Motor System I: Movement & Motor Pathways

Abstract

There are three major areas that are responsible for producing the fine motor move-ments in the human: the cerebral cortex, basal ganglia, and cerebellum. The motor cortex in the cerebral hemispheres and regions in the frontal lobe anterior to the motor–sensory strip provide voluntary control to all of the skeletal muscles in the body. Our understanding of localization in the motor system is based on studies following lesions at various levels of the neuroaxis. I. Cerebral Cortical Motor Functions In studying the motor system, we will consider reflex activity, the central generators of patterns of movement, voluntary movement, and learned movements. We will also consider two interrelated aspects: posture and movement. Under posture, we will be studying static or tonic reactions. In the following discussion, we will exam-ine motor function at the level of the spinal cord, the brain stem, and cerebral cor-tex. It is important to realize that motor functions are represented at successively higher physiological and anatomical levels of the neural axis. As we go higher in the neural axis, we are utilizing and modifying mechanisms that have been inte-grated at a lower level of the neural axis—a concept first expressed in the modern era by Huglings Jackson (Taylor, 1931). Thus, pattern generators/centers make use of the motor mechanisms involved in reflexes without the necessity of afferent input. The terms utilized in defining a reflex are presented in Table 9.1. In turn, voluntary and learned movements incorporate or impose a higher level of a more complex cortical control of these reflex and central pattern mechanisms. In subsequent sections, the role of basal ganglia and the cerebellum in modulating movement will be considered.