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How motor activity of left side controlled by right cerebral

How it is structurally described that the motor activity of left side of the body is controlled by the right cerebral hemisphere and vice-versa?

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In cerebral hemispheres, there are neurons which centrally command and control muscle movements. Such neurons are termed as superior motor neurons and they are situated in a special gyrus of both frontal lobes termed as motor gyrus (or precentral gyrus). Superior motor neurons send axons which transmit impulses to the inferior motor neurons of the spinal cord (that is, for neck, trunk and limb movements) and to the motor nuclei of cranial nerves (that is, for face, eyes and mouth movements).

The fibers cross to other side in particular regions of those axon paths. Around 2/3 of the fibers which go down the spinal cord cross at the medullar level making a structure termed as pyramidal decussation. Another (1/3) of fibers descend in similar side of their original cerebral hemisphere and cross only in the spinal cord at the level where their related motor spinal root exit. The fibers which command the inferior motor neurons of cranial nerves cross to the other side just prior to the connection with the nuclei of such nerves.

The motor fibers which descend from the superior motor neurons to inferior motor neurons of the spinal cord form pyramidal tract. Injuries in this area, for illustration, caused by spinal sections or by central or spinal tumors might lead to tetraplegia and paraplegia.

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