NervousBrain region

Cerebellum

Cerebellum

Also known as: little brain

The "little brain" in the posterior cranial fossa (≈10% of brain mass but ≈80% of its neurons); it compares intended with actual movement to coordinate posture, balance, gait, eye movements and skilled motor learning, acting on the same side of the body.

Show on body map Brain — Lateral View Brain — Midsagittal Section

Overview

Mass≈130–150 g (≈10% of brain)

Neurons≈69 billion (≈80% of all brain neurons)

Cortical layers3 (molecular, Purkinje, granular)

Climbing fibre sourceContralateral inferior olivary nucleus

The cerebellum lies beneath the tentorium cerebelli, behind the pons and medulla, and forms the roof of the fourth ventricle. It consists of two hemispheres joined by a midline vermis. Its surface is folded into thin transverse folia, and the primary fissure and posterolateral fissure divide it into three lobes:
• the anterior lobe;
• the posterior lobe;
• the flocculonodular lobe.
In sagittal section its branching white matter is called the arbor vitae ("tree of life"). The four pairs of deep cerebellar nuclei lie embedded in this white matter. Three peduncles attach the cerebellum to the brainstem: superior to the midbrain, middle to the pons and inferior to the medulla.

The cortex is uniform throughout and has three layers:
• the outer molecular layer: Purkinje dendrites, parallel fibres, and stellate and basket cells;
• the Purkinje cell layer: a single row of large GABAergic Purkinje cells, the only output of the cortex;
• the inner granular layer: tightly packed granule cells, the most numerous neurons in the brain, plus Golgi cells.
Two kinds of input reach the cortex:
• Mossy fibres (from the pontine nuclei, spinal cord and vestibular nuclei) excite granule cells, whose parallel fibres each contact thousands of Purkinje cells.
• Climbing fibres, exclusively from the contralateral inferior olive, each wrap a single Purkinje cell powerfully. They signal errors that adjust parallel-fibre synapses through long-term depression, which is the basis of motor learning.
Purkinje cells inhibit the deep nuclei, which send the final output.

Functionally the cerebellum has three divisions:
• the vestibulocerebellum (flocculonodular lobe), for balance and eye movements;
• the spinocerebellum (vermis and paravermis), for posture, gait and the execution of limb movements;
• the cerebrocerebellum (lateral hemispheres), for planning, timing and learning skilled movements, and for some cognitive functions.
Cerebellar output crosses once to the contralateral thalamus and motor cortex, and the corticospinal tract crosses back. Each half of the cerebellum therefore controls the ipsilateral side of the body.

Location: Posterior cranial fossa, beneath the tentorium cerebelli and occipital lobes, posterior to the pons and medulla (separated by the fourth ventricle).

Parts & subdivisions

Vermis and hemispheres
Midline vermis (axial musculature, posture, gait) and lateral hemispheres (limb coordination, planning). The intermediate paravermal zone controls ongoing limb movement.
Lobes
Anterior lobe and posterior lobe (separated by the primary fissure); flocculonodular lobe (separated by the posterolateral fissure).
Cerebellar cortex
Molecular, Purkinje-cell and granular layers; Purkinje cells are the sole (inhibitory, GABA) output.
Deep cerebellar nuclei
From lateral to medial: dentate (cerebrocerebellum → thalamus VL → motor cortex), emboliform and globose (= interposed; spinocerebellum → red nucleus), fastigial (vermis/vestibulocerebellum → vestibular nuclei and reticular formation).
Superior cerebellar peduncle
Mainly efferent (dentate and interposed nuclei → contralateral red nucleus and thalamus); also the ventral spinocerebellar tract (afferent).
Middle cerebellar peduncle
Largest; purely afferent from the contralateral pontine nuclei (corticopontocerebellar pathway).
Inferior cerebellar peduncle
Mostly afferent: dorsal spinocerebellar and cuneocerebellar tracts, olivocerebellar climbing fibres, vestibular fibres; plus efferents to the vestibular nuclei.
Arbor vitae
Tree-like white matter of afferent and efferent fibres seen in sagittal section.

Anatomy

Arterial supply: Superior cerebellar artery (from the top of the basilar artery: superior surface and dentate nucleus), anterior inferior cerebellar artery (from the basilar artery: anterior inferior surface, flocculus), posterior inferior cerebellar artery (from the vertebral artery: posterior inferior surface, inferior vermis).
Histology: Three-layered cortex; Purkinje cells with flat, fan-shaped dendritic trees in the plane perpendicular to the folium; granule-cell parallel fibres run along the folium, crossing the Purkinje dendrites like wires on telegraph poles.
Development: From the rhombic lips of the metencephalon (alar plate). Granule-cell precursors migrate to form a transient external granular layer that persists for about the first year after birth. This is the cell of origin of the commonest type of medulloblastoma.

Function

Coordination of the timing, force and range of voluntary movements
Maintenance of balance, posture and gait
Control of eye movements (VOR adaptation, smooth pursuit, saccade accuracy)
Motor learning and adaptation (error-driven plasticity)
Muscle tone
Contributions to cognition, language fluency and timing

Clinical significance

Cerebellar syndrome

Signs are ipsilateral to the lesion:
• dysmetria (past-pointing on finger–nose testing) and intention tremor;
• dysdiadochokinesia;
• hypotonia and pendular reflexes;
• nystagmus, maximal when looking towards the lesion side;
• scanning dysarthria;
• a wide-based ataxic gait.
Vermis lesions (e.g. alcohol-related anterior vermis degeneration) cause mainly truncal and gait ataxia with relatively normal limbs. The Romberg test is typically negative: the patient is unsteady even with eyes open.

Posterior-fossa tumours in children

Medulloblastoma (vermis, highly malignant, can seed through the CSF), pilocytic astrocytoma (cerebellar hemisphere, cystic with a mural nodule, excellent prognosis) and ependymoma (fourth ventricle) obstruct CSF flow. They present with morning headache, vomiting, ataxia and hydrocephalus.

Cerebellar haemorrhage or infarct

This is a neurosurgical emergency. Swelling in the tight posterior fossa can compress the brainstem and fourth ventricle, causing hydrocephalus and tonsillar herniation, and suboccipital decompression may be lifesaving.

Mnemonics

Deep nuclei (lateral → medial)

"Don't Eat Greasy Food" — Dentate, Emboliform, Globose, Fastigial.

Cerebellar signs

"DANISH" — Dysdiadochokinesia, Ataxia, Nystagmus, Intention tremor, Slurred/scanning speech, Hypotonia.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.