The Cerebellum

Central visual pathways | Main Anatomy Index | Functional aspects of cerebellum

• It is a semidetached mass of neural tissue that covers most of the posterior surface of the brainstem.
• It is anchored there by 3 pairs of fibre bundles called the cerebellar peduncles.

• Sensory inputs of virtually every description find their way to the cortex of the cerebellum.
• This in turn projects (via a set of deep cerebellar nuclei) to various sites in the brainstem and thalamus.
• The cerebellum is extensively concerned with the processing of sensory information.

• Although it has few ways to influence motor neurons directly, it is considered part of the motor system.
• This is as cerebellar damage results in abnormalities of equilibrium, of muscle tone, postural control, and co-ordination of voluntary movements.

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• The exterior of the cerebellum has a banded appearance (due to extensive folding).
• This folding is for increasing the cerebellar surface area.
• If the cortex could be drawn out into a flat sheet, it would be over 1 metre long.

• Deep fissures indent the cerebellar surface.
• Smaller fissures indent the walls of these deep fissures.
• The result of this is that the entire cerebellar surface is made up of cortical ridges called folia.
• These are mostly oriented transversely.

• Beneath the cortex is a mass of white matter, the medullary centre of the cerebellum.
• This is composed of fibres going to or coming from the cerebellar cortex.

• The 1st fissure to appear during development is the posterolateral fissure.
• It separates the flocculonodular lobe from the corpus cerebelli.

• In humans, the corpus cerebelli is far larger than the flocculonodular lobe.
• The posterolateral fissure is so deep that the flocculus of each side is almost pinched off from the rest of the cerebellum.

• This is a prominent landmark in sagittal sections of the cerebellum.
• It subdivides the corpus cerebelli into anterior and posterior lobes.

• The cerebellum may also be subdivided into longitudinal zones, perpendicular to the fissures.
• The most medial zone, straddling the midline, is the vermis (L. worm).

• This is on either side of the vermis.
• Each hemisphere is subdivided into a medial longitudinal strip adjacent to the vermis, called the intermediate or paravermal zone.

• The demarcation into longitudinal zones is based on function and on patterns of connections.

• The tonsils are the hemispherical portions just across the posterolateral fissure from the flocculi.
• Their vermal continuation is the uvula.

• The cerebellum is attached to the brainstem by 3 peduncles on each side.

• These are buried in the medullary centre of each side of the cerebellum.

• The most lateral is the dentate nucleus.
• This looks strikingly like the inferior olivary nucleus.
• Most of the fibres in the SCP originate from the dentate nucleus and emerge from its medially facing mouth, or hilus.

• Medial to the dentate nucleus are the emboliform and globose nuclei.
• In most non-human cerebella, the equivalent cells form a single nuclear mass called the interposed nucleus (or nucleus interpositus).
• This term is still often used in humans.

• The most medial of the deep cerebellar nuclei is the fastigial nucleus.

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• This has a uniform and fairly simple three-layered structure.

• The most superficial layer is the molecular layer.
• It consists mainly of the axons and dendrites of various cerebellar neurons.

• Deep to the molecular layer is a single layer of large neurons called Purkinje cells.

• Finally is the granular layer.
• This is composed mainly of small granule cells arranged in a stratum many cells thick.

• These are the only neurons whose axons leave the cortex.
• Each Purkinje cell has an intricate, extensive dendritic tree that is flattened out in a plane perpendicular to the long axis of the folium in which it resides.

• Although these cells are the only route out of the cerebellar cortex, few of them leave the cerebellum itself.
• Rather, they project to the deep nuclei, which in turn give rise to the cerebellar output.

• Each granule cell sends its axon into the molecular layer.
• Here it bifurcates to form a fine, unmyelinated parallel fibre that extends for about 5 mm along the long axis of the folium.
• In its course, each parallel fibre passes through and synapses on the dendritic trees of a succession of Purkinje cells (as many as 500).

• Each of us is estimated to have 10¹⁰ or more granule cells.
• Each of our 15 million Purkinje cells receives synapses from perhaps l0⁵ of them.

• This is one of the two sets of afferent fibres of the cerebellar cortex.

• A single climbing fibre ends directly on each Purkinje cell.
• They wind around the proximal portions of its dendrites (like ivy climbing a trellis).

• All of these climbing fibres arise in the contralateral inferior olivary nucleus.

• All the rest of the afferents to the cerebellar cortex are mossy fibres.
• These end on the dendrites of granule cells, so this is an indirect route to the Purkinje cells.

• There is a certain amount of overlap of these functional divisions in terms of connections.
• For example, the spinocerebellum receives afferents from pontine nuclei, and parts of it receive vestibular afferents as well.

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• The cerebellar cortex receives some of its mossy fibres from the deep cerebellar nuclei.
• The remaining mossy fibres carry information from 3 principal extracerebellar sources:

1. The vestibular nerve and nuclei;
2. The spinal cord;
3. And the cerebral cortex (via pontine nuclei).

• The climbing fibre input arises in the inferior olivary nucleus.

