Describe the functional divisions and connections of the cerebellum. Enumerate the signs of cerebellar lesion and explain why they are ipsilateral.

Anatomical and Functional Divisions

Deep Nuclei

• From lateral to medial — Dentate, Emboliform, Globose, Fastigial ("Don't Eat Greasy Food")
• Dentate is the largest and connects with the neocerebellum
• Fastigial connects with the vestibular system
• The deep nuclei provide the only output of the cerebellum — and it is excitatory, whereas the Purkinje cell output onto them is inhibitory (GABA)

Peduncles

Cortical Layers and Circuitry

• Three layers — molecular, Purkinje, granular
• Purkinje cell is the only output neurone of the cerebellar cortex; it is inhibitory (GABA)
• Two afferent inputs — climbing fibres (from the inferior olive, one per Purkinje cell, very powerful) and mossy fibres (all other afferents, via granule cells and parallel fibres)

Functions

• Maintenance of equilibrium and posture
• Regulation of muscle tone — facilitatory
• Coordination of voluntary movement — smoothness, accuracy, correct timing and sequence
• Acts as a comparator — compares the intended movement (from cortex) with the actual movement (from proprioceptors) and corrects the error
• Motor learning — acquisition of skilled movements such as cycling or writing
• Planning of movement, along with the basal ganglia

Signs of Cerebellar Lesion

• All signs are ipsilateral — because the cerebellar output crosses twice (dentatorubrothalamic decussation, then the corticospinal decussation)
• Ataxia — drunken, reeling, wide-based gait
• Intention tremor — appears and worsens on approaching a target; absent at rest
• Dysmetria (past pointing) — misjudging distance
• Dysdiadochokinesia — inability to perform rapid alternating movements
• Nystagmus — usually horizontal, worse looking to the side of the lesion
• Hypotonia and a pendular knee jerk
• Scanning (staccato) speech — slurred and explosive
• Rebound phenomenon — failure to check a sudden release
• Asynergia and decomposition of movement

Cerebellum VS Basal Ganglia and Sensory Ataxia

Cerebellum VS Basal Ganglia

Applied Aspects

• Cerebellum has no direct motor output to the cord — therefore a lesion causes incoordination but never paralysis
• Alcohol acutely depresses the cerebellum → the ataxia and slurred speech of intoxication; chronic use damages the anterior vermis
• Friedreich ataxia — hereditary; spinocerebellar tracts and posterior columns
• Medulloblastoma — commonest posterior fossa tumour of children; arises in the vermis → truncal ataxia