Cytoarchitecture of the Cerebellum

Alfred Cuschieri

Department of Anatomy, University of Malta

The cerebellum consists of foliae (folds) composed of cortex and white matter. The photomicrograph below is a section in which the grey matter stains blue, and the white matter is pale staining. It shows the layers forming the cerebellar cortex

The following micrograph of the cerebellum is stained for myelin. The white matter is darkly stained. The granular layer appears as a fine meshwork of fibers. The molecular layer is pale staining

The following photomicrograph is a silver-stained section of cerebellum . It shows prominently the Purkinje cells and the neuropil in the molecular and cellular layers.

The cerebellar cortex consists of three layers on a core of white matter:

· Molecular layer

– Consists mainly of neuropil and is the site of synapses

– Contains scanty neurons consisting of stellate and basket cells

· Purkinje cell (Piriform) layer

– Single layer of neurons

– Consists of large (25 micrometer) pear-shaped neurons

· Granular layer

- Very small(7 micrometer) granular neurons

- Very numerous – 3 to 7 million neurons per cubic mm

· White matter – forms the core of the foliae

The Purkinje cells are central neurons (everything else converges on them)

They consist of:

• A large dendritic tree in the molecular layer, which is elaborately branched and fan-shaped (branches are all in one plane) and has dendritic spines at the sites of synapses

• A large cell body

•

An axon which forms the efferent pathway from the cerebellum , and sends collaterals in the granular layer.

• GABA is the main neurotransmitter

The Purkinje cells have a definite orientation in the foliae.

The dendritic trees of the Purkinje cells are fan-shaped in the transverse plane of the foliae.

The Purkinje cells are arranged in parallel arrays (regimentation) along the foliae

The Granule cells

• Very numerous: 3-7 million / mm³

• Very small (7mm), closely packed neurons

• Heterochromatic nuclei, scanty cytoplasm

• Small dendritic tree in granule layer

• An unmyelinated axon

– Directed to molecular layer (centrifugal)

– Splits in T-shape manner to form parallel fiber

– Parallel fibers run longitudinally along folia

– Cross dendrites of many Purkinje cells

• Have glutamate as neurotransmitter

Note the phenomena of convergence and divergence in the cerebellum.

One granule cell diverges to contact many Purkinje cells.

About 400 parallel fibers of granule cells converge on one Purkinje cell.

Golgi Type II Neurons Are Small Internuncial Neurons

•Situated mostly in outer part of granular layer

•

•Dendrites extend in 3 dimensions across blocks of Purkinje cells

•

•Short axon with numerous arborizations in granular layer

•GABA neurotransmitter

Afferents to the Cerebellum Have Two Types of Endings

1. Climbing fibers

2. Mossy fibers

They have important functional implications

Climbing fibers

•Climb through granular layer

•End in a tuft of terminal branches in molecular layer

•Correspond to dendrites of Purkinje cells (1:1 relationship)

•Are the terminal fibers of the olivo-cerebellar neurons

•Glutamate neurotransmitter

Mossy Fibers

• Comprise all other cerebellar afferents

• Branch and terminate in granular layer

• Have dilated endings (rosettes)

• Synapse with

– dendrites of granule cells and

– axons of Golgi type II neurons

A cerebellar glomerulus is a complex of synapses having a mossy fibre ending as its core, synapsing with axons of Golgi type II neurons and dendrites of granule cells

Stellate and Basket Cells Are Situated Transversely in the Molecular Layer

Stellate cells

–In superficial part, oriented transversely

–Synapse with the Purkinje cell dendrites

–Inhibitory

Basket cells

–In deep part, oriented transversely

–Axons form a basket around the Purkinje cell axon hillock

–Inhibitory

The cortico-olivo-cerebellar pathway conveys direct input from the sensori-motor and visual association areas to the cerebellum via the climbing fibers.

Mossy fibers convey information required for making adjustments

• Spinocerebellar and cuneo-cerebellar

– Feedback information from muscles and joints

• Vestibulo-cerebellar

– Feedback regarding body posture & movement

• Cortico-ponto-cerebellar

– From association areas

– Motor adjustments requiring judgment, discrimination, decisions and thought

Some activities require more co-ordination than others

Cerebellar Neurons Are Stimulatory or Inhibitory on Purkinje Cells

• Climbing fibers are strongly excitatory

• Mossy fibers stimulate granule cells

• Parallel fibers of granule cells stimulate several Purkinje cells simultaneously

• Basket cells strongly inhibit Purkinje cells

• Stellate cells inhibit Purkinje cell dendrites

• Golgi Type II cells inhibit directly the mossy fiber input

Output From the Cerebellum Is Via the Purkinje Cells and Central Nuclei.

Signs of Spino-cerebellar Disease

• Ataxia

• Tremor

• Decomposition of movement

• Dysdiadokinesis

• Dysarthria and scanning speech

• Nystagmus

• Hypotonia

• Romberg Sign