Stellate cells are any neuron in the central nervous system that have a star-like shape formed by dendritic processes radiating from the cell body. Many Stellate cells are located in the molecular layer of the cerebellum. Stellate cells are derived from dividing progenitors in the white matter of postnatal cerebellum. Dendritic trees can vary between neurons. There are two types of dendritic trees in the cerebral cortex, which include pyramidal cells, which are pyramid shaped and stellate cells which are star shaped. Dendrites can aid neuron classification. Dendrites with spines are classified as spiny. Stellate cells can be aspinous, while pyramidal cells are always spiny. Most common stellate cells are the inhibitory interneurons found within the upper half of the molecular layer in the cerebellum. Cerebellar stellate cells synapse onto the dendritic arbors of Purkinje cells and send inhibitory signals. Stellate neurons are sometimes found in other locations in the central nervous system. In the somatosensory barrel cortex of mice and rats, glutamatergic spiny stellate cells are organized in the barrels of layer 4.
They receive excitatory synaptic fibres from the thalamus and process feed forward excitation to 2/3 layer of V1 visual cortex to pyramidal cells. Cortical spiny stellate cells have a'regular' firing pattern. Stellate cells are chromophobes, cells that does not stain and thus appears pale under the microscope. Cerebellar Stellate Cells are GABAergic. Stellate and basket cells originate from the cerebellar ventricular zone along with Purkinje cells and Bergmann glia Due to their similarity and stellate cells are grouped together when examined during migration given they follow the same pathway. After mitosis, these cells start in the deep layer of the white matter and migrate up through the internal granular layer and purkinje cell layer until they reach the molecular layer. During their time in the molecular layer, they change orientation and positioning until they end up in the middle portion of this layer, facing the rostrocaudal direction. Once in this layer, the stellate cells are guided to their correct placement by Bergman glial cells.
Aspinous stellate cells are GABAergic cells. Apart from visual classification of the aspinous dendrites, they can be immunohistochemically labelled with glutamic acid decarboxylase because of their GABAergic activity, colocalize with neuropeptides. Stellate ganglion NIF Search - Stellate Cell via the Neuroscience Information Framework
Tyrosine hydroxylase or tyrosine 3-monooxygenase is the enzyme responsible for catalyzing the conversion of the amino acid L-tyrosine to L-3,4-dihydroxyphenylalanine. It does so using molecular oxygen, as well as tetrahydrobiopterin as cofactors. L-DOPA is a precursor for dopamine, which, in turn, is a precursor for the important neurotransmitters norepinephrine and epinephrine. Tyrosine hydroxylase catalyzes the rate limiting step in this synthesis of catecholamines. In humans, tyrosine hydroxylase is encoded by the TH gene, the enzyme is present in the central nervous system, peripheral sympathetic neurons and the adrenal medulla. Tyrosine hydroxylase, phenylalanine hydroxylase and tryptophan hydroxylase together make up the family of aromatic amino acid hydroxylases. Tyrosine hydroxylase catalyzes the reaction in which L-tyrosine is hydroxylated in the meta position to obtain L-3,4-dihydroxyphenylalanine; the enzyme is an oxygenase. One of the oxygen atoms in O2 is used to hydroxylate the tyrosine molecule to obtain L-DOPA and the other one is used to hydroxylate the cofactor.
Like the other aromatic amino acid hydroxylases, tyrosine hydroxylase use the cofactor tetrahydrobiopterin under normal conditions, although other similar molecules may work as a cofactor for tyrosine hydroxylase. The AAAHs converts the cofactor 5,6,7,8-tetrahydrobiopterin into tetrahydrobiopterin-4a-carbinolamine. Under physiological conditions, 4a-BH4 is dehydrated to quinonoid-dihydrobiopterin by the enzyme pterin-4a-carbinolamine dehydrase and a water molecule is released in this reaction; the NADH dependent enzyme dihydropteridine reductase converts q-BH2 back to BH4. Each of the four subunits in tyrosine hydroxylase is coordinated with an iron atom presented in the active site; the oxidation state of this iron atom is important for the catalytic turnover in the enzymatic reaction. If the iron is oxidized to Fe, the enzyme is inactivated; the product of the enzymatic reaction, L-DOPA, can be transformed to dopamine by the enzyme DOPA decarboxylase. Dopamine may be converted into norepinephrine by the enzyme dopamine β-hydroxylase, which can be further modified by the enzyme phenylethanol N-methyltransferase to obtain epinephrine.
