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on to the inhibitory system rather uniform in type and properties 9 the inhibitory system by contrast possess several 10 different types of synapses originating from specifically differentiated cells and are much more difficult to track 9 there is insufficient information to precisely distinguish between excitatory and inhibitory pathways contributing to the alterations in neurotransmitter expression and cell structure changes 9 ca1 edit ca1 pyramidal cells make up a homogeneous population which together with relatives in subiculum comprise the primary output cells of the hippocampal formation 2 primary excitatory inputs are via glutamatergic ca3 schaffer collaterals both ipsi and contralateral which contact dendritic spines on the apical and basal dendrites in strata radiatum and oriens 2 additional excitatory input is via the temporoammonic system which synapses on distal apical dendrites in the stratum lacunosum moleculare 2 imaging studies following localized changes intracellular calcium from discrete synaptic inputs have shown a role for these currents in synaptic plasticity 2 there is disagreement however as to how activity dependent changes in synaptic inhibition might occur 2 studies do agree that plasticity is enhanced when inhibition is reduced 2 ca2 edit ca2 differs from other regions because it is one of the few areas to survive temporal lobe epilepsy 2 kainic acid used to model tle and related scleroses affects primarily the mossy fiber synapses in ca3 2 it is thought that at these release glutamate with administration of ka 2 ca2 and ca3 can be distinguished using histological stains because the proximal apical dendrites of ca2 do not possess dendritic spines 8 entorhinal cortex edit the entorhinal cortex ec is composed of six layers 2 superficial layer i consists largely of afferent fibers onto the apical dendrites of the cells in layers ii vi caudal levels project strongly to rostral levels within each ec area deeper layers innervate superficial layers with superficial layers innervating adjacent superficial layers entorhinal pyramidal cells of layer v receive strong input from the perirhinal cortex and sensory cortices 2 these pyramidal cells then project into the superficial entorhinal layer ii and iii cells layer v ec cells have strong recurrent excitatory synapses much like ca3 layers in the hippocampus and when provoked are capable of burst activity medial to lateral entorhinal area connections are sparse and principally project from the medial ec to the lateral ec these connections are not reciprocal 2 the majority of cells in the ec are pyramidal more than 90 of layer v cells are regular spiking with only a few burst firing and fast spiking cells 2 gaba is strong in superficial layers horizontal slice tissue preparations of both ec and hippocampus tissues show that exposure to low magnesium ion concentrations produces protracted seizure events this response is likely a result of the interconnections of layer v pyramidal cells increases in extracellular potassium in seizures are seen in deeper layers these responses are accurate reflections of in vivo animal models 2 piriform cortex edit in the piriform cortex layer i consists mostly of afferent inputs to apical dendrites of deeper cells layer i is subdivided into layers ia and ib each having its own afferents layer ii is densely packed with pyramidal and semilunar cells layer iii contains mostly pyramidal cells in its superficial part 2 in the piriform cortex the distal apical dendrites of layer iii pyramidal neurons receive extrinsic inputs which the corresponding proximal dendrites receive intrinsic inputs 5 olfactory bulb edit in each glomerulus the axons of the receptor neurons contact the apical dendrites of mitral cells which are the principal projection neurons in the olfactory bulb cell bodies of mitral cells are located in a distinct layer deep in the olfactory glomeruli 10 each mitral cell extends a primary dendrite to a single glomerulus where the dendrite gives rise to an elaborate tuft of branches onto which the primary olfactory axons synapse 10 each glomerulus in the mouse model for example contains approximately 25 mitral cells which receive innervation from approximately 25 000 olfactory receptor axons 10 the convergence increases the sensitivity of mitral cells to odor detection 10 cerebral cortex edit general edit the most superficial layer of the cortex is the molecular or plexiform layer 1 it has a dense network of tangentially oriented fibers and cells made of axons of martinotti cells and stellate cells as well as apical dendrites of pyramidal cells 1 apical dendrites from pyramidal cells in the external granular layer and more prominently the external pyramidal layer project into the molecular layer 1 there are also in the plexiform layer gabaergic synaptic connections between the apical dendrites of granular cells and the basal dendrites of the tufted cells and mitral cells 