Fundamentals of neuroscience: cells, axons, and synapses

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Distribution of Major Ions Across theMembrane of the Squid Giant AxonDistribution of Major Ions Across theMembrane of the Frog MuscleIon Cytoplasm ExtracellularK + 40020Cl - 52 560Na + 50 440Ion Cytoplasm ExtracellularK + 124 2.3Cl - 1.5 78Na + 10 109(mM)(mM)(mM)(mM)Resting potential ca. -60mVResting potential ca. -100mVWhat Forces Govern the Movements of Ions?1. Concentration Gradients. i.e. diffusionfrom high to low concentration areas.2. Electric Charge Separation. i.e. ionstend to move towards regions of oppositeelectric charge.3. Cell Membrane. i.e. ion movement isrestricted by the physical barrier imposedby the cell membrane.Selectively Permeable MembraneIn OutA -A -K +K +A -A - K + K +K +K + A -K + K +A -A -A - K +K +A - K +A - K +K + A -K + A -K +K + A -A - A -A -Selectively Permeable MembraneSelectively Permeable MembraneIn OutIn OutA -A -K + K +A - A -K +KK ++ K +A - A - K + A - A -K +K +K +KK ++K + AĀ- KA -+K +A -A- A - K +A -A -K +A - K +K +K +A -K +KA - +A - K +K +K K + A -K ++A -K + A - A -KK + A - + A -A - K + K +A -A -A - A - A -- - - + + +- - - - + + + +2
Nernst EquationRTE K =__ZFWhereln____ [K + ] o[K + ] iE K = K + Equilibrium Pot’l.R = Gas ConstantT = Absolute TemperatureZ = Valence of K +F = Faraday Constant[K + ] o,i =K + concentrationsSubstituting,E K = 26mVNerve___ 20ln400 = -75mVRelationship of Membrane Potential (V m ) to [K + ] o0V m25mV5075100Nernst1 10 [K + ] 100o mM1. How can concentration gradients forNa + , K + , and Cl - all be maintainedacross the cell membrane?2. How do these gradients interact todetermine the resting membranepotential?IonDistribution of Major Ions Across theMembrane of the Squid Giant AxonCytoplasmExtracellularFluidNernstPotentialPassive and Active Movement of Ions through the MembraneMembraneIntracellularK + K +E K - V m = -10mVExtracellularK + 400 20-75Cl - 52 560 -60Na + 50 440 +55(mM) (mM) (mV)MetabolicEnergy(ATP)Na +Na + -K +PumpK +Na +Resting Pot’l (V m ) = -75mVE Na - V m = 155mVResting potential ca. -60mVGOLDMAN EQUATIONV m =RT __FlnIf P K>> P Naand P Cl,_______________________P K[K + ] o+ P Na[Na + ] o + P Cl[Cl - ] iP K[K + ] + iP Na[Na + ] i+ P Cl[Cl - ] oRT __V m ~FTherefore,The greater the permeability and concentration ofan ion, the greater will be its contribution to V m .ln_____ P K[K + ] oP K[K + ] iwhere P = Permeability Membrane potential (Vm(Vm) ) is determined primarily by K+ andNa+. Membrane potential will be closest to the Nernst(Equilibrium) Potential of the ion with the greatestconcentrations and membrane permeability. At Rest, Membrane Potential is close to the potassiumequilibrium potential (EK+) because the membrane is mostpermeable to K+. At Rest, as EK+ is slightly more negative than Vm, , there is asteady K+ efflux, balanced by a steady Na+ influx. Thesetwo passive fluxes are balanced by active pumping of Na+and K+ in the opposite directions. Note: this is a steadystate, not an equilibrium. Under most physiological conditions the bulk concentrationsof Na+, K+ and Cl- inside and outside of the cell remainconstant.3
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Page 5 and 6: PROPAGATION OF THE ACTION POTENTIAL
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Fundamentals of neuroscience: cells, axons, and synapses

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