BISC 421 1st Edition Lecture 4 Outline of Current LectureI. The Membrane and Action PotentialsCurrent LectureThe Membrane and Action Potentials -ContinuedThe resting potential is determined by K+ permeability Resting potential is set by non-gated K+ channels These channels are always openThese are different from the K+ channels that play a role in the action potentialThe action potential is a rapid transient change in the membrane potential that occurs to conduct electrical signal •The resting membrane potential of the neuron is controlled by potassium. There are constantly open K+ channels so the resting membrane potential is very close to the K+ potential (-‐58) mV •The equilibrium potential for Na+ is +58 mV •The reason that the resting potential of the cell is negative is because of the open K+ channelsNa+ has little efect on resting membrane potential•Na+ has little efect on the resting membrane•This is because the membrane is not permeable to sodium•But if we look at the peak of the action potential we fnd that the membrane potential is perfectly dependent on Na+ concentration•The action potential is due to a change in Na+ permeabilityNa+ does efect the action potential•Happens very fast•Under low Na+ concentration, the peak is much lower than under normal conditions‐Action potentials arise from diferential permeability to ions••What controls the action potential is a change in membrane potential-‐ this happens bychanging permeability•Due to changes in concentration of ions•At rest:o Non gated K+ channels are the only ones infuencing membrane potential•Action potentialo Increase permeability of membrane to Na+ now at the peak of the action potential the permeability of Na+ is greater than K+ permeability (membrane potential is becoming more positive)•Repolarizationo Lowering permeability for Na+ makes the drive for K+ more so the membrane becomes more negative againGoldman-‐Hodgkin and Katz (GHK) equation•Can tell us the contribution of all the ions to the membrane potential (unlike Nernst whichonly tells you about one ion)•Note that Cl-‐ values are fipped due to the valence of the ionNomenclature of the action potential•What contributes most to the falling phase is the voltage gated K+ channels opening-‐ causesan undershoot•If we didn’t have these the action potential would look a lot wider and slower•Recording from voltage clamp-‐ determine what happens in membrane permeability if we change membrane potential•Can make it 0 by injecting current into the inside-‐ aka inject positive currentFlow of current during voltage steps•Inward current-‐ positive ions into cell•Outward current-‐ positive ions out of the cell•Capacitive current-‐ all membranes have charge in themCurrent produced at diferent potentials•Voltage dependent currents: depends on whether or not the current will fow when positive or negative•Currents are due to voltage gated channels•Dependency of the amplitude of the currents on what the membrane potential actually wasEarly and late currents•-‐65-‐0 under normal conditions are dependent on Na+•without sodium you don’t get this current•used a toxin to block potassium channels and found that the early inward current stays the same butthe late outward current is blockedMembrane conductance changes underlying the action potential are time-‐ and voltage-‐dependentthe reason why the action potential is a transient event is because the Na+ current itself is
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