Within the posterior cranial fossa, the cerebellum is covered by the tentorium cerebelli of the dura mater (eFig. 9.17) and connects to the posterior surface of the brainstem via the superior, middle, and inferior cerebellar peduncles (eFig. 9.62). Anteriorly, the cerebellum forms the roof of the fourth ventricle (eFig. 9.14). On its surface, the cerebellum has several convoluted folds, or folia, separated by fissures. Two of these fissures serve as landmarks to divide the cerebellum into three lobes. Superiorly, the primary fissure separates the anterior lobe from the posterior lobe (eFig. 9.61). Anteriorly and inferiorly, the posterolateral fissure defines the structures of the flocculonodular lobe, which includes the flocculus from each hemisphere and nodule of the vermis (eFig. 9.63). A third fissure, the horizontal fissure, borders the superior and inferior surfaces of the cerebellum (eFig. 9.64).
Each fold or folia of the cerebellar cortex has a central core of white matter covered by a thin layer of gray matter superficially. In sections parallel to the median plane, the branching pattern of the folia can be appreciated; this is often referred to as the arbor vitae (eFig. 9.28). Deep within the white matter of each hemisphere are four masses of cerebellar nuclei. From lateral to medial they are the dentate, emboliform, globose, and fastigial (eFig. 9.65). Note that the emboliform and globose are collectively referred to as the interposed nuclei. Output from the cerebellum originates from one of these four nuclear complexes before leaving through the superior cerebellar peduncle, predominantly. In general, the output from each cerebellar hemisphere coordinates movement on the ipsilateral side of the body.
Functionally, the cerebellar cortex can be divided into three areas. The vermis in the midline influences movements along the axis of the body, including the neck, trunk, abdomen, and pelvis (eFig. 9.66). Adjacent to the vermis, the intermediate zone controls muscles of the distal upper and lower limbs. The lateral zone participates in motor planning to coordinate sequential movements of the entire body. Input to these functional areas of cerebellar cortex come from the cerebral cortex, spinal cord, and brainstem by passing predominantly through the middle and inferior cerebellar peduncles. Fibers entering the cerebellum proceed as mossy fibers (from various regions) or climbing fibers (from olivary nucleus). Mossy fibers form excitatory synapses with dendrites of the granule cells, in the granule cell layer (eFig. 9.67). From here, the granule cells send axons to the molecular layer, where they bifurcate into parallel fibers that run longitudinally in the folia. The parallel fibers then synapse on Purkinje cells in the outermost Purkinje cell layer (eFig. 9.67). Climbing fibers project to the Purkinje cell layer and form powerful excitatory connections with the Purkinje cells. The Purkinje cells then project to the deep cerebellar nuclei (eFig. 9.67).
As mentioned earlier, the cerebellum receives input from the cerebral cortex, brainstem, and spinal cord. Input from the cerebral cortex to the cerebellum is primarily involved in voluntary muscle control and coordination of movement. Axonal projections from the cerebral cortex destined for the cerebellum descend through the internal capsule and terminate on pontine nuclei (eFig. 9.68). The axons from the pontine nuclei then cross over as transverse fibers to enter the contralateral cerebellum through the middle cerebellar peduncle.