Wk 6: Cerebellum

The cerebellum is the **largest part of the hindbrain**, originating from the **dorsal aspect of the brainstem** and overlying the **fourth ventricle**.

The cerebellum is connected to the brainstem by three pairs of fiber bundles: the **superior cerebellar peduncle** (joins with the midbrain), **middle cerebellar peduncle** (joins with the pons), and **inferior cerebellar peduncle** (joins with the medulla).

The cerebellum consists of two laterally located **hemispheres**, joined in the midline by the **vermis**. The superior surface lies beneath the **tentorium cerebelli**.

The **tentorium cerebelli** is a dura fold that extends over the **posterior cranial fossa**, separating the **occipital** and **temporal** cerebral hemispheres from the cerebellum and infratentorial brainstem.

The cerebellum is divided into three lobes: the **anterior lobe**, **posterior lobe**, and **flocculonodular lobe**.

The **primary fissure** separates the **anterior lobe** from the **posterior lobe** on the superior surface.

The **posterolateral fissure** demarcates small regions of the hemisphere (the **flocculus**) and the vermis (the **nodule**), together forming the **flocculonodular lobe** on the inferior surface.

The **horizontal fissure** (or horizontal sulcus) is the largest and deepest fissure, dividing the cerebellum into an upper and lower portion.

The cerebellum has an outer layer of **gray matter** (the cerebellar cortex) and an inner core of **white matter**, which bifurcates into irregular, branch-like projections called the **arbor vitae**.

The **arbor vitae** or "tree of life" refers to the white matter structure of the cerebellum that bifurcates into branch-like projections.

The cerebellar cortex is divided into three layers: the **molecular layer** (outer), **Purkinje cell layer** (intermediate), and **granular layer** (inner).

The cerebellum contains four bilaterally paired nuclei, which serve as the primary output of the cerebellum. These nuclei have important connections with the cerebellar cortex, brainstem, and thalamus.

**Mossy fibers** provide status reports on various systems of the body, while **climbing fibers** carry error messages. Both are critical for motor learning and enter the cerebellum through the cerebellar peduncles.

Afferent projections come from the **spinal cord** (spinocerebellar fibers), **vestibular nuclei** (vestibulocerebellar fibers), **pons** (pontocerebellar fibers), and **inferior olivary nucleus** (olivocerebellar fibers).

The cerebellum is functionally divided into three regions: **spinocerebellum**, **cerebrocerebellum**, and **vestibulocerebellum**, each with different motor and sensory roles.

The cerebellum is implicated in nonmotor cognitive-linguistic processes, such as **receptive** and **expressive language processing**, through its connections with the cerebral cortex, particularly the frontal lobe.

A **unilateral lesion** in the cerebellum causes symptoms on the same side of the body (**ipsilateral impairments**), including **incoordination of the arm and leg** and an **unsteady gait**.

A **midline lesion** of the cerebellum results in a loss of **postural control**.

**Bilateral dysfunction** of the cerebellum, caused by conditions such as **alcoholic intoxication**, **multiple sclerosis**, or **cerebellar ataxia**, can lead to slowness of speech (**dysarthria**), **incoordination of both arms**, and an **unsteady, wide-based gait**.

**Nystagmus** is an involuntary rhythmic motion of the eyes, often seen in patients with **multiple sclerosis** and associated with cerebellar lesions.

**Charcot's triad** consists of **nystagmus**, **scanning dysarthric speech**, and **intention tremor**, and is highly diagnostic of **multiple sclerosis**.