Unformatted text preview: Membrane Potentials October 4, 2010 PSL302Y: Lecture 10, by Prof. MacKay! Mon., Oct. 4, 2010 Membrane Potential - An enzyme ion pump must work to create a concentration gradient of ion species across themembrane is most permeable to K+ `resting' cell membrane resting - Semi-permeable membrane allows one ion species to diffuse across more than others ion - Diffusion of that of cell, down concentration K+ diffuses out ion species down its conc gradient creates electrical gradient
gradient, via `2-pore family' of K+ channels pump cations accumulate outside membrane, , Na+/K+-dependent ATPase: enzyme that moves Na+ out of cell, and K+ into cell leaving a net negativity inside membrane Resting Potential Na+/K+ - For each ATP molecule broken down, 2 Na+ ions are pumped out and 2 K+ pumped in - Consumes 1/3 of energy needs of body - Na/K inequality -> potential diffce of -10mV Resting Potential - K+ leaks out through 2-pore K channel, then pumped back in via primary active transport Equilibrium potential Potential Resting - `Resting' membrane is most permeable to K+ ion - K+ diffuses out of cell - down conc gradient - via `2-pore family' of K+ channels - Cations accumulate outside membrane = net negativity inside membrane
+ mbrane Potentials - - - October 4, 2010 2 2-pore K channel
(K+ leakage) Equilibrium Potential - At eq: elcal work to repel outwd cation diffusion = chemcal work of diffusion down conc gradt - Membrane potential at eq is determined by the conc gradient - Can be calculated usng the Nernst eqtn Equilibrium potential At equilibrium, electrical work to repel equilibrium outward cation diffusion equals chemical work of diffusion down conc. gradient membrane potential at equilibrium is 1 of 4 PSL302Y: Lecture 10, by Prof. MacKay! Mon., Oct. 4, 2010 Membrane Potentials Nernst equation October 4, 2010 EK+ = (RT/F) ln([K+]o /[K+]i) = -90 mV - The eqtn gives the potential diffce across the membrane, inside wrt outside, at eq - The result is valid iff one ion species (K+ in this case) is diffusing across the membrane - Note: RT/F; R: gas constant, T: temp (constant in endotherms), F: Faraday's constant Goldman equation - Membrane is most permeable to K+, but Na+ and Cl- ions are also diffusing somewhat - Nernst equation difficult to use b/c membrane permeable to other ions as well - Other ion concentration ratios are quite small but have a significant effect on membrane potential - Actual membrane potential can be calculated from an expanded eqtn containing a term for each diffiusable ion species - From Goldman eqtn: Em = -70 mV Goldman Equation Em RT F ln PK PK K K o i PN a N a PN a N a o i PC l C l PC l C l i o Membrane Potentials - Cations' ratios: Outside over Inside, but for anion ratio: I over O (effective diffusion in opposite direction of cations)
October 4, 2010 Na+ equilibrium potential - If the membrane properties change to make the membrane most permeable to Na+, then there is a net Na+ current inwd - At eq, there is a net cation accumulation inside the membrane - Membrane potential is +ve inside wrt outside: ENa+ = +50 mV Na+ equilibrium potential
- Na+ diffuses inward (into cell): positive accumulation of charge inside membrane - Large enough accumulation of positive charge w/i cell repels further net diffusion of Na If the membrane properties change to make + into cell = voltage-gated Na+ channel - Potential due to Na+ alone calculated by Nernst equation Na+, then the membrane most permeable to Na+ channel - To generate a signal, membrane increasesis aconductance by opening a channel at equilibrium, there its net cation permeable only+to Na+ ion = voltage-gated Na+ channel Na Channel accumulation inside the membrane - Opened by depolarizing membrane to a threshold potential of about -50mV there is a net Na+ current inward membrane potential is positive To generate a signal, membrane increases signal its conductance byoutside: opening a channel ENa+ = +50 mV permeable only to Na+ ion this is a voltage-gated Na+ channel opened by depolarizing membrane to a p y p g threshold potential of about -50 mV inside w.r.t. 2 of 4 Membrane Potentials October 4, 2010 PSL302Y: Lecture 10, by Prof. MacKay! Mon., Oct. 4, 2010 Inward-current Na+ channel occurs at high numbers in "excitable tissue" Membrane after Inactivation gate stops Na+ influx (short-lived) shortlyPotentials depolarization Action Potential Action Potential - Na+ channels occur in high density w/i `excitable' membranes Na+ - When channels occur in high membrane potential surges channels open, density within `excitable' membranes upwardchannels are ENa+ membrane mV when towards open, = +50 - Butpotential surges towards ENa+ = 50 mV channels rapidly inactivate but-channels rapidly inactivate p y Na+ inactivation leaves K+ leakage as main Na+ inactivation leaves K+ leakage as main Membrane Potentials current = resting potential restored current, and resting potential is restored - Threshold initially reached (-70mV -> -50mV) (later)
