The cerebellum is composed of a highly convoluted cerebellar cortex and a core of white matter containing the cerebellar nuclei. This structure is anchored to the brainstem via the cerebellar peduncles. The cerebellum is located superior to the brainstem, inferior to the tentorium cerebelli, and internal to the occipital bone. The cerebellum has a superior surface apposed to the tentorium and a convex inferior surface that abuts the inner surface of the occipital bone.

The cerebellum receives input from many areas of the  neuraxis and influences motor performance through connections with the dorsal thalamus and, ultimately, the motor cortices. Lesions of these pathways result in characteristic motor dysfunctions, which may involve either proximal (axial) or distal musculature. These deficits are actually the result of altered activity in the motor cortex and its descending brainstem and spinal projections, which influence lower motor neurons of the spinal cord.

Basic Structural Features

Cerebellar Peduncles

The cerebellum is connected to the brainstem by three pairs of cerebellar peduncles. The inferior cerebellar peduncle is composed of a larger part, the restiform body, and a smaller portion, the juxtarestiform body. The restiform body is the large ridge on the dorsolateral aspect of the medulla rostral to the level of the obex. This bundle contains mainly fibers that arise in the spinal cord or medulla. The juxtarestiform body is located in the wall of the fourth ventricle. This bundle is composed primarily of fibers that form reciprocal connections between the cerebellum and vestibular structures.

The basilar pons, which is located inferior to the exiting roots of the trigeminal nerve, is continuous into the middle cerebellar peduncle (brachium pontis), which is located superior to the exiting roots of the fifth cranial nerve. These exiting roots represent the boundary between the basilar pons and the middle cerebellar peduncle. This large peduncle mainly conveys pontocerebellar fibers arising from the pontine nuclei of the basilar pons into the cerebellum.

The superior cerebellar peduncle (brachium conjunctivum) sweeps rostrally out of the cerebellum and penetrates into the midbrain just caudal to the exit of the trochlear nerve. Within the midbrain, these fibers cross the midline as the decussation of the superior cerebellar peduncle at the level of the inferior colliculus. This bundle contains predominantly cerebellar efferent fibers that originate from neurons of the cerebellar nuclei and distribute to the diencephalon and brainstem.