Granule cells are the smallest cells found in the brain and are an extremely small type of neuron. Granule cells are found within the granular layer of the cerebellum (which is also known as layer II and IV layer), the inner most layer of cerebellar cortex with the middle layer being the Purkinje cell layer and the outermost being the Molecular layer), the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, and in the olfactory bulb.
Cerebellar granule cells account for nearly half of the neurons in the central nervous system. Granule cells receive excitatory input from mossy fibers originating from pontine nuclei. Cerebellar granule cells send parallel fibers up through the Purkinje layer into the molecular layer where they branch out and spread through Purkinje cell dendritic arbors. These parallel fibers form thousands of excitatory Granule-cell-Purkinje-cell synapses onto the intermediate and distal dendrites of Purkinje cells using glutamate as a neurotransmitter.
Layer 4 granule cells of the cerebral cortex receive driving inputs from thalamus and convey driving inputs largely to supragranular layers 2-3, but also to infragranular layers of the cerebral cortex.
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