Since L-DOPA is the precursor for the neurotransmitters dopamine and adrenaline, tyrosine hydroxylase is therefore found in the cytosol of all cells containing these catecholamines. This initial reaction catalyzed by tyrosine hydroxylase has been shown to be the rate limiting step in the production of catecholamines; the enzyme is specific, not accepting indole derivatives -, unusual as many other enzymes involved in the production of catecholamines do. Tryptophan is a poor substrate for tyrosine hydroxylase, however it can hydroxylate L-phenylalanine to form L-tyrosine and small amounts of 3-hydroxyphenylalanine; the enzyme can further catalyze L-tyrosine to form L-DOPA. Tyrosine hydroxylase may be involved in other reactions as well, such as oxidizing L-DOPA to form 5-S-cysteinyl-DOPA or other L-DOPA derivatives. Tyrosine hydroxylase is a tetramer of four identical subunits; each subunit consists of three domains. At the carboxyl terminal of the peptide chain there's a short alpha helix domain that allows tetramerization.
The central ~300 amino acids make up a catalytic core, in which all the residues necessary for catalysis are located, along with a non-covalently bound iron atom. The iron is held in place by two histidine residues and one glutamate residue, making it a non-heme, non-iron-sulfur iron-containing enzyme; the amino terminal ~150 amino acids make up a regulatory domain, thought to control access of substrates to the active site. In humans there are thought to be four different versions of this regulatory domain, thus four versions of the enzyme, depending on alternative splicing, though none of their structures have yet been properly determined, it has been suggested that this domain might be an intrinsically unstructured protein, which has no defined tertiary structure, but so far no evidence has been presented supporting this claim. It has however been shown that the domain has a low occurrence of secondary structures, which doesn't weaken suspicions of it having a disordered overall structure.
As for the tetramerization and catalytic domains their structure was found with rat tyrosine hydroxylase using X-ray crystallography. This has shown how its structure is similar to that of phenylalanine hydroxylase and tryptophan hydroxylase. Tyrosine hydroxylase activity is increased in the short term by phosphorylation; the regulatory domain of tyrosine hydroxylase contains multiple serine residues, including Ser8, Ser19, Ser31 and Ser40, that are phosphorylated by a variety of protein kinases. Ser40 is phosphorylated by the cAMP-dependent protein kinase. Ser19 is phosphorylated by the calcium-calmodulin-dependent protein kinase. MAPKAPK2 has a preference for Ser40, but phosphorylates Ser19 about half the rate of Ser40. Ser31 is phosphorylated by ERK1 and ERK2, increases the enzyme activity to a lesser extent than for Ser40 phosphorylation; the phosphorylation at Ser19 and Ser8 has no direct effect on tyrosine hydroxylase activity. But phosphorylation at Ser19 increases the rate of phosphorylation at Ser40, leading to an increase in enzyme activity.
Phosphorylation at Ser19 causes a two-fold increase of activity, through a mechanism that requires the 14-3-
A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, whose axon projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs muscles and glands. There are two types of motor neuron -- lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and directly onto lower motor neurons; the axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, gamma motor neurons. A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch; as a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch.
The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, tension rises smoothly reaching a plateau. Motor neurons begin to develop early in embryonic development, motor function continues to develop well into childhood. In the neural tube cells are specified to either ventral-dorsal axis; the axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis. This homeodomain is known as the motor neural progenitor domain. Transcription factors here include Pax6, OLIG2, Nkx-6.1, Nkx-6.2, which are regulated by sonic hedgehog. The OLIG2 gene being the most important due to its role in promoting Ngn2 expression, a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development.
Further specification of motor neurons occurs when retinoic acid, fibroblast growth factor, TGFb, are integrated into the various Hox transcription factors. There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character. In the spinal column, Hox 4-11 sort motor neurons to one of the five motor columns. Upper motor neurons originate in the motor cortex located in the precentral gyrus; the cells that make up the primary motor cortex are Betz cells. The axons of these cells descend from the cortex to form the corticospinal tract. Corticomotorneurons are neurons in the primary cortex which project directly to motor neurons in the ventral horn of the spinal cord. Axons of corticomotorneurons terminate on the spinal motor neurons of multiple muscles as well as on spinal interneurons, they are unique to primates and it has been suggested that their function is the adaptive control of the distal extremities including the independent control of individual fingers.
Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas. Nerve tracts are bundles of axons as white matter. In the spinal cord these descending tracts carry impulses from different regions; these tracts serve as the place of origin for lower motor neurons. There are seven major descending motor tracts to be found in the spinal cord: Lateral corticospinal tract Rubrospinal tract Lateral reticulospinal tract Vestibulospinal tract Medial reticulospinal tract Tectospinal tract Anterior corticospinal tract Lower motor neurons are those that originate in the spinal cord and directly or indirectly innervate effector targets; the target of these neurons varies, but in the somatic nervous system the target will be some sort of muscle fiber. There are three primary categories lower motor neurons, which can be further divided in sub-categories. According to their targets, motor neurons are classified into three broad categories: Somatic motor neurons Special visceral motor neurons General visceral motor neurons Somatic motor neurons originate in the central nervous system, project their axons to skeletal muscles, which are involved in locomotion.
The three types of these neurons are the alpha efferent neurons, beta efferent neurons, gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system to the periphery. Alpha motor neurons innervate extrafusal muscle fibers, which are the main force-generating component of a muscle, their cell bodies are in the ventral horn of the spinal cord and they are sometimes called ventral horn cells. A single motor neuron may synapse with 150 muscle fibers on average; the motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories: Main Article: Motor Unit Slow motor units stimulate small muscle fibers, which contract slowly and provide small amounts of energy but are resistant to fatigue, so they are used to sustain muscular contraction, such as keeping the body upright, they gain their energy via oxidative hence require oxygen. They are called red fibers. Fast fatiguing motor units stimulate larger muscle groups, which apply large amounts of force but fatigue quickly.
They are used for tasks that require large brief bursts on energy, such as jumping or
Basket cells are inhibitory GABAergic interneurons of the brain, found throughout different regions of the cortex and cerebellum. Basket cells are multipolar GABAergic interneurons that function to make inhibitory synapses and control the overall potentials of target cells. In general, dendrites of basket cells are free branching, contain smooth spines, extend from 3 to 9 mm. Axons are branched, ranging in total from 20 to 50mm in total length; the branched axonal arborizations give rise to the name as they appear as baskets surrounding the soma of the target cell. Basket cells form axo-somatic synapses. By controlling the somas of other neurons, basket cells can directly control the action potential discharge rate of target cells. Basket cells can be found throughout the brain, in among other the cortex, amygdala, basal ganglia, the cerebellum. In the cortex, basket cells have sparsely branched axons giving off small pericellular, basket-shaped elaborations at several intervals along their length.
Basket cells make up 5-10% of total neurons in the cortex. There are three types of basket cells in the cortex, the small and nest type: The axon of a small basket cell arborizes in the vicinity of that same cell's dendritic range, this axon is short. In contrast, large basket cells innervate somata in different cortical columns due to a long axon; the nest basket cells are an intermediate form of the small and large cells, their axons are confined to the same cortical layer as their somata. Nest basket cells have "radiating axonal collaterals" between that of small basket cells, they are included as basket cells. Hippocampal basket cells target proximal dendrites of pyramidal neurons. Similar to their counterparts in the cortex, hippocampal basket cells are parvalbumin-expressing and fast-spiking. In the CA3 region of the hippocampus, basket cells can form recurrent inhibition loops with pyramidal cells. Projections from a pyramidal cell will innervate the basket cell, which in turn has a projection back onto the original pyramidal cells.
Since basket cells are inhibitory, this generates a closed loop that can help dampen excitatory responses. In the cerebellum, the multipolar basket cells have branching dendrites, which are dilated and knotty. Basket cells synapse on the cell bodies of Purkinje cells and make inhibitory synapses with Purkinje cells. Cerebellar basket cell axons fire inhibitory neurotransmitters such as GABA to Purkinje cell axons, inhibits the Purkinje cell. Purkinje cells send inhibitory messages to the deep cerebellar nuclei and are responsible for the sole output of motor coordination from the cerebellar cortex. With the work of the basket cell, Purkinje cells do not send the inhibitory response for motor coordination and motor movement occurs. Cell Centered Database - Cerebellar basket cell NIF Search - Basket Cell via the Neuroscience Information Framework
The cerebellum is a major feature of the hindbrain of all vertebrates. Although smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or larger. In humans, the cerebellum plays an important role in motor control, it may be involved in some cognitive functions such as attention and language as well as in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The human cerebellum does not initiate movement, but contributes to coordination and accurate timing: it receives input from sensory systems of the spinal cord and from other parts of the brain, integrates these inputs to fine-tune motor activity. Cerebellar damage produces disorders in fine movement, equilibrium and motor learning in humans. Anatomically, the human cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres, its cortical surface is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex.
