Pyramidal system :
Clinically, the most important tract is the lateral corticospinal tract, because it is responsible for controlling movement of the upper and lower extremities. Cell bodies of upper motor neurons forming this tract are located in the primary motor cortex (eFig. 9.54). Axons of these upper motor neurons converge in the corona radiata and descend through the posterior limb of the internal capsule to reach the crus cerebri of the midbrain. These axons continue through the anterior aspect of the pons as small bundles to accommodate the transverse pontocerebellar fibers, which are also present in this location. Once the fibers reach the medulla, they are again grouped together and form a large swelling known as the pyramid (eFig. 9.54). At the caudal medulla, before transitioning into the spinal cord, most of the axons decussate over to the contralateral side to form the lateral corticospinal tract (eFig. 9.54). The remaining axons will stay ipsilateral and form the anterior corticospinal tract, a tract included in the medial motor systems.
After decussating and forming the lateral corticospinal tract, the axons descend through the spinal cord to synapse on the cell bodies of lower motor neurons in the lateral portion of the anterior horn gray matter. Axons of these lower motor neurons then exit the spinal cord through the anterior root (eFig. 9.54).