Lab 9 - Descending Pathways to the Spinal Cord

The Corticospinal Pathway - Pons (continued)

Figure 1 is a transverse section through the midbrain at the level of the inferior colliculus, near the pons-midbrain junction. Locate the inferior colliculus, and medial longitudinal fasciculus.

At this level of the brainstem, the pontine nuclei and transverse pontocerebellar fibers begin to break up the compact corticofugal fiber bundles of the cerebral peduncles (crus cerebri). The transverse pontocerebellar fibers are also beginning to surround the longitudinal corticofugal fibers anteriorly to form the middle cerebellar peduncle. Some of the corticopontine fibers are beginning to terminate in the pontine nuclei. The cerebral cortex influences the cerebellum via this corticopontocerebellar pathway.

Figure 2 is a transverse section through the pons at the level of the facial and abducens nerve roots. Locate the fourth ventricle, facial colliculus, medial longitudinal fasciculus, facial nerve rootlets and middle cerebellar peduncle.

The corticofugal fibers (i.e., the corticospinal, corticobulbar and corticopontine fibers) are located within the basilar pons. Some of the corticoreticular fibers in the corticopontine and corticobulbar tracts are analogous to the corticospinal tracts; i.e., they arise from the same cortical areas, terminate in the pons and medulla and influence the cranial motor nuclei directly or indirectly through interneurons in the reticular formation.