Cerebellum: Anatomical Regions

The cerebellum, also known as the "little brain," is located in the posterior cranial fossa, inferior to the tentorium cerebelli and dorsal to the brainstem. It plays a significant role in motor control, coordination, and proprioception.

Cerebellar Structure

Externally, the cerebellum features a highly convoluted surface with numerous folia (narrow ridges) separated by shallow sulci (grooves). The cerebellum is divided into two hemispheres by a thin median structure known as the vermis. The vermis primarily coordinates axial and proximal limb musculature. The hemispherical division also delineates the cerebellum's internal organization into distinct lobes, each with specific functions. The anterior and posterior lobes, which are demarcated by the primary fissure, are involved in regulating muscular movements and coordination. The flocculonodular lobe, located inferiorly and set apart by the posterolateral fissure, plays a pivotal role in maintaining balance and equilibrium.

The outer layer of the cerebellum is called the cerebellar cortex and is composed of gray matter. The cerebellar cortex contains three layers — the molecular layer, the Purkinje cell layer, and the granular layer. The molecular layer consists of stellate and basket cells, which provide inhibitory input to Purkinje cells. The Purkinje cell layer contains large, flask-shaped neurons with elaborate branching dendritic trees that relay motor impulses. These dendrites receive inputs from two main sources — parallel fibers, which originate from granule cells in the cerebellar cortex, and climbing fibers, which originate from neurons in the inferior olivary nucleus of the brainstem. Finally, the granular layer is densely packed with small, excitatory granule cells, which receive input from mossy fibers and give rise to parallel fibers that synapse onto Purkinje cells.

Beneath this layer of folia lies the white matter, known as the arbor vitae, due to its tree-like appearance. It consists of myelinated axons coursing through the cerebellar cortex, connecting different regions within the cerebellum and facilitating communication between them. Embedded within the white matter are four pairs of deep cerebellar nuclei — the fastigial, globose, emboliform, and dentate nuclei. These nuclei receive input from the cerebellar cortex and send output to various brainstem nuclei and the thalamus.

Connectivity with the Brainstem

The cerebellum connects with the brainstem through three pairs of bundled white matter tracts known as the cerebellar peduncles.

• Superior Peduncles: These form the connection between the cerebellum and the midbrain, diencephalon, and cerebrum. They function to integrate motor commands with sensory inputs.
• Middle Peduncles: The middle peduncles are the largest of the three and primarily transport motor information from the pons to the cerebellum. They are vital for planning and executing movements.
• Inferior Peduncles: These are essential for conveying sensory information from the medulla to the cerebellum. They aid in adjusting motor actions based on proprioceptive feedback.