Motor disorders

© Cambridge University Press 2014. he inal pathway for voluntary movement in mammals consists of the following elements: (1) the motor neuron, with its cell body in the ventral horn of the spinal cord and its axonal projection into the periphery; (2) the neuromuscular junction, which is the functional synapse between nerve and muscle; and (3) the muscle itself. Together these components form the “motor unit, ” i. e., one motor neuron and the muscle ibers it innervates. here is extensive sub-specialization among motor units, with one motor neuron innervating two to three ibers in fast-acting tonic muscles like the extraocular muscles, and 200 to 300 individual ibers in postural muscles. An individual anterior horn cell receives many modulatory inluences. The neurophysiological “pattern generator” for voluntary and relex motor control is modulated by input from descending corticospinal tract ibers, by local inluences from interneurons, and by reciprocally innervated contralateral motor neurons. Despite tens of millions of years of evolutionary divergence the basic components of this system have been conserved between rodents and Homo sapiens, making mouse models an essential tool in the elucidation of the pathogenesis of disorders of the motor system.