CEREBELLUM

Cerebellum is a part of the hindbrain. Its main function is to coordinate motility and muscle tension. A surface is covered with a system of fissurae and foliae. The cortex has a trilaminar structure. The superficial layer, just under the meninges, is called the molecular layer containing some nervous cells.  The second layer, ganglionic layer contains the largest cerebellar neurons, the Purkinje cells. Their perikarya are arranged in one discontinuous layer between the light molecular and dark granular layers (the latter contains densely packed granular neurons). Histologically these layers can be easily distinguished (Fig. 1). Below the cortex is the pale medulla containing nerve fibres. The medulla occurs in a central area of gyri that gives rise to a well apparent pattern called „arbor vitae“ (tree of life) that allows an easy identification of a slide.

Fig. 1 A. 3D recostruction of the  cerebellum. A typical morphology of the cerebellum (arbor vitae) comes from heterogeneity of its surface that reflects ramification of the medulla that enters gyri centrum; a surface is covered by the cortex that comprises three layers. An outer surface is covered by meninges. Krstic, R. V. Human microscopic anatomy; Microphotograph: Jaroslav Mokrý		Fig. 1 B. Histological structure of the cerebellar cortex. Semithin section shows all three layers: I. stratum moleculare, II. stratum gangliosum, III. str. granulosum; HE.   Microphotograph: Jaroslav Mokrý

Purkinje cells that are very specific to the cerebellum are classified as multipolar neurons as they have two main dendrites (that multiply as entering the molecular layer) and one axon running from the cell basis to central cerebellar area.  Purkinje cells are flattened, densely arranged and oriented perpendicularly to a longitudinal axis (Fig. 2 A). Dendrites are long and richly ramified; their surface is enlarged with a large number of dendritic spines (Fig. 2 B) – approx. 60,000 are on a surface of a single cell. Another characteristic structure of the molecular layer is the so called parallel fibre. It arises from a division of the axon of the small granular neuron in a shape of letter „T“ – for that reason it is also called as the T fibre. Parallel fibres run along the gyrus axis; one fibre contacts approx. 350 Purkinje cells and makes synapses with other elements of the molecular layer. Approx. 200,000 T fibres pass through arborisation of a single Purkinje cells. Presynaptic elements of T fibres make en passant axodendritic synapses with dendritic spines of the Purkinje cells; one neuron can bear up to 200,000 synapses. 

Fig. 2 A.Relationship of parallel fibre and dendrites of Purkinje cells. Small granular cells with a body in the granular layer sends a T fibre that runs longitudinally with a gyrus axis and contacts dendrites of Purkinje cells and other elements in the molecular layer. Krstic, R. V. Human microscopic anatomy.		Fig. 2 B. Ultrastructure of a dendrite of Purkinje cell. Numerous dendrtitic spikes can be visualized with  impregnation (upper part) or in TEM (lower part); en passant axodendritic synapses of parallel fibres contact the spikes. Dendrite is shown in light blue colour. Krstic, R. V. Human microscopic anatomy.