles.

Alpha motor neurons reside mainly in the ventral horn of the spinal cord and in various cranial nerve nuclei of the brainstem. They receive excitatory and inhibitory input from higher motor centers, interneurons, and sensory receptors, and they transmit action potentials to muscle fibers via their axons. A motor unit consists of one motor neuron and all the muscle fibers it innervates; the size of the motor unit, along with recruitment and rate coding, determines the force and precision of muscle contraction.

The neuromuscular junction is the synapse where the motor neuron releases acetylcholine to stimulate nicotinic receptors on the muscle membrane, producing an endplate potential and, if sufficient, a contraction. Gamma motor neurons adjust the sensitivity of muscle spindles, thereby modulating reflexes and proprioception.

Clinical relevance includes degeneration or dysfunction of lower motor neurons, as seen in amyotrophic lateral sclerosis and spinal muscular atrophy, which can cause muscle weakness, atrophy, fasciculations, and diminished reflexes. Upper motor neuron lesions affect the control of these neurons and can cause spasticity. Motoneurons develop from the ventral neural tube and extend axons to their muscle targets during development and throughout life.