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Project 1:Excitatory synaptic transmissionMost information transfer between neurons in our brains occurs at specialized contacts called chemicalsynapses. Synapses that are made by approximately three quarters of all neurons in the brain are excitatorysynapses, which release an amino acid neurotransmitter called glutamate onto their postsynaptic targets.The postsynaptic neuron has receptors on its cell membrane capable of sensing the glutamate andresponding by opening ion channels. The receptor and ion channel is the same protein, which is referred togenerically as a ligand-gated ion channel or ionotropic receptor. The ones specific for the neurotransmitterglutamate are called ionotropic glutamate receptors and their channels are selective for positively chargedions (e.g. Na+). The passage of Na+ions down their electrochemical gradient through these channels resultsin depolarization of the resting membrane potential closer to action potential firing threshold – hence thesynapse being referred to as ‘excitatory’. The ionotropic glutamate receptors typically playing this role arethe AMPA-type glutamate receptors (AMPARs) (Figure 1A)Figure 1.Panel A (top) illustrates action potential-triggered synaptic transmission at an excitatory chemicalsynapse from the vertebrate central nervous system. Note that only one synapse is depicted in this figure andthe remaining parts of the neurons are omitted. ‘Pre’ refers to the presynaptic neuron and ‘Post’ refers to thepostsynaptic neuron. Panel A (bottom) illustrates the effect that excitatory synaptic transmission has on theresting membrane potential. Panel B illustrates how AMPA receptors are maintained at synapses in thepostsynaptic neuron.The way by which neurotransmitter is released makes chemical synaptic transmission a quantized process,meaning that neurotransmitter release occurs in discrete steps. Within the presynaptic terminal, theneurotransmitter is packaged into hundreds of membranous vesicles, where each vesicle is referred to as aquantum. Release of neurotransmitter requires that one or more vesicles (or quanta) undergo fusion withthe cell membrane by a process called exocytosis. Whether a quantum of glutamate is released is a matterof probability; while the presynaptic cell is resting, individual quanta are randomly and spontaneouslyreleased at low frequency, while action potential firing dramatically increases the probability of release,typically causing the release of multiple quanta. The size of the postsynaptic response to a single quantum

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Term
Spring
Professor
miss ayesha
Tags
mEPSCs

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