The cerebellum lies on the dorsal side of the brainstem in the posterior cranial fossa. Its inferior surface facing the brainstem forms the roof of the fourth ventricle. A double fold of the dura mater (tentorium cerebelli) is inserted between the cerebellum and the occipital lobes. The cerebellum is connected to the brainstem by three pairs of fibre bundles - cerebellar peduncles: superior cerebellar peduncles (brachia conjunctiva), middle cerebellar peduncles (brachia pontis) and inferior cerebellar peduncles (restiform bodies). The roof of the fourth ventricle is completed by two plates – the superior medullary velum (white matter between the brachia conjunctiva) and inferior medullary velum (tela choroidea of the fourth ventricle between the corpora restiformia)
V – superior medullary velum, S superior cerebellar peduncle, M – middle cerebellar peduncle, I - inferior cerebellar peduncle
From external aspect the cerebellum consists from the central portion – vermis cerebelli and two lateral hemispheres (cerebellar hemispheres). Between them is the paravermal zone.
Hemispheres - pink, paravermal zone - blue, vermis- green.
The cerebellar cortex is intensively folded into folia and so most of the cortex is hidden in grooves between the folia. This is best seen on the sagittal section with the conspicuous white cerebellar „tree of life“ – arbor vitae . The total cortical area of cerebellum is as extensive as one half of the area of the much larger cerebral hemispheres.
Deep in the white matter (substantia medullaris) are embedde the cerebellar nuclei: the fastigial nucleus (close to midline, connected to vestibular nuclei), the emboliform and the globose nucleus (more laterally, connected with the spinal cord and brainstem) and the dentate nucleus (the most lateral, it has shape of a crumpled band of the grey matter and it is connected mostly to pontine nuclei and thalamus.
Dentate nucleus- blue arowheads
Developmental aspect of the cerebellar classification. We can distinguish 3 cerebellar lobes according to phylogenesis and afferent connections:
1) Archicerebellum - The phylogenetically oldest part of the cerebellum is a flocculonodular lobe, the smallest part of the cerebellum on the inferior surface of the cerebellum. It was developed by growing of vestibular aferents into the roof plate of the metencephalon, so it is called vestibulocerebellum.
2) Paleocerebellum – It is a younger part of the cerebellum – the anterior lobe (cranially from the primary fissure) and it forms approximately 1/3 of the cerebellum. It recieves most of aferents from the spinal cord, so it is called spinonocerebellum.
3) Neocerebellum – It is the youngest and in mammals the largest part os the cerebellum - the posterior lobe. It was developed by growing aferents from the pontine nuclei, so it is called pontocerebellum. In the pontine nuclei are relayed the cortico-ponto-cerebellar tracts, so the neocerebellum is also called corticocerebellum.
Cerebellar cortex follows the course of sulci and folia. Three layers of it are evident in the histological section.
On the surface is placed stratum moleculare (molecular layer). It contains few cells (stellate and basket cells) but many nerve fibres and synapses (especially those between granule cells axons and Purkynje cells dendrites).
In the middle is stratum gangliosum (gangliose layer) – a single layer of large pear-shaped neurons (Purkinje cells) with dendrites profusely branching into the molecular layer and axons directed towads cerebellar nuclei.
The deepest layer is stratum granulare (granular layer) with two types of neurons – small granular cells with axons running toward the molecular layer and medium size Golgi cells (inhibitory interneurons) with short axons terminating on the granular cells in the cerebellar glomerulus.
They are three different types of afferent nerve terminations in the cerebellum. Mossy fibres are the most abundant afferent fibres of the cerebellum (axons of cells of the spinal cord, reticular formation, pontine nuclei etc.) with terminations, on the granular cells in so called cerebellar glomerulus (empty area in the granular layer with a complex synaptic contact of granule cells, Golgi cells and afferent fibres). The climbing fibres are axons of cells of the inferior olivary nuclei running through the granular layer to the Purkynje cells dendrites. Multilaminar fibres come from the hypothalamus and brain stem „chemical“ nuclei and terminate in all layers of cerebellar cortex.
Cerebellar nuclei are the main output nuclei of the cerebellum. The Purkynje cells axons and collaterals of all cerebellar afferents are interpolated in them.
All vestibular, spinal cord and pontine afferents to the cerebellum terminate in the cerebellar cortex. Intrinstic circuits process and interpole the signals by axons of Purkynje cells to the cereballar nuclei. Only small part of the old cerebellar cortex projects directly to the brainstem (to the lateral vestibular nucleus of Deiters. Mossy and climbing fibres and granular cells are excitatory, other cells are inhibitory.
Microscopic aspect of the cerebellar cortex (HE)
molecular layer – green arrow, ganglionic layer (Purkinje cells) – blue arrows, granular layer – red arrow