1 some of the apical dendrites from the pyramidal cells in the cerebral cortex may be up to 10μm in diameter 11 the apical dendrite of a large pyramidal neuron in the cerebral cortex may contain thousands of spines 11 spines in the cerebral cortex vary in size by several orders of magnitude from one region to another smallest have a length of 0 2μm and a volume of about 0 04 cubic micrometres and the largest a length of 6 5μm and a volume of 2 cubic micrometres 11 neocortex edit pyramidal cells are the majority class of cells in the neocortex 2 they have high density of dendritic spines prominent apical dendrites and axons that project out of the cortex as well as locally within it 2 soma for these appear in all layers except i 2 spiny stellate cells are distinguished from pyramidal cells here by the absence of the apical dendrite and the fact that their axons also do not leave the cortex 2 these cells are thought to begin as pyramidal neurons and then retract their apical dendrites and axons 2 cerebellum edit a defining characteristic of purkinje cells in the cerebellum is the apical dendrite 10 development edit dendritic arbor formation for pyramidal neurons in the cortices occurs progressively beginning in late embryonic stages of development and extending well into post natal periods 2 many dendrites of pyramidal neurons in deep layers branch and form connections in layer iv while some extend to more superficial layers pyramidal cell dendrites in layer iii branch to form arbors in layer i thalamocortical afferents will make synaptic contact with dendrites in layer iv while myriad of other inputs will meet dendrites in layer i the post synaptic structure is driven in part by signals from incoming afferent fibers and through life there is plasticity in the synapses 2 the formation of these arbors is regulated by the strength of local signals during development 3 several patterns in activity control the development of the brain action potential changes in the retina hippocampus cortex and spinal cord provide activity based signals both to the active neurons and their post synaptic target cells spontaneous activity originating within neuronal gap junctions the cortex sub plate and sensory inputs are all involved in the cell signaling that regulates dendrite growth 3 useful models of dendritic arbor formation are the xenopus tadpoles which are transparent in early stages of larval development and allow for dye labeled neurons to be repeatedly imaged in the intact animal over several weeks 3 it has been observed from this and other models that there are rapid dendritic branch additions and retractions which lengthen the overall dendrite and accumulate more branches this mirrors the development of axonal branches both have a lifetime of approximately 10min 3 this activity decreases as neurons mature signals including glutamate from axon branches may increase branch additions 3 within the xenopus tadpole model several signaling systems have been studied for example in optical tectal neurons dendrite arbor growth occurs approximately at the onset of retinal input 3 many on the caudal tectate have silent synapses which are modulated only by n methyl d aspartate nmda receptors as neurons mature alpha amino 3 hydroxy 5 methyl 4 isoxazole ampa receptors are added increasing synaptic transmission neuron and dendrite development are nmda dependent 3 rapidly growing dendrite arbors are more dynamic than slowly growing ones and dendrites themselves play an active role in their own development 3 it has been shown in studies that transport of hcn hyperpolarization activated cyclic nucleotide gated channel isoforms to dendritic fields of ca1 pyramidal neurons in the hippocampus occurs in an age specific manner in the developing hippocampus 12 among the signals studied in this system is camkii a calcium calmodulin regulated serine threonine kinase which is required for induction by not expression of long term potentiation 3 camkii mrna is targeted to dendrites and both protein synthesis and enzyme activity are increased by strong synaptic input 3 expression in xenopus indicates that it is associated with the transition to slowed arbor growth this suggests that activity promotes the reduction of dendrite branch growth and retraction stabilizing the arbor configuration 3 the following pattern emerges for this system branches with nmda only receptors mature and recruit ampars which stabilize the branches 3 these stable branches then add new branches with nmdar only synapses which either stabilize through ampars or retract ampar additions are present in adults and account for synaptic plasticity 3 camkii strengthening of signals results from the selective trafficking of glur1 ampars into synapses in long term depression ltd the glur subunits of ampars undergo endocytosis 3 temporal differences in signaling over the course of neuron maturation suggest that the most promising studies of arbor development and synaptogenesis in the future are going