Motor pathways contain upper motor neurons, synapses in the brainstem or spinal cord, and lower motor neurons. Nerve cell bodies or upper motor neurons lie in the motor strip of the cerebral cortex and in several brainstem nuclei; their axons synapse with motor nuclei in the brainstem (for cranial nerves) and in the spinal cord (for peripheral nerves). Lower motor neurons have cell bodies in the spinal cord, termed anterior horn cells; their axons transmit impulses through the anterior roots and spinal nerves into peripheral nerves, terminating at the neuromuscular junction.
Three kinds of motor pathways impinge on the anterior horn cells: the cor-ticospinal tract, the basal ganglia system, and the cerebellar system. There are additional pathways originating in the brainstem that mediate flexor and extensor tone in limb movement and posture; most notable in coma (see Table 16-16, p. 622).
All of these higher motor pathways affect movement only through the lower motor neurons—sometimes called the "final common pathway." Any movement, whether initiated voluntarily in the cortex, "automatically" in the
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