Q. How is it structurally explained that the motor activity of the left side of the body is controlled by the right cerebral hemisphere and the motor activity of the right side of the body is controlled by the left cerebral hemisphere?
In the cerebral hemispheres there are neurons that centrally control and command muscle movements. These neurons are called superior motor neurons and they are located in a special gyrus of both frontal lobes called as motor gyrus or precentral gyrus. The superior motor neurons send axons that transmit impulses to the inferior motor neurons of the spinal cord for trunk, neck and limb movements and to the motor nuclei of the cranial nerves for eyes, face and mouth movements).
The fibers cross to the other side in specific areas of those axon paths about 2/3 of the fibers that go down the spinal cord cross at the medullar level forming a structure known as pyramidal decussation. The other (1/3) of fibers descends in the same side of their original cerebral hemisphere and cross only inside the spinal cord at the level where their associated motor spinal root exit and the fibers that command the inferior motor neurons of the cranial nerves cross to the other side just before the connection with the nuclei of these nerves.
The motor fibers that descend from the superior motor neurons to the inferior motor neurons of the spinal cord form the pyramidal tract. Injuries in this tract, for instance, caused by spinal sections or by spinal or central tumors may lead to tetraplegia and paraplegia.