Lab 9 - Descending Pathways to the Spinal Cord

The Corticospinal Pathway - Medulla

Figure 1 is a transverse section through the brain stem at the level of the pontomedullary junction. Locate the fourth ventricle, vestibular nuclei, medial longitudinal fasciculus, inferior cerebellar peduncle, inferior olivary complex, medial lemniscus, central tegmental tract, and pyramids.

The corticospinal fibers, along with the corticobulbar fibers form two large compact bundles, the pyramids, at the base of the medulla. At this level of the brain stem, most of the corticopontine fibers have terminated within the pons tegmentum and in the pontine nuclei of the basilar pons and will not be present at lower levels.

Figure 2 is a transverse section through the medulla at the level of the caudal portion of the fourth ventricle. Locate the fourth ventricle, vestibular nuclei, inferior cerebellar peduncle, inferior olivary nucleus, pyramids, medial longitudinal fasciculus, medial lemniscus and spinal trigeminal tract and nucleus.

The corticospinal fibers and the remaining corticobulbar fibers continue to form the pyramids of the medulla. Note that corticobulbar fibers leave the pyramids along the entire course of the medulla to terminate in the medullary reticular formation and in certain cranial nerve nuclei.

Figure 3 is a transverse section through the caudal medulla near its junction with the cervical spinal cord. Recall the location of the spinal trigeminal tract and nucleus. Most of the corticospinal fibers are, at this level, crossing the midline in the pyramidal decussation and shifting posteriorly. The pyramidal decussation takes up the entire anteromedial field of this section. Just anterior to the central gray (which surrounds the central canal - the caudal extension of the ventricular system), the crossed corticospinal fibers can be seen in transverse section starting to form the lateral corticospinal tract. Anterior to the crossed corticospinal fibers, the crossing fibers can be seen cut longitudinally as they course across the midline. From 75% to 90% of the corticospinal fibers cross and form the crossed lateral corticospinal tract. Most of the remaining fibers descend uncrossed and form the anterior corticospinal tract below this level. At lower levels of the medulla, the lateral corticospinal fibers shift laterally to join the posterior spinocerebellar, spinothalamic and rubrospinal fibers, which together form the lateral funiculus of the spinal cord.