Key Points

Conscious movement is initiated by upper motor neurons in an area of the frontal cortex designated as MI.

Pyramidal cells in the MI issue motor commands based on input from the adjacent motor association area designated as MII, as well as from prefrontal cortex, motor nuclei in the thalamus, and primary (SI) and association (SII) sensory cortex.

The firing rate of an individual pyramidal cell correlates with the force of the resulting muscle contraction; the average firing rate of a given cluster of pyramidal cells correlates with the direction of resulting limb movement. The Mil motor association area is further divided into a supplemental motor area, which coordinates bilaterally coordinated movements, and a premotor area, which orients the body for upcoming movement.

The prefrontal cortex initiates the initial planning stages of the movement, and the sensory association area (SII) focuses attention on parts of the body to be involved in the movement.

The primary motor cortex consists of a strip of cortical tissue in the frontal lobe (Fig. 1). On the surface, it is separated from the primary somatosensory cortex by the central gyrus. However, these two cortical areas are functionally connected by a bridge of tissue, the para-central lobule, that follows the contour of the central sulcus and links the two areas. Cells in both motor and sensory strips are arranged somatotopically, providing both a motor homunculus and a sensory homunculus that are in register with each other. As discussed in the preceding chapters, the primary sensory cortex is the primary receiving area for somatosensory information, and its activity correlates with conscious perception of the sensation; however, higher order processing of sensory information occurs in association areas of the parietal cortex. A corollary can be seen in the arrangement of the motor pathway. The primary motor area is the site of initiation of conscious movement; however, events preceding the generation of the initiation signal occur elsewhere in motor association areas in the frontal cortex and in the parietal lobe in combination with sensory processing. Additional components of planned movements occur in

A. Lateral view of right cerebral cortex

Ml PMA SMa

B. Medial view of right cerebral cortex SMA Ml w

C. Origin of corticofugal paths

D. Somatic destination of motor cortex neurons ventra/ CS7-

ventra/ CS7-

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