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18-Dec-2017 22:46 by 2 Comments

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Neurons are cells that are specialized to receive, propagate, and transmit electrochemical impulses.

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These vesicles are pooled together in terminal boutons of the presynaptic neuron.Once bounded with Ca2 , the vesicles dock and fuse with the presynaptic membrane, and release neurotransmitters into the synaptic cleft by a process known as exocytosis.The neurotransmitters then diffuse across the synaptic cleft and bind to postsynaptic receptors embedded on the postsynaptic membrane of another neuron.Metabotropic receptors on the other hand activate second messenger cascade systems that result in the opening of ion channel located some place else on the same postsynaptic membrane.Although slower than ionotropic receptors that function as on-and-off switches, metabotropic receptors have the advantage of changing the cell's responsiveness to ions and other metabolites, examples being gamma amino-butyric acid (inhibitory transmitter), glutamic acid (excitatory transmitter), dopamine, norepinephrine, epinephrine, melanin, serotonin, melatonin, and substance P.Synaptic plasticity in both excitatory and inhibitory synapses has been found to be dependent upon postsynaptic calcium release Two molecular mechanisms for synaptic plasticity (researched by the Eric Kandel laboratories) involve the NMDA and AMPA glutamate receptors.

Opening of NMDA channels (which relates to the level of cellular depolarization) leads to a rise in post-synaptic Ca2 concentration and this has been linked to long-term potentiation, LTP (as well as to protein kinase activation); strong depolarization of the post-synaptic cell completely displaces the magnesium ions that block NMDA ion channels and allows calcium ions to enter a cell – probably causing LTP, while weaker depolarization only partially displaces the Mg2 ions, resulting in less Ca2 entering the post-synaptic neuron and lower intracellular Ca2 concentrations (which activate protein phosphatases and induce long-term depression, LTD).When ionotropic receptors are activated, certain ion species such as Na to enter the postsynaptic neuron, which depolarizes the postsynaptic membrane.If more of the same type of postsynaptic receptors are activated, then more Na will enter the postsynaptic membrane and depolarize cell.The Hodgkin–Huxley model of an action potential in the squid giant axon has been the basis for much of the current understanding of the ionic bases of action potentials.Briefly, the model states that the generation of an action potential is determined by two ions: Na and K .There are two families of receptors: ionotropic and metabotropic receptors.