Cerebellar Cytoarchitecture

All afferent and efferent projections of the cerebellum traverse the ICP, MCP, or SCP Most afferent input enters the cerebellum in the ICP and MCP; most efferent outflow leaves in the SCP (Figure IV-6-2 and Table IV-6-2).

Internally, the cerebellum consists of an outer cortex and an inner medulla.

The three cell layers of the cortex are the molecular layer, the Purkinje layer, and the granule cell layer.

The molecular layer is the outer layer and is made up of basket and stellate cells as well as parallel fibers, which are the axons of the granule cells. The extensive dendritic tree of the Purkinje cell extends into the molecular layer.

The Purkinje layer is the middle and most important layer of the cerebellar cortex. AH of the inputs to the cerebellum are directed toward influencing the firing of Purkinje cells, and only axons of Purkinje cells leave the cerebellar cortex. A single axon exits from each Purkinje cell and projects to one of the deep cerebellar nuclei or to vestibular nuclei of the brain stem.

The granule cell layer is the innermost layer of cerebellar cortex and contains Golgi cells, granule cells, and glomeruli. Each glomerulus is surrounded by a glial capsule and contains a granule cell and axons of Golgi cells, which synapse with granule cells. The granule cell is the only excitatory neuron within the cerebellar cortex. All other neurons in the cerebellar cortex, including Purkinje, Golgi, basket, and stellate cells, are inhibitory.

The medulla contains the deep cerebellar nuclei.

Cortical surface

Cortical surface

To Deep Cerebellar Nuclei and Vestibular Nucleus

Figure IV-6-2. Cerebellar Cytoarchltecture

medical

From medial to lateral, the deep cerebellar nuclei in the medulla are the fastigial nucleus, interposed nuclei, and the dentate nucleus.

Table rV-6-3. Cerebellum: Cell Types

Name

Target (Axon Termination)

Transmitter

Function

Purkinje cell

Deep cerebellar nuclei

GABA

Inhibitory*

Granule cell

Purkinje cell

Glutamate

Excitatory

Stellate cell

Purkinje cell

GABA

Inhibitory

Basket cell

Purkinje cell

GABA

Inhibitory

Golgi cell

Granule cell

GABA

Inhibitory

"Purkinje cells are the only outflow from the cerebellar cortex.

"Purkinje cells are the only outflow from the cerebellar cortex.

Two kinds of excitatory input enter the cerebellum in the form of climbing fibers and mossy fibers. Both types influence the firing of deep cerebellar nuclei by axon collaterals.

Climbing fibers originate exclusively from the inferior olivary complex of nuclei on the contralateral side of the medulla. Climbing fibers provide a direct powerful monosynaptic excitatory input to Purkinje cells.

Mossy fibers represent the axons from all other sources of cerebellar input. Mossy fibers provide an indirect, more diffuse excitatory input to Purkinje cells.

All mossy fibers exert an excitatory effect on granule cells. Each granule cell sends its axon into the molecular layer, where it gives off collaterals at a 90-degree angle that run parallel to the cortical surface (i.e., parallel fibers). These granule cell axons stimulate the apical dendrites of the Purkinje cells. Golgi cells receive excitatory input from mossy fibers and from the parallel fibers of the granule cells. The Golgi cell in turn inhibits the granule cell, which activated it in the first place.

The basket and stellate cells, which also receive excitatory input from parallel fibers of granule cells, inhibit Purkinje cells.

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