motorneuronene

Motor neurones are nerve cells in the central nervous system that transmit commands from the brain and spinal cord to skeletal muscles, forming the final common pathway that initiates voluntary movement. Their axons extend into peripheral nerves and terminate at neuromuscular junctions on muscle fibers.

Lower motor neurons reside in the ventral horn of the spinal cord and in motor nuclei of the brainstem. They are divided into alpha motor neurons, which innervate extrafusal muscle fibers responsible for generating force, and gamma motor neurons, which innervate intrafusal fibers of muscle spindles to regulate spindle sensitivity and reflexes.

Motor commands originate from upper motor neurons and interneurons in the cortex and brainstem. These signals travel down the corticospinal and related tracts to synapse on lower motor neurons, whose axons then conduct action potentials to the neuromuscular junction. At the neuromuscular junction, the motor neuron releases acetylcholine, which binds nicotinic receptors on the muscle fiber. This triggers an end-plate potential and, if sufficient, an action potential that leads to muscle contraction.

A motor unit comprises a single motor neuron and the set of muscle fibers it innervates. The size of motor units varies by muscle, enabling a range of force production and precision.

Clinical relevance: Loss or dysfunction of motor neurons causes weakness and atrophy of affected muscles, reduced reflexes, and fasciculations, termed lower motor neuron signs. Upper motor neuron involvement produces spasticity and hyperreflexia. Diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy affect motor neurons and produce combined or selective motor symptoms. Diagnosis relies on clinical examination, electrophysiology, and imaging in some cases.