Anterior brainstem white matter dissection. The pyramidal pathway or corticospinal tract originates mostly from upper motor neurons (UMNs) in the precentral gyrus (primary motor cortex) with additional contributions from the premotor, supplementary, and somatosensory cortices.  The fibers descend in sequence through the centrum semiovale,  corona radiata, and posterior limb of the internal capsule before entering the brainstem. In the brainstem, the corticospinal fibers descend in the middle one-third of the midbrain crus cerebri surrounded on both sides by corticopontine fibers, then enters the basis pons while bisecting the transverse pontocerebellar fibers, and finally forms the pyramids on the ventral surface of the medulla oblongata.  At the medulla-spinal cord junction, ~85-90% of the fibers cross over in the pyramidal decussation, then continue into the lateral column of the spinal cord as the crossed lateral corticospinal tract. The pontocerebellar fibers originate from neurons in the contralateral basis pons then enter and form the middle cerebellar peduncle (MCP) to reach the cerebellum. (Image courtesy of M Nunez)