Somatomotor System

Motor function is mediated by several areas of the prefrontal cortex, including the primary motor cortex, the motor planning areas (premotor area, supplementary motor area, Broca's area, and the area corresponding to Broca's area in the right hemisphere), and the cortical eye fields.

The primary motor cortex (MI) functions to mediate control of voluntary movements. It is located anterior to the central sulcus in the precentral gyrus (Fig. 6C; Brodmann area 4) and is the origin for some of the fibers of the corticospinal tract. All these fibers originate in layer 5 of the six-layered primary motor cortex and are pyramidal-type neurons. Primary motor cortex is an example of agranular cortex.

Similar to the primary somatosensory cortex, MI is somatotopically organized such that the lower body representation lies on the medial surface of the cortex, within the longitudinal fissure, whereas the upper body and face representations lie laterally along the pre-central gyrus. In addition, there are large, disproportionate areas for the hand and mouth. MI receives input from numerous cortical areas, including SI (Brodmann areas 1-3), SII (Brodmann area 5), and premotor and supplementary motor areas (area 6), as well as input from the cerebellum, basal ganglia, and ventrolateral nucleus of the thalamus. The supplementary and premotor areas are collectively termed the motor planning areas and are responsible for organizing and planning complex movements.

The supplementary motor area (superior Brodmann area 6) is found anterior to the lower leg area of the primary motor cortex (medial surface of the medial frontal gyrus). It receives input from the prefrontal association cortex for planning of complex bimanual and sequential movements and for coordinating motor responses to sensory stimuli. Furthermore, efferent projections from the supplementary motor area are sent to the spinal cord and brain stem in addition to the primary motor cortex.

The premotor area (inferior Brodmann area 6), anterior to the primary motor area on the lateral surface of the prefrontal cortex, receives information from the prefrontal association cortex and sends output to the brain stem, basal ganglia, and cerebellum, in addition to the primary motor cortex. The premotor area is important for planning and control of visually guided movements. Located just rostral to the premotor area in the prefrontal cortex is the frontal eye field (Brodmann area 8), which controls voluntary conjugate movements of the eyes, independent of visual stimuli.

Finally, there are two areas that control the motor planning of speech and gestures associated with speech. Broca's area is located anterior to the pre-motor area in the inferior prefrontal cortex, usually in the left hemisphere. It is responsible for the motor programming of speech, whereas the corresponding area in the contralateral hemisphere (usually the right) is concerned with the planning of nonverbal communication such as gestures.

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