The brainstem is a stalklike structure within the posterior cranial fossa of the skull connecting the forebrain and spinal cord (eFig. 9.31). From rostral to caudal, the brainstem consists of the midbrain, pons, and medulla oblongata. Broadly speaking, the brainstem has three main functions: (1) it is a conduit for tracts ascending and descending through the CNS; (2) it houses cranial nerve nuclei III to XII (note that the CNXI is in the cervical spinal cord); and (3) it is the location for reflex centers related to respiration, cardiovascular function, and regulation of consciousness. Externally, each portion of the brainstem has a distinct appearance and structural features that define its many functional roles.
Approximately 2 cm in length, the midbrain connects the forebrain to the pons caudally and the cerebellum posteriorly (eFig. 9.32A). Along the midline of the anterior surface there is a deep depression, the interpeduncular fossa, which has several perforations on its surface where small vessels perforate through the floor of the fossa. On either side of the interpeduncular fossa are the crus cerebri (eFig. 9.32B). Projecting medially from the crus is cranial nerve III, the oculomotor nerve (eFig. 9.33A). The most prominent features on the posterior surface are the superior and inferior colliculi (eFig. 9.32A). Between the inferior colliculi, cranial nerve IV, the trochlear nerve, emerges, crosses the midline, and wraps around the lateral aspect of the midbrain (eFig. 9.32A). Laterally, the superior brachium and inferior brachium can be seen as they project in an anterolateral direction from the superior and inferior colliculi.
Caudal to the midbrain, the pons is approximately 2.5 cm in length and connects the midbrain to the medulla (eFig. 9.32B). The midline of the anterior surface has a shallow groove, the basilar groove, where the basilar artery resides along its course (eFig. 9.32B). On either side of the basilar groove, the pons has a prominent convex shape as a result of the vast number of fibers bridging through the pons to the cerebellum through the middle cerebellar peduncle. Along the anterolateral surface of the pons, cranial nerve V, the motor and sensory root of the trigeminal nerve, emerges (eFig. 9.33B). At the junction of the pons and medulla, from medial to lateral, cranial nerves VI (abducent), VII (facial), and VIII (vestibulocochlear) emerge (eFig. 9.33C). The posterior surface of the pons faces the cerebellum and forms the floor of the fourth ventricle (eFig. 9.32A). Along the midline of the posterior surface is a prominent bump, the facial colliculus, which represents the relationship between the facial nerve fibers as they wind around the nucleus of the abducent nerve (eFig. 9.33C).