These parallel grooves conceal the fact that the cerebellar cortex is a continuous thin layer of tissue folded in the style of an accordion. Within this thin layer are several types of neurons with a regular arrangement, the most important being Purkinje cells and granule cells; this complex neural organization gives rise to a massive signal-processing capability, but all of the output from the cerebellar cortex passes through a set of small deep nuclei lying in the white matter interior of the cerebellum. In addition to its direct role in motor control, the cerebellum is necessary for several types of motor learning, most notably learning to adjust to changes in sensorimotor relationships. Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum; these models derive from those formulated by David Marr and James Albus, based on the observation that each cerebellar Purkinje cell receives two different types of input: one comprises thousands of weak inputs from the parallel fibers of the granule cells.
The basic concept of the Marr–Albus theory is that the climbing fiber serves as a "teaching signal", which induces a long-lasting change in the strength of parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial. At the level of gross anatomy, the cerebellum consists of a folded layer of cortex, with white matter underneath and a fluid-filled ventricle at the base. Four deep cerebellar nuclei are embedded in the white matter; each part of the cortex consists of the same small set of neuronal elements, laid out in a stereotyped geometry. At an intermediate level, the cerebellum and its auxiliary structures can be separated into several hundred or thousand independently functioning modules called "microzones" or "microcompartments"; the cerebellum is located in the posterior cranial fossa. The fourth ventricle and medulla are in front of the cerebellum, it is separated from the overlying cerebrum by a layer of leathery dura mater, the tentorium cerebelli.
Anatomists classify the cerebellum as part of the metencephalon, which includes the pons. Like the cerebral cortex, the cerebellum is divided into two hemispheres. A set of large folds is, by convention, used to divide the overall structure into 10 smaller "lobules"; because of its large number of tiny granule cells, the cerebellum contains more neurons than the total from the rest of the brain, but takes up only 10% of the total brain volume. The number of neurons in the cerebellum is related to the number of neurons in the neocortex. There are about 3.6 times as many neurons in the cerebellum as in the neocortex, a ratio, conserved across many different mammalian species. The unusual surface appearance of the cerebellum conceals the fact that most of its volume is made up of a tightly folded layer of gray matter: the cerebellar cortex; each ridge or gyrus in this layer is called a folium. It is estimated that, if the human cerebellar cortex were unfolded, it would give rise to a layer of neural tissue about 1 meter long and averaging 5 centimeters wide—a total surface area of about 500 square cm, packed within a volume of dimensions 6 cm × 5 cm × 10 cm.
Underneath the gray matter of the cortex lies white matter, made up of myelinated nerve fibers running to and from the cortex. Embedded within the white matter—which is sometimes called the arbor vitae because of its branched, tree-like appearance in cross-section—are four deep cerebellar nuclei, composed of gray matter. Connecting the cerebellum to different parts of the nervous system are three paired cerebellar peduncles; these are the superior cerebellar peduncle, the middle cerebellar peduncle and the inferior cerebellar peduncle, named by their position relative to the vermis. The superior cerebellar peduncle is an output to the cerebral cortex, carrying efferent fibers via thalamic nuclei to upper motor neurons in the cerebral cortex; the fibers arise from the deep cerebellar nuclei. The middle cerebellar peduncle is connected to the pons and receives all of its input from the pons from the pontine nuclei; the input to the pons is from the cerebral cortex and is relayed from the pontine nuclei via transverse pontine fibers to the cerebellum
A spinal interneuron, found in the spinal cord, relays signals between sensory neurons, motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in a region of grey matter in the spinal cord; the grey column of the spinal cord appears to have groups of small neurons referred to as spinal interneurons, that are neither primary sensory cells nor motor neurons. The versatile properties of these spinal interneurons cover a wide range of activities, their functions include the processing of sensory input, the modulation of motor neuron activity, the coordination of activity at different spinal levels, the relay of sensory or proprioceptive data to the brain. There has been extensive research on the identification and characterization of the spinal cord interneurons based on factors such as location, structure and function, it is difficult to characterize every aspect of the neuronal anatomy of a vertebrate's spinal cord.
