The metencephalon consists of the pons and cerebellum. The pons is the middle segment of the brainstem, the caudal part being the medulla and the rostral portion being the midbrain. Although comprising only about 1.3% of the brain by weight, the pons has many important functions. The motor and sensory nuclei and the exit points of cranial nerves V to VIII are associated with the pons. The cerebellum is not part of the brainstem but rather is considered a suprasegmental structure because it is located superior to the brainstem. The cerebellum is comparatively large, comprising about 10.5% of the total brain weight. Functionally, it is part of the motor system. The blood supply to the pons and cerebellum arises from branches of the basilar and cerebellar arteries.

External Features

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, 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. 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.

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). 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. The abducens, facial, and vestibulocochlear nerves emerge in medial to lateral sequence along the pons-medulla junction. 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. 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.

Cerebellum

The cerebellum is located posterior to the brainstem and fills much of the posterior fossa. It is attached to the brainstem by three pairs of cerebellar peduncles (superior, middle, and inferior). In sagittal section, the human cerebellum appears wedge shaped, with its superior surface apposed to the tentorium cerebelli and its inferior surface curving toward the foramen magnum.

The cerebellum consists of anterior, posterior, and flocculonodular lobes; each lobe, in turn, is composed of lobules. Lobes and lobules are separated from each other by fissures. Lobules are made up of yet smaller folds of cerebellar cortex called folia (singular, folium). Cerebellar folia, lobules, and lobes can often be followed across the midline from one side of the cerebellum to the other.

Each cerebellar lobe (and lobule) is also divided into rostrocaudally oriented regions of cortex commonly called the vermis (medial), intermediate (paravermis), and hemisphere (lateral) zones. The vermal zone is approximately 1.0 cm across at its widest point. The hemisphere is expansive in the human cerebellum and is separated from the vermis by the intermediate zone.

Four cerebellar nuclei are located in the white matter core of each hemisphere. From medial to lateral, they are the fastigial, globose, emboliform, and dentate nuclei. These cells receive input from branches of cerebellar afferent fibers and from Purkinje cells located in the cerebellar cortex. In turn, axons of cerebellar nuclear cells provide the main output signals of the cerebellum.

Vasculature of the Pons and Cerebellum

The basilar artery and its branches serve basilar and tegmental areas of the pons. The superior cerebellar artery, through its medial and lateral branches, distributes to the superior surface of the cerebellum and most of the cerebellar nuclei; the inferior surface of the cerebellum is served by anterior and posterior inferior cerebellar arteries.