to occur in intact brain systems 3 another model studied in apical dendrite development is the rat injection of tetanus toxin into neonatal rats has shown that growth of apical dendrites occurs normally during signal deprivation while basal dendrite growth is restricted this indicates that neural activity is critical to new dendrite formation 13 however animal models may be insufficient to elucidate the complexity of these systems pyramidal cells in ca1 for example are 30 times as thick in humans as they are in rats 14 the entorhinal cortex is also subdivided into as few as 8 and as many as 27 sections in humans depending on the system used whereas there are only 2 in rats and 7 in monkeys 14 the connections of the dentate gyrus and entorhinal cortex are also more sophisticated in humans 14 in rats and cats a very large reciprocal connection exists between the entorhinal cortex and the olfactory system 2 in primates this connection is absent and there are highly differentiated connections between the multimodal parasensory and paralimbic cortices and the ec which are not as evident in rats and cats 2 the increased size of the primate subiculum may proportionally enhance its effects on the entorhinal cortex 2 sexual dimorphism edit pyramidal cell dendritic arbor formation in the anterior cingulate cortex layers 2 3 is more complex in males and in contrast the orbital prefrontal regions dendritic arborization is greater in females suggesting a fundamental difference in the prefrontal organization in males and females 15 in rats for example exposure to estrogen either exogenously or endogenously during proestrous leads to increases in ca1 spine densities 10 15 these differences may be due to the presence of gonadal hormones which have been demonstrated to influence cell structure in the hippocampus treatment with testosterone has been shown to affect cortical neuron structure 15 pathology edit stress response and ptsd edit dendritic spines post synaptic structures receiving mainly excitatory input are sensitive to experiences in development including stress episodes or drugs studies have shown that prenatal stress reduces complexity length and spine frequency of layer ii iii pyramidal apical dendrites in rat and primate models dendritic atrophy has been described in hippocampal formation and prefrontal cortex in both models 15 chronic stress has been shown to reduce the arbor complexity and total dendritic length of apical dendrite trees of ca3 pyramidal neurons in the hippocampus as well 16 17 chronic stress induced changes in behavior have usually been attributed to changes in the hippocampus which is a primary neural target of glucocorticoids and is involved in many of the behaviors altered by corticosteroid administration 5 17 both chronic stress and corticosteroid administration result in extensive atrophy of apical dendrites of pyramidal neurons in hippocampal area ca3 and these dendrites do not atrophy when cyanoketone a corticosteroid blocker is given 5 this dendrite atrophy is mediated by both glutaminergic and serotonergic systems administration of either nmda receptor antagonist cgp 43487 or serotonin uptake inhibitor tianeptine prevents atrophy 5 cell death has been reported to prolonged treatment 17 stress hormones in small doses do not themselves cause damage but magnify effects of other dangerous agents including excitotoxins hypoglycemia hypoxia and ischemia 17 damaging effects of stress in these neurons are thought to be related to expression of brain derived neurotrophic factor bdnf the expression of which is reduced in stressed conditions and increased with the administration of anti depressants 17 the prefrontal cortex is also a target for the glucocorticoids in stress 3h dexamethasone binds to receptors in frontal and prefrontal cortex at about 75 of concentration of hippocampus 5 endogenous regulation of corticosteroid receptors is indicated by altered binding of the previously mentioned compound in the prefrontal cortex with administration of corticosteroids 5 furthermore regulation of stress activities involves the prefrontal cortex lesions in rat prefrontal cortices impair spontaneous alternation radial maze performance and passive avoidance 5 in primates these impair inhibition of line of sight responses 5 chronic administration of corticosteroids decreases 5 ht1a receptor binding 5 ht2 receptor binding serotonin levels and expression of neural cell adhesion molecule a cell surface macromolecule involved in regulating aspects of synapse stabilization 5 these changes indicate structural change follows stress hormone elevation studies of dendritic morphological changes indicate that elevation of stress hormones in layer ii iii of the prefrontal cortex causes no observable change in the structure or distribution of basal dendrites 5 the apical dendrites however show a significant redistribution in stress hormone treated animal brains which is measured using scholl analysis 5 scholl analysis estimates the...