Lab 2 - Internal Organization of the Brain

Pons and Cerebellum - Coronal and Sagittal Sections

The pons and cerebellum form the metencephalon. The cranial nerves with nuclei in the pons include the trigeminal, the abducens, the facial, and the vestibulocochlear nerves. These nuclei are located within the pontine tegmentum along with the reticular formation, longitudinal fiber bundles and other important cell groups.

The longitudinal fibers you see cut in cross section are the Corticofugal Fibers that we followed from the cortex down to the corona radiata, internal capsule, and cerebral peduncles. In the pons, these fibers are surrounded by the Pontine Nuclei. Some of these corticofugal fibers (corticopontine fibers) will synapse at the pontine nuclei and give rise to axons that decussate to form the Middle Cerebellar Peduncles. These fibers (pontocerebellar fibers), which are located in the interior of the pons, terminate within the cerebellum and are involved in the interaction of the cerebral cortex and cerebellum in regulating motor function. These fibers constitute most of the base of the pons. The remaining corticofugal fibers (corticospinal fibers) constitute the pyramidal tract and form the pyramids of the medulla.

The reticular formation receives the terminals of corticoreticular fibers (a sub-population of the corticofugal fibers). Some of these reticular formation neurons, in turn, send their axons to cranial nerve nuclei and are part of a system exerting cortical influences on these nuclei.

The posterior surface of the pons forms the rostral end of the floor of the fourth ventricle. The stria medullaris (not to be confused with the stria medullaris of the thalamus) marks the inferior boundary between the pons and medulla. As mentioned above, the anterior surface of the pons is formed by the fibers of the middle cerebellar peduncles. Notice the shallow groove running along the anterior midline that normally contains the basilar artery.

Notice that the cerebral aqueduct widens to form the fourth ventricle and that the superior cerebellar peduncles form the walls of the fourth ventricle. The superior cerebellar peduncles carry the axons of cerebellar neurons to the red nucleus and thalamus and thus influence structures of the motor system. The tegmental portion of the pons extends from the base up to the floor of the fourth ventricle. The tegmentum contains the cranial nerve nuclei, longitudinal fiber systems in continuity with those in the midbrain and medulla, as well as the reticular formation and other cell groups. Notice that the abducens (VI) nucleus and the facial nerve root, which flanks the abducens nucleus, form the elevation called the facial colliculus. The sulcus limitans forms a shallow groove between the facial colliculus and the more lateral sensory vestibular region. Review the points of attachment of the trigeminal, facial, vestibulocochlear and abducens nerves.