● The corticospinal (pyramidal) tract. The corticospinal tracts mediate voluntary movement and integrate skilled, complicated, or delicate movements by stimulating selected muscular actions and inhibiting others. They also carry impulses that inhibit muscle tone, the slight tension maintained by normal muscle even when it is relaxed. The corticospinal tracts originate in the motor cortex of the brain (Fig. 1). Motor fibers travel down into the lower medulla, where they form an anatomical structure resembling a pyramid.

There, most of these fibers cross to the opposite or contralateral side of the medulla, continue downward, and synapse with anterior horn cells or with intermediate neurons. Tracts synapsing in the brainstem with motor nuclei of the CNs are termed corticobulbar.

● The basal ganglia system. This exceedingly complex system includes motor pathways between the cerebral cortex, basal ganglia, brainstem, and spinal cord. It helps to maintain muscle tone and to control body movements, especially gross automatic movements such as walking.

● The cerebellar system. The cerebellum receives both sensory and motor input and coordinates motor activity, maintains equilibrium, and helps to control posture.

FIGURE 1. Motor pathways: corticospinal and corticobulbar tracts.

责任编辑：admin