The Principal Motor Pathways

■ 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. 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 cranial nerves 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.

basal ganglia, or reflexly in the sensory receptors, must ultimately be translated into action via the anterior horn cells. A lesion in any of these areas will affect movement or reflex activity.

When the corticospinal tract is damaged or destroyed, its functions are reduced or lost below the level of injury. When upper motor neurons are damaged above the crossover of its tracts in the medulla, motor impairment develops on the opposite or contralateral side. In damage below the crossover, motor impairment occurs on the same or ipsilateral side of the body. The affected limb becomes weak or paralyzed, and skilled, complicated, or delicate movements are performed especially poorly when compared to gross movements. Muscle tone is increased and deep tendon reflexes are exaggerated.

Damage to the lower motor neurons causes ipsilateral weakness and paralysis, but in this case muscle tone and reflexes are decreased or absent.

Disease of the basal ganglia system or cerebellar system does not cause paralysis, but can be disabling. Damage to the basal ganglia system produces changes in muscle tone (most often an increase), disturbances in posture and gait, a slowness or lack of spontaneous and automatic movements termed bradykinesia, and a variety of involuntary movements. Cere-bellar damage impairs coordination, gait, and equilibrium, and decreases muscle tone.

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