Contrarily, lower motor neurons directly innervate skeletal muscle and have cell bodies in the anterior horn of the spinal cord (ventral horn) and at cranial nerve nuclei. Upper motor neurons are first-order neurons regulated by the neurotransmitter glutamate, are found in the primary motor cortex (precentral gyrus), and terminate in the spinal cord or brainstem, unable to leave the central nervous system (CNS). Upper motor neurons relay information from the brain to the spinal cord and brainstem, where they activate lower motor neurons, which directly stimulate muscles to contract. The collaborative effect of the UMN with the LMN is crucial in facilitating voluntary movement.
Although these entities share familiar nomenclature, they each serve distinct functions in steering spinal mechanics. The constellation of motor pathways within the human central and peripheral nervous system involves two entities that guide voluntary movement: upper motor neurons (UMN) and lower motor neurons (LMN).
