Cerebellum
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Direct answer
The cerebellum is the largest part of the hindbrain, lying in the posterior cranial fossa below the tentorium cerebelli and behind the pons and medulla, connected to the brainstem by three pairs of peduncles. It is the organ of coordination: it compares intended movement with actual performance and corrects errors of rate, range, force and direction. Despite being asked about often, its signs are ipsilateral because its output pathways cross twice.
What you must remember
- Phylogenetic parts: archicerebellum (flocculonodular lobe, also called vestibulocerebellum) for equilibrium; paleocerebellum (anterior lobe and parts of the vermis, the spinocerebellum) for muscle tone and posture; neocerebellum (bulk of the posterior lobe, the pontocerebellum) for fine skilled movements.
- Fissures: the primary fissure separates anterior from posterior lobe; the posterolateral fissure separates the flocculonodular lobe; the vermis joins the two hemispheres in the midline, and the tonsils project inferiorly and herniate through the foramen magnum in raised intracranial pressure.
- Peduncles: inferior (restiform body) connects to the medulla and carries olivocerebellar, dorsal spinocerebellar and vestibulocerebellar fibres in; the middle (brachium pontis) connects to the pons and is purely afferent, carrying contralateral pontocerebellar fibres; the superior (brachium conjunctivum) connects to the midbrain and is mainly efferent, carrying cerebellothalamic fibres that decussate in the caudal midbrain.
- Deep nuclei, medial to lateral: fastigial (vestibular outflow), globose and emboliform, and the large dentate nucleus to the thalamus; all output of the cerebellar cortex reaches these nuclei.
- Cortex: molecular layer with stellate and basket cells, a single Purkinje cell layer, and a granular layer packed with granule cells — the most numerous neurons of the central nervous system; Purkinje cells are the only efferent neurons of the cortex and are inhibitory (GABA-ergic).
- Afferents: mossy fibres end on granule cells; climbing fibres from the inferior olivary complex wrap round Purkinje cell dendrites directly.
- Signs: ipsilateral hypotonia and ataxia, dysmetria, intention tremor, dysdiadochokinesia, nystagmus and scanning speech; a vermis lesion causes truncal ataxia, a flocculonodular lesion vertigo and nystagmus.
Common confusion
Because the lateral hemispheres work with the opposite cerebral cortex and the dentatorubrothalamic fibres decussate, students wrongly predict contralateral signs. Cerebellar lesions are ipsilateral. Also do not swap the peduncles: the middle peduncle is the thick, purely afferent one to the pons, whereas the superior peduncle is the main efferent route — a common MCQ trap. Finally, intention tremor is cerebellar; a resting, pill-rolling tremor belongs to parkinsonism.
Exam-focused takeaway
Theory papers ask for the connections of the cerebellum through the three peduncles, the functions of archi-, paleo- and neocerebellum, and differences between cerebellar and basal gangliar tremor. In the viva, define dysdiadochokinesia and demonstrate the finger-nose test; in clinics, link vermis lesions to medulloblastoma in children.
Frequently asked questions
Which is the purely afferent peduncle of the cerebellum?
The middle cerebellar peduncle (brachium pontis), carrying pontocerebellar fibres from the contralateral pontine nuclei; the superior peduncle is predominantly efferent.
Name the deep cerebellar nuclei from medial to lateral.
Fastigial, globose, emboliform and dentate nuclei; the dentate nucleus projects through the superior peduncle to the contralateral red nucleus and thalamus.
Why are cerebellar signs ipsilateral?
The major output pathways decussate twice — once in the superior cerebellar peduncle and again in the corticospinal pathway — so a hemispheric lesion affects the same side of the body.
What is dysdiadochokinesia?
Impairment of rapid alternating movements such as supination and pronation, a classic sign of cerebellar disease, accompanied by intention tremor and dysmetria.
Which part of the cerebellum is connected to the vestibular apparatus?
The flocculonodular lobe or archicerebellum, through the inferior peduncle; its lesions cause vertigo, nystagmus and truncal imbalance.