This difficulty is due not only to its structural complexity but to the morphology and the connectivity of neurons. For instance, in the spinal cord of a 19-day-old rat embryo, at least 17 different subclasses of interneurons with ipsilateral axon projections were found. In addition, 18 types of commissural interneurons have been identified on the basis of morphology and location. In particular, the cell bodies of the spinal interneurons are found in the grey matter of the spinal cord, which contains the motor neurons. In 1952, the grey matter of the cat's spinal cord was investigated, it was shown to have ten distinct zones referred to as Rexed laminae; the lamination pattern was observed in several species including humans. Rexed laminae VII and VIII are locations. In the mouse's dorsal alar plate, six progenitor domains give rise to dI1-dI6 neurons and two classes of dorsal interneurons. In addition, in the ventral half of the neural tube, four classes of interneurons known as V0, V1, V2, V3 neurons are generated.
V0 neurons are commissural neurons that extend their axons rostrally for 2-4 spinal cord regions in the embryonic spinal cord. V3 neurons are excitatory commissural interneurons; the V1 neurons are inhibitory interneurons with axons that project rostrally. V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ispilaterally and caudally across multiple spinal cord regions; the class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons. The integration of the sensory feedback signals and central motor commands at several levels of the central nervous system plays a critical role in controlling movement. Research on cat's spinal cord has shown that at the spinal cord level sensory afferents and descending motor pathways converge onto common spinal interneurons. Human studies since the 1970s have documented how this integration of motor commands and sensory feedback signals is used to control muscle activity during movement.
During locomotion, the sum of convergent inputs from the central pattern generator, sensory feedback, descending commands and other intrinsic properties turned on by different neuromodulators give rise to the activity of the interneurons. Further, this interneuronal activity was either recorded directly or inferred from the modulation of response in their postsynaptic targets, most motoneurons; the most efficient way to gate sensory signals in reflex pathways is to control the firing level of interneurons. For example, during locomotion, the interneuronal activity is modulated via excitation or inhibition depending on the reflex pathways. Thus, different patterns of interneuronal activity will determine which pathways are open, blocked, or modulated; the sensory information, transmitted to the spinal cord is modulated by a complex network of excitatory and inhibitory interneurons. Different neurotransmitters are released from different interneurons, but the two most common neurotransmitters are GABA, the primary inhibitory neurotransmitter and glutamate, the primary excitatory neurotransmitter.
Acetylcholine is a neurotransmitter that activates interneurons by binding to a receptor on the membrane. Renshaw cells are among the first identified interneurons; this type of interneuron projects onto α-motoneurons, where it establishes inhibition by expressing its inhibitory neurotransmitter glycine. However, some reports have indicated that Renshaw cells synthesize calcium-binding proteins calbindin-D28k and parvalbumin. Further, during spinal reflex, Renshaw cells control the activity of the spinal motoneurons, they are excited by the axon collaterals of the motor neurons. In addition, Renshaw cells make inhibitory connections to several groups of motor neurons, Ia inhibitory interneurons as well as the same motor neuron that excited them previously. Furthermore, the connection to the motor neurons establishes a negative feedback system at may regulate the firing rate of the motor neurons. Moreover, the connections to the Ia inhibitory interneurons may modulate the strength of the reciprocal inhibition to the antagonist motor neuron.
Joints are controlled by two opposing sets of muscles called extensors and flexors that must work in synchrony to allow proper and desired movement. When a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent from working against the agonist muscle; the spinal interneuron called Ia inhibitory interneuron is responsible for this inhibition of the antagonist muscle. The Ia afferent of the muscle spin
The name granule cell has been used for a number of different types of neuron whose only common feature is that they all have 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, 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 to infragranular layers of the cerebral cortex.
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 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 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 10 μm. Axon: Each granule cell sends a single axon onto the Purkinje cell dendritic tree; the axon has an 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 to allow signalling functions necessary for granule cell development to occur.
The granule cells, produced by the rhombic lip, are found in the granule cell layer of the cerebellar cortex. They are numerous, they are characterized by a 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. 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 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 packed in the granular cell layer of the dentate gyrus; 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 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. The main intrinsic granule cell in the vertebrate olfactory bulb lacks an axon; 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. 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 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. 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; 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 dependent on processes carried out by the granular layer. Therefore, the function of granule cells determines the cerebellar function as a whole.
Granule cell dendrites synapse with distinctive unmyelinated axons which Santiago Ramón y Cajal called mossy fibers Mossy fibers and golgi cells both make synaptic connections with gr