At mid-pontine levels the superior cerbellar peduncles form the lateral walls of the expanding fourth ventricle (eFig. 9.38). Also at this level, the prominent middle cerebellar peduncles can also be seen, along with the motor nucleus and principal sensory nucleus of cranial nerve V, the trigeminal nerve.
In the caudal aspect of the pons, the abducent nucleus can be appreciated just lateral to the medial longitudinal fasciculus (eFig. 9.39). Also present at this level is the medial vestibular nucleus in addition to the anterior and posterior cochlear nuclei.
From rostral to caudal the internal structures of the medulla will be briefly described from three levels: (1) level of the inferior olivary nucleus, (2) decussation of the internal arcuate fibers, and (3) decussation of the pyramids. Nuclei visible in a transverse section at the level of the inferior olivary nucleus include those associated with cranial nerves VIII (vestibulocochlear), IX (glossopharyngeal), X (vagus), XI (spinal accessory), and XII (hypoglossal) (eFig. 9.40). At this level, the medial lemniscus maintains a vertical position immediately adjacent to the midline. Dorsal to the medial lemniscus are the medial longitudinal fasciculus and hypoglossal nucleus. A large distinguishing structure at this rostral level is the laterally oriented inferior cerebellar peduncle, which connects to the cerebellum posteriorly.
Continuing caudally at the level of the internal arcuate fibers, the dorsal aspect of the medulla is populated by the gracile nucleus medially, followed by the cuneate and spinal trigeminal nucleus laterally. Ventral to the cuneate and gracile nuclei, their neuronal axons can be seen decussating as internal arcuate fibers to form the medial lemniscus near the midline of the medulla. Lateral to the internal arcuate fibers and ventral to the spinal trigeminal tract, the spinocerebellar and anterolateral tracts can be seen along the perimeter of the medulla (eFig. 9.41).
Before transitioning into the spinal cord, the pyramidal decussation can be observed along the midline of the caudal medulla (eFig. 9.36). Dorsal to these decussating fibers, the gracile and cuneate nuclei begin to emerge as their fasciculi continue rostrally. Note that the spinal accessory nucleus (CN XI) is located in the cervical spinal cord and not the medulla.
Vascular supply to the brainstem, and other structures in the posterior cranial fossa, is provided by branches off of the vertebrobasilar system of arteries. As mentioned in the “Cerebral Vasculature” section, the vertebrobasilar system begins with the vertebral arteries bilaterally, which arise from the subclavian arteries and ascend through the foramen transversaria of cervical vertebrae C6 to C2 in the neck. At the pontomedullary junction, the vertebral arteries fuse to form the single basilar artery. The basilar artery continues rostrally and terminates as the paired posterior cerebral arteries at the pontomesencephalic junction.
Before merging and forming the basilar artery, each vertebral artery gives rise to a posterior inferior cerebellar artery (PICA) and posterior spinal artery and contributes to the formation of the anterior spinal artery (eFig. 9.42). The PICA perfuses the lateral aspect of the medulla and the inferior portion of the cerebellum. The medial and anterior portions of the medulla receive vascular supply from the paramedian branches off of the vertebral and anterior spinal arteries.