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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 3, 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.

Structure
Granule cells in different brain regions are both functionally and anatomically diverse: the main 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 have a structure typical of a neuron consisting of dendrites, a soma 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 micron.

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 synchronization and to allow signalling functions necessary for granule cell development to occur.

Cerebellum granule cell
Main article: Cerebellum granule cell

The granule cells are found in the granule cell layer of the cerebellar cortex and 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 granule cell
The principal cell type of the dentate gyrus is the granule cell. The dentate 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 just at the hippocampal fissure or at the ventricular surface. The granule cells are tightly packed in the molecular 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 inhibitory 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 enterss 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.

Neuronal network
Granule cells receive excitatory input from 3 or 4 mossy fibers originating from pontine nuclei. Mossy fibres make an excitatory connection onto granule cells which cause the granule cell 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 synchronisation which results in excitatory signals. In the cerebella cortex there are a variety of inhibitory neurons (interneurons). The only excitatory neurons present in the cerebella 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.

Mossy fiber input
The granule cells also give rise to distinctive unmyelinated axons which Santiago Ramón y Cajal called mossy fibers Mossy fibers and golgi cells both make synaptic connections with granule cells. Together these cells form the glomeruli.

Granule cells are subject to feed-forward inhibition: Granule cells are exciting Purkinje cells but they are also causing the excitation of cells (GABA interneurons) which will inhibit Purkinje cells.

Granule cells are also subject to feedback inhibition: Golgi cells receive excitatory stimulus from granule cells and in turn the golgi cells send back inhibitory signals to the granule cell.

Mossy fiber input codes are conserved during synaptic transmission between granule cells, suggesting that innervation is specfic to the input that is received. Granule cells do not just relay signals from mossy fibers, rather they perform various, intricate transformations which are required in the spatiotemporal domain.

Each granule cell is receiving an input from two different mossy fibre inputs. The input is thus coming from two different places as opposed to the granule cell receiving multiple inputs from the same source.

The differences in mossy fibers that are sending signals to the granule cells directly effects the type of information that granule cells translate to Purkinje cells. The reliability of this translation will depend on the reliability of synaptic activity in granule cells and on the nature of the stimulus being received. The signal a granule cell receives from a Mossy fiber depends on the function of the mossy fiber itself. Therefore granule cells are able to integrate information from the different mossy fibers and generate new patterns of activity.

Climbing fiber input
Different patterns of mossy fibre input will produce unique patterns of activity in granule cells that can be modified by a teaching signal conveyed by the climbing fibre input. David Marr and James Albus suggested that the cerebellum operates as an adaptive filter, altering motor behaviour based on the nature of the sensory input.

Since multiple (~200,000) granule cells synapse onto a single Purkinje cell, the affects of each parallel fiber can be altered in response to a “teacher signal” from the climbing fibre input.

Specific functions of different granule cells
David Marr suggested that the granule cells encode combinations of mossy fiber inputs. In order for the granule cell to respond, it needs to receive active inputs from multiple mossy fibers. The combination of multiple inputs results in the cerebellum being able to make more precise distinctions between input patterns than a single mossy fiber would allow. . The cerebellar granule cells also play a role in orchestrating the tonic conductances which control sleep in conjunction with the ambient levels of GABA which are found in the brain.
 * Cerebellum granule cells

Loss of dentate gyrus neurons from the hippocampus results in spatial memory deficits. Therefore dentate granule cells are thought to function in the formation of spatial memories. Young and old dentate granule cells have distinct roles in memory function. Adult-born granule cells function in pattern separation whereas old granule cells contribute to rapid pattern completion.
 * Dentate granule cells

Pyramidal cells from the primary auditory cortex project directly on to the cochlear nucleus. This is important in the acoustic startle reflex, in which the pyramidal cells modulate the secondary orientation reflex and the granule cell input is responsible for appropriate orientation. This is because the signals received by the granule cells contain information about the head position. Granule cells in the dorsal cochlear nucleus play a role in the perception and response to sounds in our environment.
 * Dorsal cochlear granule cells

Inhibition generated by granule cells, the most common GABAergic cell type in the olfactory bulb, plays a critical role in shaping the output of the olfactory bulb. There are two types of excitatory inputs received by GABAergic granule cells; those activated by an AMPA receptor and those activated by a NMDA receptor. This allows the granule cells to regulate the processing of the sensory input in the olfactory bulb .The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense of smell.
 * Olfactory bulb granule cells

Critical factors for function
Calcium dynamics are essential for several functions of granule cells such as changing membrane potential, synaptic plasticity, apoptosis, and regulation of gene transcription. The nature of the calcium signals that control the presynaptic and postsynaptic function of the olfactory bulb granule cells spines is mostly unknown.
 * Calcium

Granule neurons have high levels of the neuronal isoform of Nitric oxide synthase. This enzyme is dependent on the presence of calcium and is responsible for the production of Nitric Oxide (NO). This neurotransmitter is a negative regulator of granule cell precursor proliferation which promotes the differentiation of different granule cells. NO regulates interactions between granule cells and glia and is essential for protecting the granule cells from damage. NO is also responsible for neuroplasticity and motor learning.
 * Nitric oxide

Role in Disease
TrkB is responsible for the maintenance of normal synaptic connectivity of the dentate granule cells. TrkB also regulates the specific morphology (biology) of the granule cells and is thus said to be important in regulating neuronal development, neuronal plasticity, learning, and the development of epilepsy. The TrkB regulation of granule cells is important in preventing memory defecits and limbic epilepsy. This is due to the fact that dentate granule cells play a critical role in the function of the entorhinal-hippocampal circuitry in health and disease. Dentate granule cells are situated to regulate the flow of information into the hippocampus, a structure required for normal learning and memory.
 * Altered morphology of dentate granule cells

Both epilepsy and depression show a disrupted production of adult-born hippocampal granule cells. Epilepsy is associated with increased production - but aberrant integration - of new cells early in the disease and decreased production late in the disease. Aberrant integration of adult-generated cells during the development of epilepsy may impair the ability of the dentate gyrus to prevent excess excitatory activity from reaching hippocampal pyramidal cells, thereby promoting seizures. Long-lasting epileptic seizure stimulate dentate granule cell neurogenesis. These newly born dentate granule cells may result in aberrant connections that result in the hippocampal network plasticity associated with epileptogenesis.
 * Decreased granule cell neurogenesis

Patients suffering from Alzheimer's have shorter granule cell dendrites. Furthermore, the dendrites were less branched and had fewer spines than those in patients not suffering with Alzheimer's. However, granule cell dendrites are not an essential component of senile plaques and these plaques have no direct effect on granule cells in the dentate gyrus. The specific neurofibrillary changes of dentate granule cells occur in patients suffering from Alzheimer's, Lewy body variant and progressive supranuclear palsy.
 * Shorter granule cell dendrites