Cerebellum

The cerebellum, Latin for "little brain," sits in the back of the skull and fine-tunes movement. Though it makes up only about 10% of the brain's volume, it contains roughly 80% of the brain's neurons, reflecting its dense, highly organized circuitry.

Location and Gross Structure

The cerebellum lies in the posterior cranial fossa, tucked behind the fourth ventricle, the pons, and the medulla oblongata, beneath the occipital lobes of the cerebrum. It consists of two hemispheres joined in the middle by a narrow worm-like structure called the vermis. Its surface is folded into fine parallel ridges (folia). The cerebellum divides into three lobes—anterior, posterior, and flocculonodular—separated by distinct fissures, and connects to the brainstem through three pairs of cerebellar peduncles carrying information in and out.

Key Functions

The cerebellum does not initiate movement; instead, it coordinates and refines it. It compares the intended movement (commanded by the cerebrum) with sensory feedback about what the body is actually doing, then smooths and corrects the action in real time. The vermis governs trunk posture and balance, the intermediate zones adjust limb movements, and the lateral hemispheres help plan and time complex, sequential motions. The cerebellum also maintains muscle tone, coordinates eye movements, and supports motor learning, such as acquiring a new skill. Importantly, each cerebellar hemisphere influences the same (ipsilateral) side of the body.

Clinical Relevance

Cerebellar damage—from stroke, tumor, trauma, alcohol toxicity, or degenerative disease—produces a cluster of signs collectively called ataxia: unsteady, wide-based gait, clumsy limb movements, and intention tremor that worsens as a target is approached. Other findings include hypotonia (reduced muscle tone), nystagmus (rhythmic eye oscillation), slurred (scanning) speech, and dysdiadochokinesia, the inability to perform rapid alternating movements. Because effects are ipsilateral, the side of the body affected points to the side of cerebellar injury.

This information is educational and not a substitute for medical advice.