Cerebellum

Cerebellar hemispheres:

• Paired lateral masses, primarily involved in the coordination of ipsilateral limb movement (given the cerebellum's predominantly ipsilateral somatotopic organisation and connections, a critical point distinguishing cerebellar lesions from cerebral hemisphere lesions, which produce contralateral deficits, see Applied Anatomy).

Vermis:

• The midline, worm-shaped structure connecting the two hemispheres, primarily involved in the coordination of axial/trunk musculature and gait.

Lobes and fissures:

• The cerebellar surface is extensively folded into thin folia, separated by fissures, with two major fissures dividing the cerebellum into three lobes.

Anterior lobe:

• Separated from the posterior lobe by the primary fissure; functionally corresponds predominantly to the spinocerebellum.

Posterior lobe:

• The largest lobe, separated from the flocculonodular lobe by the posterolateral fissure; functionally corresponds predominantly to the cerebrocerebellum, with vermal portions contributing to the spinocerebellum.

Flocculonodular lobe:

• The smallest, most phylogenetically ancient lobe, comprising the paired flocculi and the midline nodulus; functionally corresponds to the vestibulocerebellum.

Vestibulocerebellum:

• Anatomically the flocculonodular lobe; receives direct vestibular input (from the vestibular nuclei/vestibular nerve), controlling balance, posture, and eye movement coordination (particularly the vestibulo-ocular reflex), its lesion classically produces truncal ataxia (difficulty maintaining trunk/postural balance) and nystagmus.

Spinocerebellum:

• Anatomically the vermis and adjacent paravermal (intermediate) cerebellar zones; receives proprioceptive input from the spinal cord (via the spinocerebellar tracts) and some visual/auditory input, controlling axial muscle tone/posture (vermis) and coordinating ongoing limb movement (intermediate zone) via comparison of intended vs actual movement.

Cerebrocerebellum (neocerebellum):

• Anatomically the lateral cerebellar hemispheres (the largest, most recently evolved functional division, particularly expanded in humans); receives input exclusively from the cerebral cortex (via the pontine nuclei), involved in the planning and fine coordination of skilled voluntary movement, and in motor learning, its lesion classically produces the limb ataxia/dysmetria/intention tremor pattern most classically associated with "cerebellar signs" in clinical teaching.

• Four paired deep cerebellar nuclei, embedded within the white matter, representing the cerebellar cortex's sole output pathway (all cerebellar cortical Purkinje cell output projects to these nuclei, which then project onward to other brain regions, the cerebellar cortex itself has no direct extracerebellar output).

Dentate nucleus:

• The largest and most lateral deep nucleus, receiving output from the lateral cerebellar hemispheres (cerebrocerebellum), projecting via the superior cerebellar peduncle to the contralateral red nucleus and thalamus (ventral lateral nucleus), the principal output pathway for skilled voluntary movement coordination.

Emboliform and globose nuclei (together, the "interposed nuclei"):

• Located medial to the dentate nucleus, receiving output from the intermediate (paravermal) cerebellar zone (part of the spinocerebellum), also projecting via the superior cerebellar peduncle.

Fastigial nucleus:

• The most medial deep nucleus, receiving output from the vermis (spinocerebellum) and the flocculonodular lobe (vestibulocerebellum), projecting predominantly via the inferior cerebellar peduncle to the vestibular nuclei and reticular formation, involved in postural/axial motor control.

Three paired fibre bundles connecting the cerebellum to the brainstem, each with a broadly characteristic (though not absolute) afferent/efferent composition.

Superior cerebellar peduncle:

• Predominantly efferent (cerebellar output), carries the dentate and interposed nuclei's projections to the contralateral red nucleus and thalamus; also carries a smaller afferent component (the anterior spinocerebellar tract).

Middle cerebellar peduncle:

• Purely afferent: the largest peduncle, carrying pontocerebellar fibres from the contralateral pontine nuclei (the cortico-ponto-cerebellar pathway).

Inferior cerebellar peduncle:

• Predominantly afferent: carries the posterior spinocerebellar tract, olivocerebellar fibres (from the inferior olivary nucleus), and vestibulocerebellar fibres; also carries a smaller efferent component (fastigial nucleus projections to the vestibular nuclei).

Simplified mnemonic: superior peduncle = predominantly OUT (efferent); middle peduncle = purely IN (afferent, the largest); inferior peduncle = predominantly IN (afferent) with some out.

• Cortico-ponto-cerebellar (via the middle cerebellar peduncle), carries cerebral cortex "intention" information, informing the cerebellum of planned voluntary movement.
• Spinocerebellar tracts (posterior via inferior peduncle, anterior via superior peduncle), carry proprioceptive/unconscious limb position information from the spinal cord.
• Vestibulocerebellar (via inferior peduncle), carries balance/head position information from the vestibular apparatus/nuclei.
• Olivocerebellar (via inferior peduncle), carries climbing fibre input from the inferior olivary nucleus, important for motor learning/error correction signalling.

• Dentate/interposed nuclei → superior cerebellar peduncle → contralateral red nucleus and thalamus (VL nucleus) → motor cortex: the principal pathway by which the cerebellum influences voluntary movement, via this indirect cortical route (the cerebellum, like the basal ganglia, has no direct projection to spinal motor neurons).
• Fastigial nucleus → inferior cerebellar peduncle (predominantly) → vestibular nuclei/reticular formation: influencing postural/axial control more directly via brainstem pathways rather than solely via the cortex.

Ataxia:

• A broad term for incoordination of voluntary movement, the hallmark clinical sign of cerebellar dysfunction, encompassing the more specific signs, importantly, cerebellar ataxia occurs despite preserved muscle strength, distinguishing it from weakness-related incoordination (a key clinical/anatomical distinction, since the cerebellum's role is coordination, not force generation).

Dysmetria:

• Impaired judgment of distance/range during movement (overshooting or undershooting a target), classically tested by the finger-to-nose test, reflects cerebrocerebellar/lateral hemisphere dysfunction.

Intention tremor:

• A tremor that worsens as a movement approaches its target (in contrast to the resting tremor of Parkinsonism, which improves with movement (a key distinguishing feature)) reflects cerebrocerebellar dysfunction, part of the same lateral hemisphere lesion pattern as dysmetria.

Nystagmus:

• Involuntary, rhythmic eye oscillation, classically associated with vestibulocerebellar (flocculonodular lobe) dysfunction, given this region's direct vestibular connections and role in coordinating eye movement/gaze stability.

Cerebellar hemisphere versus vermis lesions:

• Hemisphere (lateral) lesions: produce ipsilateral limb ataxia, dysmetria, and intention tremor (given the cerebellum's predominantly ipsilateral limb connections, via the double-crossing arrangement of cerebellar input/output pathways), affecting the limbs on the same side as the lesion.
• Vermis (midline) lesions: produce truncal/gait ataxia (a wide-based, unsteady gait) with relatively preserved limb coordination, reflecting the vermis's role in axial rather than appendicular (limb) motor control, this hemisphere-vs-vermis distinction is the key anatomical/clinical localisation principle for cerebellar lesions, directly paralleling (though distinct in mechanism from) the general principle that different cerebellar functional zones control different aspects of movement (axial vs appendicular).