Motor neuron

A motor neuron is a nerve cell that carries signals from the brain or spinal cord to muscles and glands, producing voluntary movement, reflexes and muscle tone.

Upper motor neurons begin in the motor cortex and brainstem and travel down the spinal cord, where they connect to lower motor neurons whose long fibers leave the cord and end at the neuromuscular junction on muscle fibers. There they release acetylcholine, which makes the muscle contract. Damage to upper motor neurons produces weakness with stiffness and exaggerated reflexes, while damage to lower motor neurons produces weakness with wasting, twitching and reduced reflexes, a distinction neurologists use during examination.

Motor neuron diseases, of which amyotrophic lateral sclerosis is the best known, involve progressive degeneration of these cells and are assessed by clinical examination, electromyography, nerve conduction studies and imaging. Spasticity in multiple sclerosis and spinal cord injury also reflects altered control of motor neurons. Cannabinoid receptors are present on neurons in the motor pathways, and cannabinoids have been studied for spasticity and in animal models of motor neuron disease; human evidence outside spasticity is early.