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11-20. THE MOTOR PATHWAYS

Motor pathways begin in the brain. They descend the neuraxis in bundles of a number of specific neuron processes called motor fiber tracts. Commands originating in the right half of the brain leave the CNS through peripheral nerves on the left side. Commands from the left half of the brain leave the CNS on the right side. Therefore, the right half of the brain controls the left side of the body and the left half of the brain controls the right side of the body. For example, the actions of the right hand are controlled by the left half of the brain. (In those people who are right-handed, we refer to the left half of the brain as being dominant.)

a. Pyramidal Motor Pathways. A pyramidal motor pathway is primarily concerned with volitional (voluntary) control of the body parts, in particular the fine movements of the hands. Because control is volitional, the pathways can be used for neurological screening and testing. These pathways are called pyramidal because their neuron processes contribute to the makeup of a pair of structures in the base of the brain known as the pyramids.

b. Extrapyramidal Motor Pathways. An extrapyramidal pathway is primarily concerned with automatic (nonvolitional) control of body parts for purposes of coordination. Extrapyramidal pathways use many intermediate relays before reaching the effector organs. The cerebellum of the brain plays a major role in extrapyramidal pathways; the cerebellum helps to integrate patterned movements of the body.

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