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    Neuron

    • Neuron
      PurkinjeCell.jpg
      Drawing of neurons in the pigeon cerebellum, by Spanish neuroscientist Santiago Ramón y Cajal in 1899. (A) denotes Purkinje cells and (B) denotes granule cells, both of which are multipolar.
      Identifiers
      MeSH D009474
      NeuroLex ID sao1417703748
      TA A14.0.00.002
      TH H2.00.06.1.00002
      FMA 56566
      Anatomical terminology
      []
      Structure of a typical neuron
      Neuron (peripheral nervous system)

      A neuron (/ˈnjʊərɒn/ NYEWR-on or /ˈnʊərɒn/ NEWR-on, also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information through electrical and chemical signals. These signals between neurons occur via synapses, specialized connections with other cells. Neurons can connect to each other to form neural networks. Neurons are the core components of the brain and spinal cord of the central nervous system (CNS), and of the ganglia of the peripheral nervous system (PNS).

      There are several types of specialized neurons.Sensory neurons respond to stimuli such as touch, sound or light and all other stimuli affecting the cells of the sensory organs that then send signals to the spinal cord and brain.Motor neurons receive signals from the brain and spinal cord to cause muscle contractions and affect glandular outputs.Interneurons connect neurons to other neurons within the same region of the brain, or spinal cord in neural networks.



      1. AMPA and Kainate receptors both function as cation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission
      2. NMDA receptors are another cation channel that is more permeable to Ca2+. The function of NMDA receptors is dependant on Glycine receptor binding as a co-agonist within the channel pore. NMDA receptors do not function without both ligands present.
      3. Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability.
      Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons causing NMDA and AMPA receptor activation more so than would normally be the case outside of stress conditions, leading to elevated Ca2+ and Na+ entering the post synaptic neuron and cell damage. Glutamate is synthesized from the amino acid glutamine by the enzyme glutamate synthase.
      Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons causing NMDA and AMPA receptor activation more so than would normally be the case outside of stress conditions, leading to elevated Ca2+ and Na+ entering the post synaptic neuron and cell damage. Glutamate is synthesized from the amino acid glutamine by the enzyme glutamate synthase.
      • The soma is the body of the neuron. As it contains the nucleus, most protein synthesis occurs here. The nucleus can range from 3 to 18 micrometers in diameter.
      • The dendrites of a neuron are cellular extensions with many branches. This overall shape and structure is referred to metaphorically as a dendritic tree. This is where the majority of input to the neuron occurs via the dendritic spine.
      • The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve signals away from the soma (and also carries some types of information back to it). Many neurons have only one axon, but this axon may—and usually will—undergo extensive branching, enabling communication with many target cells. The part of the axon where it emerges from the soma is called the axon hillock. Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily excited part of the neuron and the spike initiation zone for the axon: in electrophysiological terms it has the most negative action potential threshold. While the axon and axon hillock are generally involved in information outflow, this region can also receive input from other neurons.
      • The axon terminal contains synapses, specialized structures where neurotransmitter chemicals are released to communicate with target neurons.
      • Unipolar or pseudounipolar: dendrite and axon emerging from same process.
      • Bipolar: axon and single dendrite on opposite ends of the soma.
      • Multipolar: two or more dendrites, separate from the axon:
        • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.
        • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.
      • Anaxonic: where axon cannot be distinguished from dendrites.
      • Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells.
      • Golgi II: neurons whose axonal process projects locally; the best example is the granule cell.
      • Afferent neurons convey information from tissues and organs into the central nervous system and are sometimes also called sensory neurons.
      • Efferent neurons transmit signals from the central nervous system to the effector cells and are sometimes called motor neurons.
      • Interneurons connect neurons within specific regions of the central nervous system.
      • Tonic or regular spiking. Some neurons are typically constantly (or tonically) active. Example: interneurons in neurostriatum.
      • Phasic or bursting. Neurons that fire in bursts are called phasic.
      • Fast spiking. Some neurons are notable for their high firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells.
      • Cholinergic neurons—acetylcholine. Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as a ligand for both ligand-gated ion channels and metabotropic (GPCRs) muscarinic receptors. Nicotinic receptors, are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind nicotine. Ligand binding opens the channel causing influx of Na+ depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized from choline and acetyl coenzyme A.
      • GABAergic neurons—gamma aminobutyric acid. GABA is one of two neuroinhibitors in the CNS, the other being Glycine. GABA has a homologous function to ACh, gating anion channels that allow Cl ions to enter the post synaptic neuron. Cl causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (recall that for an action potential to fire, a positive voltage threshold must be reached). GABA is synthesized from glutamate neurotransmitters by the enzyme glutamate decarboxylase.
      • Glutamatergic neurons—glutamate. Glutamate is one of two primary excitatory amino acid neurotransmitter, the other being Aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor (often referred to as GPCR).
      • Dopaminergic neurons—dopamine. Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA. Dopamine is connected to mood and behavior, and modulates both pre and post synaptic neurotransmission. Loss of dopamine neurons in the substantia nigra has been linked to Parkinson's disease. Dopamine is synthesized from the amino acid tyrosine. Tyrosine is catalyzed into levadopa (or L-DOPA) by tyrosine hydroxlase, and levadopa is then converted into dopamine by amino acid decarboxylase.
      • Serotonergic neurons—serotonin. Serotonin (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory. Of the four 5-HT receptor classes, 3 are GPCR and 1 is ligand gated cation channel. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by aromatic acid decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft.
      • Kandel E.R., Schwartz, J.H., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York.
      • Bullock, T.H., Bennett, M.V.L., Johnston, D., Josephson, R., Marder, E., Fields R.D. 2005. The Neuron Doctrine, Redux, Science, V.310, p. 791–793.
      • Ramón y Cajal, S. 1933 Histology, 10th ed., Wood, Baltimore.
      • Richard S. Snell: Clinical neuroanatomy (Lippincott Williams & Wilkins, Ed.6th 2006) Philadelphia, Baltimore, New York, London.
      • Roberts A., Bush B.M.H. 1981. Neurones Without Impulses. Cambridge University Press, Cambridge.
      • Peters, A., Palay, S.L., Webster, H, D., 1991 The Fine Structure of the Nervous System, 3rd ed., Oxford, New York
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