The Pons and Cerebellum

Basal and Alar Plates

The basal and alar plates of the brainstem extend from the medulla rostrally into the developing pons. The cranial nerve motor nuclei found in the pons (trigeminal, abducens, facial, and superior salivatory) originate from the basal plate and are located medial to the sulcus limitans ( Fig. 12.2 A, B ). The functional components of these motor neurons include somatic efferent (SE) (trigeminal, facial, and abducens) and visceral efferent (VE) (superior salivatory).

The cranial nerve sensory nuclei located in the pons include portions of the trigeminal and vestibulocochlear nuclei and the rostral tip of the solitary nucleus. These nuclei originate from the alar plate and are found lateral to the sulcus limitans ( Fig. 12.2 A, B ). Their functional components are somatic afferent (SA) for the trigeminal nucleus and vestibular and cochlear nuclei and visceral afferent (VA) for the solitary nucleus. This recognizes the fact that taste may be generally regarded as a special sense. The portion of the posterior pons that contains these motor and sensory nuclei as well as the reticular formation and several ascending and descending tracts is the pontine tegmentum ( Fig. 12.2 B ).

The anterior area of the developing pons is invaded by large numbers of descending cortical fibers. Although some will terminate here, others pass through to more caudal targets. Immature alar plate neurons also migrate into this anterior pontine region to form the basilar pontine nuclei. These nuclei, their axons, and the descending fibers passing to and through this area collectively form the basilar pons ( Fig. 12.2 B ).

Cerebellum

The cerebellum develops from the rhombic lips of the pontine alar plates. These lips expand posteromedially toward each other until they meet at the midline and fuse to form the cerebellar plate ( Fig. 12.2 A, B ), which is the rudiment of the cerebellum. As development progresses, the cerebellum is divided by transverse fissures into lobes and lobules. The first of these fissures to appear is the posterolateral fissure, which separates the flocculonodular lobe caudally from the corpus cerebelli rostrally. The primary fissure, the second to appear, divides the corpus cerebelli into the anterior and posterior lobes ( Fig. 12.2 B-D ). Internal changes, such as development of the cerebellar cortex and nuclei, take place concurrently with these external events.

Basilar Pons

The portion of the brainstem lying between the midbrain rostrally and the medulla caudally is the pons ( pons, Latin for “bridge”). Anteriorly and laterally ( Fig. 12.3 ), the pons consists of a massive bundle of transversely oriented fibers that enter the cerebellum as the middle cerebellar peduncle ( brachium pontis ). The exit of the trigeminal nerve marks the transition from the basilar pons, which is anterior to the trigeminal root, to the middle cerebellar peduncle, which lies posterior to the exit of the trigeminal nerve ( Figs. 12.3 and 12.4 ). Rostrally, the large axonal bundles forming the crus cerebri of the midbrain extend into the basilar pons. Caudally, some of these axons emerge to form the pyramids of the medulla ( Fig. 12.4 A ).

The cranial nerves that emerge from the pons are the trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII). The trigeminal nerve exits laterally and is composed of a large sensory root (the portio major ) and a small motor root (the portio minor ) ( Fig. 12.4 A ). The portion of the trigeminal nerve that traverses the subarachnoid space between the pons and the trigeminal ganglion forms a landmark that is visible on magnetic resonance imaging at this level ( Fig. 12.4 B ). The abducens, facial, and vestibulocochlear nerves emerge in medial to lateral sequence along the pons-medulla junction ( Fig. 12.3 ). Although cranial nerve VII is commonly called the facial nerve, it is actually composed of two roots, the facial nerve (SE fibers) and the intermediate nerve (VA, VE, and SA fibers). The vestibulocochlear nerve (SA fibers) emerges posterolaterally and, with the facial and intermediate nerves and labyrinthine artery, occupies the internal acoustic meatus.

Rhomboid Fossa of the Pons

The rhomboid fossa forms the floor of the fourth ventricle. Its caudal portion is located in the medulla, and its larger, more rostral area is in the pons. The posterior surface of the pontine tegmentum, which forms the floor of the fourth ventricle, is visible only when the cerebellum is detached from the brainstem ( Fig. 12.5 ). This part of the ventricular floor is characterized by the facial colliculus located between the median fissure and the superior fovea of the sulcus limitans and by the vestibular area located lateral to the sulcus limitans. The facial colliculus is formed by the underlying abducens nucleus and internal genu of the facial nerve, and the vestibular area marks the location of the vestibular nuclei. The brachium pontis and the brachium conjunctivum form the lateral walls of the fourth ventricle in the pons; the roof is formed by the anterior medullary velum, by a small part of the cerebellum, and by a portion of the tela choroidea ( Fig. 12.6 ; see also Fig. 10.2 ).