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lecture 4

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University of Toronto St. George
Doug Mac Kay

04 SynapseSeptember 17 2007Postsynaptic Receptorstransmitter agent diffuses across synapse and binds to a specific site on a receptor protein embedded in postsynaptic membranebinding of transmitter causes a change in shape of the receptor proteinreceptor can be either ionotropic or metabotropicoreceptor determines what happens in the cell after a transmitter bindsthe same transmitter can cause different events depending on what receptor the transmitter binds toIonotropic effectsligand binding opens an ion channelion channel may be specific for cations Na KEPSPexcitatory postsynaptic potential depolarizingomix of Na 50 mV and K 90 mV channelsomembrane depolarizes toward 20 mVthe potential never reaches this because the number of current sources is too small to compete with K leakage channels and NaK pumpsion channel may be specific for Cl or K ionIPSPinhibitory postsynaptic potential hyperpolarizingoK has a hyperpolarizing effect 90 mV oClis more concentrated on the outside membrane 70 mV Clhas an inward concentration gradient when channels are openE70 mV eq hold resting potential when Clchannels are opened at resting potential nothing happens when Clchannels are opened during depolarizationfrom EPSP the influx of Clwill counteract the EPSP and bring the membrane potential to 70 mVduration of PSP is about 2040 msothis is the time that it takes for ligand to bind to receptors and be removedreceptor determines effect not transmitterACh bind to receptors to generate an EPSPoNa and K channels are opened and those ions diffuse inSpread of PostSynaptic PotentialsPSPs are generated in inexcitable membrane of neuronal dendrites and cell bodiesPSP has to use the cable properties of the dendrites and soma to arrive to the initial segment of axonnearest excitable membrane is initial segment of axonopassive conduction of PSPs to initial segment where they must summate to depolarize membrane to threshold levelssynapses are used for signaling process summate inhibitory and excitatory signalsolots of interaction between individual PSPsthis it the point of having synapsesoif you want to carry a signal from one place of the nervous system to another dont break it into synapsesjust carry the axon the whole distance quicker than having a bunch of synapsesindividual PSPs are tinyEPSPsgenerated on dendrites farthest away from the initial segment of axonosignal is small 1 mV and has to spread to initial segment of axon 1
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