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Left and right cerebral hemisphere

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

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In the cerebral hemispheres, there exist neurons which centrally command and control the movements of the muscle. These neurons are known as the superior motor neurons and they are situated in special gyrus of both the frontal lobes called as motor gyrus or precentral gyrus. The superior motor neurons send axons which transmit some impulses to inferior motor neurons of the spinal cord and to the motor nuclei of the cranial. The fibers cross to another side in the specific areas of those axon paths. Around 2/3 of fibers which go down the spinal cord cross at the medullar level forming the structure called as the pyramidal decussation. Another (1/3) of fibers decreases in the same side of their original cerebral hemisphere and cross only in the spinal cord at level where their linked motor spinal root exit. The fibers which command the inferior motor neurons of cranial nerves cross to another side just before the connection with the nuclei of these nerves.The motor fibers which descend from the superior motor neurons upto the inferior motor neurons of spinal cord form the pyramidal tract. Injuries in this tract, such as, caused due to spinal sections or by the central or spinal tumors generally lead to tetraplegia and paraplegia.

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