Granule cell

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. 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 s and circuits in the cerebellum. (+) represent excitatory synapse, while (-) represent inhibitory synapses. 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. Mossy fiber input on cerebellar granule cells Granule cell dendrites also synapse with distinctive unmyelinated axons which Santiago Ramón y Cajal called mossy fibers Mossy fiber input codes are conserved during synaptic transmission between granule cells, suggesting that innervation is specific 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. 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. 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. ;Dentate 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 and of episodic memories. Immature and mature dentate granule cells have distinct roles in memory function. Adult-born granule cells are thought to be involved in pattern separation whereas old granule cells contribute to rapid pattern completion. ;Dorsal cochlear 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. ;Olfactory bulb 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. Critical factors for function ;Calcium Calcium dynamics are essential for several functions of granule cells such as changing membrane potential, synaptic plasticity, apoptosis, and regulation of gene transcription. ==Role in disease==

Altered morphology of dentate granule cells 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 deficits 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. Epilepsy is associated with increased production - but aberrant integration - of new cells early in the disease and decreased production late in the disease. Shorter granule cell dendrites Patients with Alzheimer's have shorter granule cell dendrites. Furthermore, the dendrites were less branched and had fewer spines than those in patients not 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 with Alzheimer's, Lewy body variant and progressive supranuclear palsy. == See also ==