The name granule cell has been used for a number of different types of neurons whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex. Cerebellar granule cells account for the majority of neurons in the human brain. These granule cells receive excitatory input from mossy fibers originating from pontine nuclei. Cerebellar granule cells project up through the Purkinje layer into the molecular layer where they branch out into parallel fibers that 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 inputs from the thalamus and send projections to supragranular layers 2–3, but also to infragranular layers of the cerebral cortex.
Structure Granule cells in different brain regions are both functionally and anatomically diverse: the only thing they have in common is smallness. For instance, olfactory bulb granule cells are GABAergic and axonless, while granule cells in the dentate gyrus have glutamatergic projection axons. These two populations of granule cells are also the only major neuronal populations that undergo adult neurogenesis, while cerebellar and cortical granule cells do not. Granule cells (save for those of the olfactory bulb) have a structure typical of a neuron consisting of dendrites, a soma (cell body) and an axon. Dendrites: Each granule cell has 3 – 4 stubby dendrites which end in a claw. Each of the dendrites are only about 15 μm in length. Soma: Granule cells all have a small soma diameter of approximately 10 μm. Axon: Each granule cell sends a single axon onto the Purkinje cell dendritic tree. The axon has an extremely narrow diameter: ½ micrometre. Synapse: 100–300,000 granule cell axons synapse onto a single Purkinje cell. The existence of gap junctions between granule cells allows multiple neurons to be coupled to one another, allowing multiple cells to act in synchrony, and allows signalling functions necessary for granule cell development to occur.
Cerebellar granule cell
The granule cells, produced by the rhombic lip, are found in the granule cell layer of the cerebellar cortex. They are small and numerous. They are characterized by a very small soma and several short dendrites which terminate with claw-shaped endings. In the transmission electron microscope, these cells are characterized by a darkly stained nucleus surrounded by a thin rim of cytoplasm. The axon ascends into the molecular layer where it splits to form parallel fibers.
Dentate gyrus granule cell The principal cell type of the dentate gyrus is the granule cell. The dentate gyrus granule cell has an elliptical cell body with a width of approximately 10 μm and a height of 18μm. The granule cell has a characteristic cone-shaped tree of spiny apical dendrites. The dendrite branches project throughout the entire molecular layer, and the furthest tips of the dendritic tree end at the hippocampal fissure or at the ventricular surface. The granule cells are tightly packed in the granular cell layer of the dentate gyrus.
Dorsal cochlear nucleus granule cell The granule cells in the dorsal cochlear nucleus are small neurons with two or three short dendrites that give rise to a few branches with expansions at the terminals. The dendrites are short with claw-like endings that form glomeruli to receive mossy fibers, similar to cerebellar granule cells. Its axon projects to the molecular layer of the dorsal cochlear nucleus where it forms parallel fibers, also similar to cerebellar granule cells. The dorsal cochlear granule cells are small excitatory interneurons which are developmentally related and thus resemble the cerebellar granule cell.
Olfactory bulb granule cell The main intrinsic granule cell in the vertebrate olfactory bulb lacks an axon (as does the accessory neuron). Each cell gives rise to short central dendrites and a single long apical dendrite that expands into the granule cell layer and enters the mitral cell body layer. The dendrite branches terminate within the outer plexiform layer among the dendrites in the olfactory tract. In the mammalian olfactory bulb, granule cells can process both synaptic input and output due to the presence of large spines.
Function
Neural pathways and circuits of the cerebellum
Cerebellar granule cells receive excitatory input from 3 or 4 mossy fibers originating from pontine nuclei. Mossy fibers make an excitatory connection onto granule cells, which causes the granule cells to fire an action potential. The axon of a cerebellar granule cell splits to form a parallel fiber which innervates Purkinje cells. The vast majority of granule cell axonal synapses are found on the parallel fibers. The parallel fibers are sent 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 dendrites of Purkinje cells. This connection is excitatory as glutamate is released. The parallel fibers and ascending axon synapses from the same granule cell fire in synchrony which results in excitatory signals. In the cerebellar cortex there are a variety of inhibitory neurons (interneurons). The only excitatory neurons present in the cerebellar cortex are granule cells. Plasticity of the synapse between a parallel fiber and a Purkinje cell is believed to be important for motor learning. The function of cerebellar circuits is entirely dependent on processes carried out by the granular layer. Therefore, the function of granule cells determines the cerebellar function as a whole.
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