The cerebellum consists of the cortex and the medulla. The cerebellar, cortex consists of three layers-outer molecular, intermediate Purkinje and inner granular. The outer molecular layer contains two types of neurons-the stellate and basket cells. It also contains large number of parallel fibers, which are the axons of the granule cells.
The intermediate Purkinje layer is composed of a single layer of Purkinje cells. The Purkinje cells are stimulated by the parallel fibers, and their axons stimulate the deep cerebellar nuclei.
The inner granular layer contains the cell bodies of granule cells and Golgi cells. Out of five types of neurons in the cerebellar cortex, only the granule cell is excitatory, it releases the excitatory neurotransmitter glutamate. All the other cells are inhibitory and release the inhibitory neurotransmitter GAB A.
The cerebellar input consists of two types of sensory fibers, the climbing fibers and mossy fibers. The climbing fibers are derived from the cells of the inferior olivary nucleus.
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All other afferents reach the cerebellum as the mossy fibers. Thus a climbing fiber directly excites a single Purkinje cell, whereas a mossy fiber through granule cells and parallel fibers fires thousands of Purkinje cells. Both the climbing and mossy fibers of sensory input exert excitatory influence.
The deep cerebellar nuclei are the fastigial, globose, emboli form and dentate nuclei. These nuclei receive axon terminals of Purkinje cells from the cerebellar cortex and collaterals from the climbing and mossy fibers. The axons of the deep cerebellar nuclei are projected as the final efferent pathways to the thalamus, red nucleus, brain stem reticular nuclei, inferior olivary and vestibular nuclei.
The fastigial nuclei receives projections from the vermal cortex and their efferents are connected with the vestibular and medullary reticular nuclei. The globose and emboli form nuclei a collectively called the nucleus interpositus. Both of them receive projections from the paravermal cortex.
Their efferents establish connection with the contra-lateral red nucleus, brain stem reticular nuclei and inferior olivary nucleus. Each dentate nucleus receives projections from the hemispheric or lateral cortex. The axons of dentate nucleus form dentatorubrothalamic fibers.