Motorneurons

Motor neurons are nerve cells that convey impulses from the central nervous system to muscles, initiating and regulating voluntary and reflexive movements. They are classified as lower motor neurons, which directly innervate skeletal muscle, and upper motor neurons, which originate in the cerebral cortex or brainstem and modulate lower motor neuron activity. The cell bodies of lower motor neurons reside in the ventral horns of the spinal cord or in cranial nerve motor nuclei, and their axons project to neuromuscular junctions in skeletal muscles. At the neuromuscular junction, the nerve terminal releases acetylcholine, which binds to nicotinic receptors on the motor end plate to trigger contraction. Alpha motor neurons innervate extrafusal fibers, while gamma motor neurons regulate the sensitivity of muscle spindles by innervating intrafusal fibers.

Motor neurons participate in reflex circuits; simple reflexes involve direct activation of alpha motor neurons, while upper motor neurons influence lower motor neurons via interneurons and descending pathways, contributing to voluntary movement, tone, and posture. Peripheral axons are myelinated by Schwann cells, enabling rapid conduction.

Clinical relevance: degeneration or loss of motor neurons occurs in several disorders. Amyotrophic lateral sclerosis involves progressive loss of both upper and lower motor neurons, causing weakness, spasticity, fasciculations, and atrophy. Spinal muscular atrophy primarily affects lower motor neurons. Diagnosis relies on clinical assessment, electromyography, and nerve conduction studies; treatment is supportive and may include medications for symptoms, physical therapy, and assistive devices.