Cerebellum: Anatomy, Functions, and Ataxia Diagnosis

Overview

The cerebellum ("little brain") sits in the posterior cranial fossa, dorsal to the pons and medulla. Despite containing more neurons than the rest of the brain combined, it does not initiate movement. Instead, it fine-tunes motor commands, coordinates balance, and contributes to motor learning and certain cognitive functions.

Gross Anatomy

Three Anatomical Lobes

Anterior lobe: Spinocerebellum; receives proprioceptive input from the body. Paleocerebellum.
Posterior lobe: Largest; cerebrocerebellum; involved in motor planning and coordination of skilled voluntary movements. Neocerebellum.
Flocculonodular lobe: Vestibulocerebellum; balance and eye movements. Archicerebellum.

Two Hemispheres and Vermis

The midline vermis controls truncal posture and gait. The lateral hemispheres coordinate ipsilateral limb movements.

Foliations and Arbor Vitae

The highly folded cerebellar cortex creates the "tree of life" (arbor vitae) appearance on sagittal section — branching white matter surrounded by cortex.

Three Functional Divisions

Vestibulocerebellum (flocculonodular lobe): Balance, eye movements (VOR)
Spinocerebellum (vermis + paravermis): Posture, gait, muscle tone
Cerebrocerebellum (lateral hemispheres): Motor planning, coordination of skilled movements, motor learning

Deep Cerebellar Nuclei (medial to lateral)

Fastigial: Vestibular nuclei — balance
Globose + Emboliform = Interposed: Red nucleus and thalamus — limb coordination
Dentate: Largest; thalamus and motor cortex — motor planning

Microscopic Anatomy: Three Cortical Layers

Molecular layer (outer): Sparse cells, parallel fibers
Purkinje cell layer (middle): Single row of giant Purkinje neurons with elaborate dendritic trees. Only output from cerebellar cortex (inhibitory, GABAergic to deep nuclei)
Granular layer (inner): Densely packed granule cells; mossy fibers enter here

Input Pathways

Mossy fibers: From spinocerebellar tracts, pontine nuclei, vestibular nuclei. Synapse on granule cells.
Climbing fibers: From inferior olive. Synapse directly on Purkinje cells (one climbing fiber per Purkinje cell — powerful teaching signal).

Clinical Signs of Cerebellar Lesions

The cerebellum coordinates ipsilateral movements, so signs appear on the same side as the lesion.

Vermis Lesion

Truncal ataxia, wide-based gait
Seen in chronic alcoholism (anterior vermis degeneration)

Cerebellar Hemisphere Lesion

Limb ataxia (incoordination)
Dysmetria: Past-pointing on finger-to-nose test
Intention tremor: Tremor that worsens approaching target
Dysdiadochokinesia: Inability to perform rapid alternating movements
Rebound phenomenon: Stewart-Holmes test positive
Dysarthria: Scanning, "drunk" speech
Nystagmus: Coarse, towards the side of lesion

Flocculonodular Lobe Lesion

Truncal ataxia, titubation (rhythmic head/trunk tremor)
Nystagmus, vertigo
Seen in medulloblastoma in children (compresses the vermis)

Cerebellar Ataxia Causes

Acute: Stroke (PICA territory), hemorrhage
Chronic alcohol use: Anterior vermis degeneration
Paraneoplastic: Anti-Yo antibodies (ovarian, breast cancer)
Genetic: Friedreich ataxia (frataxin gene, GAA repeat)
Toxic: Phenytoin, mercury
MS plaques: Cerebellar white matter

Key Takeaway

The cerebellum is the brain's quality-control center for movement. Pure cerebellar lesions cause incoordination without weakness — a key distinguishing feature from upper motor neuron lesions. Always test finger-to-nose, heel-to-shin, rapid alternating movements, and gait in any neurologic exam.