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Lecture 5 Synaptic Transmission

Miniature post synaptic potentials due to spontaneous

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miniature post-synaptic potentials due to spontaneous release of NT always same size equivalent to single vesicle fusion events o EPSPs or IPSPs are integer multiples of miniPSPs Postsynaptic Potential Size o size of response following detection of NT o Ipsp = N * P0 * G * Vdiff o Ipsps = current of EPSP or IPSP (i.e. size) o N = number of channels o P0 = probability of opening o G= single channel conductance o Vdiff= (i.e. driving force) = Eion – Vm o any factor influencing these variables will influence the magnitude of the post- synaptic potential; e.g. different subunits results in different conductance Transmitter gated ion channels – nAChR (ion channel) o Nicotinc Acetylcholine receptor (nAChR) – sensitive to nicotine 5 polypetide subunits 4 membrane spans per subunit
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only alpha subunit binds Ach (2 /5 subunits bind) – causes structural change selective for Na+ and K+ typically produces EPSPs; Why... o Current flow through a nAChR o Iion = g (Eion -Vm) o At rest (Vm = -65 mV): E na+ - V m = about 130 mV E k+ - V m = 15 mV o the higher driving force on Na+ (at rest) produces far greater inward Na+ current than the outward K+ current. o Thus, nAChR activation leads to depolarization at rest Vm. o At peak of AP (at 60 mV) , opposite effect. Look at graph Integration of Post-synaptic Signals o Neurons have 100's – 1000's of inputs (synapes) o inputs are a mixture of ionotropic receptors that produce excitatory and inhibitory PSPs (as well as metabotropic receptors – later) o integration is the NET result of all inputs Integration of EPSPs o spatial summation from different neurons o temporal summation from the same neuron Integration of EPSP with IPSPs o summate spacially Site of synapse and input strength o axon dendritic & axon somatic axon terminals that are on the soma tend to impact on post-synaptic cells b/c input is closer to axon hillock (i.e. closer to anatomical site of electrical integration; thus, less signal degradation). Signal degradation o magnitude of the signal declines as you move away from the point of depolarization i.e. like an axon, dendrites are leaky dendrites typically can not regenerate depolarization Inhibitory Shunting o excitatory synapse – receptor coupled ion channel for Na+ or Ca+ o inhibitory synapse – receptor coupled ion channel for Cl- or K+ Excitable Dendrites o dendrites with voltage-gated channels can maintain depolarization magnitude (i.e. prevent signal degradation or amplifies signal) Metabotrobic Receptors o enzyme linked receptor systems o NT bonded to a receptor that will affect enzymes in the intracellular side of system. Will change biochemical processes that occur within the cell. o Structure of receptors
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metabotropic receptors are comprised of single subunit cytoplasmic portion of receptor couples to intracellular enzymes Ionotropic vs. Metabotropic receptors: o Relation to Ion Channels/Changes in Membrane Potential o direct couling (ionotropic receptors) = “fast” neurotransmission o indirect coupling (metabotropic receptors) = “slow” neurotransmission; tends to have more complex effects on post-synaptic cell i.e. “modulates” slow offset, slower effects.
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