Table is based on Nolte (1993) The Human Brain. Table 9, p. 348

• Some primary vestibular afferents enter the cerebellum through the ICP and end as mossy fibres in the nodulus and uvula.
• A larger number of secondary fibres, arising in the vestibular nuclei, follow the same course to the flocculonodular lobe and most of the vermis, bilaterally.

• A great deal of somatosensory information (from various mechanoreceptors of the skin, muscles, and joints) reaches the vermal and paravermal cortex.

• Some of it reaches the cerebellum directly via the spinocerebellar tracts and the cuneocerebellar tract.
• Some arrives indirectly by way of the reticular formation.

• Similar information from the head also reaches the cerebellum from the trigeminal system.
• The bulk of it arises in the rostral 2/3 of the spinal nucleus (interpolar and oral nuclei).

• The anterior spinocerebellar tract travels in the SCP.
• All the rest of the somatosensory input from both body and head traverses the ICP.

• These projection ends somatotopically in an interesting way.
• Each part of the body is mapped 3 times onto the cerebellar cortex.

• It is mapped once ipsilaterally in a pattern mostly contained in the anterior lobe.
• It is mapped again with some bilateral representation in the posterior lobe.
• In each of the 3 somatotopic maps, the head is nearest the primary fissure and the trunk is adjacent to the midline.

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• The majority of the fibres of the basis pedunculi (approx. 20 million of 21 million fibres) end in ipsilateral pontine nuclei.
• The pontine nuclei of one side contain about 12 million cells that project through the MCP to virtually all parts of the cerebellar cortex.

• Almost all of these fibres cross the midline in the basal pons.
• Thus, they end on the contralateral side of the cerebellum.
• A few fibres (particularly those to the vermis) end ipsilaterally.

• Of the cortical projections to the pontine nuclei, contributions from the vicinity of the central sulcus predominate:

1. The motor and premotor cortex;
2. The somatosensory cortex;
3. And adjacent parts of the parietal lobe.

• There are projections from other parts of the cortex, such as auditory, visual and association areas.

• The vermis and intermediate zone preferentially receive their cortical input from the motor cortex of the precentral gyrus.
• The pathway is somatotopically organised so that the pontocerebellar fibres end in the same pattern as do those carrying information from the spinal cord.

• The lateral parts of the cerebellar hemispheres receive most of their cortical input from premotor, somatosensory, and association areas of the cerebral cortex.

• All olivary efferents emerge medially, and enter the contralateral ICP, and blanket the entire contralateral cerebellar cortex with climbing fibres.

• The information these climbing fibres convey comes from diverse sources including:

1. The spinal cord;
2. The red nucleus;
3. The cerebral cortex;
4. And the cerebellum itself.

• Fibres from the ipsilateral red nucleus form the bulk of the central tegmental tract.
• They are the numerically most important olivary input.

• Spinal inputs are all crossed and reach the inferior olivary complex both:

1. Directly (via spinoolivary fibres);
2. And indirectly (through relays in the posterior column nuclei).

• A few fibres from the cerebral cortex of both sides, mostly from motor cortex, also reach the olive.

• Finally, there is a topographically highly organised projection from the contralateral dentate and interposed nuclei to the inferior olivary complex.

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• The output of the cerebellar cortex is entirely in the form of the axons of Purkinje cells.
• Some of these, arising in the flocculonodular lobe and in parts of the vermis of both anterior and posterior lobes as well, leave the cerebellum via the ICP.
• They end in the vestibular nuclei.

• This provides the only reasonably direct access the cerebellar cortex has to motor neurons of the spinal cord (via the vestibulospinal tracts).

• All other Purkinje axons end in the deep cerebellar nuclei.
• This is done in a medial-to-lateral fashion.

Vermis --> Fastigial nucleus

Paravermal zone --> Interposed nuclei

Lateral hemisphere --> dentate nucleus

• This output is directed primarily to the brainstem.
• It ends in the vestibular nuclei of both sides and in the reticular formation, mainly contralaterally.

• Fibres that end ipsilaterally go through the ICP.
• Those to contralateral targets cross the midline within the cerebellum, loop over the SCP as the uncinate fasciculus (or hook bundle).
• They then descend through the contralateral ICP.

• A few fibres also project to the contralateral VL/VA complex of the thalamus and to the contralateral cervical spinal cord.

• The major output from the cerebellum is from these two nuclei, and leaves the cerebellum as the bulk of the SCP.
• This peduncle joins the brainstem in the rostral pons.
• Some fibres turn caudally as the descending limb of the SCP and end in the reticular formation and the inferior olivary nucleus.

• Most of the fibres, however, continue rostrally, decussate in the midbrain, and reach the red nucleus.
• Here, many of the fibres from the interposed nucleus (and a minority of dentate fibres) terminate.

• The remaining fibres pass through or around the red nucleus.
• They join the thalamic fasciculus, and end in the VL/VA complex of the thalamus.

• The interposed nucleus preferentially influences the red nucleus.
• The dentate nucleus preferentially influences the thalamus.

• From the thalamic cells, dentate information is conveyed to motor and premotor cortex.
• Information from the interposed nucleus is conveyed to the limb areas of motor cortex.

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