In the precentral gyrus, or primary motor cortex, components are represented in maps for the action of certain muscle groups on the opposite side of the body (homunculus) (Fig. 2). There is bilateral representation for central groups of muscles that work in unison (e.g. upper face, pharynx). Strong sensory input into the motor cortex and feedback from basal ganglia and cerebellar modulating centers allow for sensorimotor integration and modulation so that movements are smooth and accurate. Integrated postural movements are represented in the adjacent premotor and supplementary motor areas which also receive sensory feedback. Communication occurs between these cortical motor areas, and each sends descending pathways down the corticospinal tracts to the brainstem and spinal cord. Apart from the direct communications of the primary motor cortex with the cell bodies of lower motor neurons, these primary and supplementary centers also act indirectly via brainstem centers that have motor pathways to local spinal or brainstem segmental circuits, and eventually to the lower motor neuron cell bodies.
Fig. 2 The principal motor control areas of the cortex are shown on the convexity (in front) and the medial aspects (behind) of the cerebral hemispheres (Reproduced with permission from Kandej eL&L (1991))
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