Motor activity of left side of body or right side of body

Question

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

Answer 1

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

The fibers cross to other side in particular areas of those axon paths. About 2/3 of fibers which go down the spinal cord cross at medullar level forming a structure which is known as pyramidal decussation. The other (1/3) of fibers descends in same side of their original cerebral hemisphere and cross only in the spinal cord at level where their allocated motor spinal root exit. The fibers which command inferior motor neurons of cranial nerves cross to the other side just before the connection with nuclei of these nerves.

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