Na+ inactivation October 4, 2010 Na+ influx October 4, 2010 Refractory Period - Na+ channels remain inactivated until membrane potential drops below `threshold' Refractory Period - Then channels reconfigure to original state and 7 membrane becomes excitable again Na+ channels remain inactivated until membrane potential drops below `threshold' - Absolute r.p.: none of the channels then channels reconfigure to original state reconfigured (time btwn Na+ inactivation and membrane becomes excitable again absolute r.p.: none of channels reconfigured p g and before potential drops below relative r.p.: some but not all of channels threshold -> impossible to generate are reconfigured (generally 2-5 ms duration) another action potential) - Relative r.p.: some but not all of channels are reconfigured (generally 2-5ms duration: time btwn potential drops below threshold but not yet to original potential -> some action potential possible during period) - Action potential not always same size: depends on number of Na+ channels involved, Depolarization Block i.e. During relative r.p., action potential stunted the closer it occurs to absolute r.p. Depolarization Block - If the membrane is kept depolarized (e.g. Excess - Membrane remains in absolute r.p. until normal resting polarity restored = paralysis! If the membrane is kept depolarized (e.g. (e g excess [K+]o), then Na+ inactivation persists membrane remains in absolute refractory [K+]0), then Na+ inactivationis restored persists state until normal resting polarity 8 3 of 4 9 PSL302Y: Lecture 10, by Prof. MacKay! opened by depolarization to threshold pot. slower kinetics than Na+ channels maximum outward K+ current occurs after Mon., Oct. 4, 2010 Na+ inactivation causes a hyperpolarization after action pot. - If you inject K+ into blood plasma, obliterate potential across membrane and Na+ inactivation persists = signaling not possible! After-hyperpolarization - In some membrane locations, voltage-gated K+ channels are found October 4, 2010 - Opened by depolarization in threshold potential - Slower kinetics than Na+ channels - Max outward K+ current occurs AFTER Na+ inactivation: repolarizes membrane after action potential - Causes a hyperpolarization after action potential, i.e. Reaching almost -80mV When a If only 2-pore K channels involved, - patch of excitable membrane generates anrepolarization would be slower, then action potential, zone of reversedgradual and would not source of polarity serves as overshoot - Voltage-gated K channels inactivate themselves: membrane reaches original potential entials Impulse conduction depolarizing current for adjacent membrane Impulse conduction Na+ channels opened in adjacent membrane - When a patch of excitable membrane generates an action potential, zone of rvsed Membrane Potentials therefore,serves as source andepolarizing current for adjacent membrane polarity once started, of a.p. will - Na+ channels opened in adjacent membrane propagate from its origin across the rest of - Therefore, once started, an a.p. will propagate from its origin across the rest of the cell the cell
A.P. 10 October 4, 2010 + - - - - - - - - - - depolarizing current Typical Neuron -Depolarizing current discharges ve charge along membrane in the local vicinity of the action potential embrane Potentials October 4, 2010 Excitable cells - Most cells are not `excitable', i.e. they do not generate action potentials (lack voltagegated Na+ channels) - Neurons w/ long `axons' and muscle cells/fibers generate propagating action potentials - Action potential not transferred along axon very efficiently...a lot lost, but it's enough Typical cells ExcitableNeuron Most cells are not `excitable', i e they do excitable i.e. not generate action potentials (lack voltagegated Na+ channels) neurons with long `axons' and muscle cells / fibers generate propagating action potentials 4 of 4 12 ...
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This note was uploaded on 03/27/2012 for the course PSL PSL300 taught by Professor Mackayfrench during the Fall '11 term at University of Toronto.
- Fall '11