Electrochemical Potentials A. Factors responsible 1. ion concentration gradients on either side of the membrane - maintained by active transport

Electrochemical Potentials A. Factors responsible 2. selectively permeable ion channels

B. Gradients not just chemical, but electrical too 1. electromotive force can counterbalance diffusion gradient 2. electrochemical equilibrium

C. Establishes an equilibrium potential for a particular ion based on Donnan equilibrium

Nernst equation 1. What membrane potential would exist at the true equilibrium for a particular ion? - What is the voltage that would balance diffusion gradients with the force that would prevent net ion movement? 2. This theoretical equilibrium potential can be calculated (for a particular ion). RT [Na + ] out [Na + ] in E Na = zF ln ___ R = Gas constant T = Temp K z = valence of X F = Faraday’s constant For K + around -90mV For Na + around +60mV

Resting Membrane Potential A. V rest 1. represents potential difference at non-excited state -normally around -70mV in neurons 2. not all ion species may have an ion channel 3. there is an unequal distribution of ions due to active pumping mechanisms - contributes to Donnan equilibrium - creates chemical diffusion gradient that contributes to the equilibrium potential

B. Ion channels necessary for carrying charge across the membrane 1. the  the concentration gradient, the greater its contribution to the membrane potential 2. K + is the key to V rest (due to increased permeability) Resting Membrane Potential

C. Role of active transport E Na is +55 mV in human muscle V m is mV in human muscle Resting Membrane Potential

Action Potentials large, transient change in V m depolarization followed by repolarization propagated without decrement consistent in individual axons “all or none”

Action Potentials A. Depends on 1. ion chemical gradients established by active transport through channels 2. these electrochemical gradients represent potential energy 3. flow of ion currents through “gated” channels - down electrochemical gradient 4. voltage-gated Na + and K + channels

Action Potentials B. Properties 1. only in excitable cells - muscle cells, neurons, some receptors, some secretory cells

Action Potentials B. Properties 2. a cell will normally produce identical action potentials (amplitude)

Action Potentials B. Properties 3. depolarization to threshold - rapid depolarization - results in reverse of polarity - or just local response (potential) if it does not reach threshold

Action Potentials B. Properties a. threshold current (around -55 mV) b. AP regenerative after threshold (self-perpetuating)

Action Potentials B. Properties 4. overshoot: period of positivity in ICF 5. repolarization a. return to V rest b. after-hyperpolarization

Action Potentials C. Refractory period 1. absolute 2. relative a. strong enough stimulus can elicit another AP b. threshold is increased

Action Potentials D. ∆ Ion conductance - responsible for current flowing across the membrane

Action Potentials D. ∆ Ion conductance 1. rising phase:  in g Na overshoot approaches E Na (E Na is about +60 mV) 2. falling phase:  in g Na and  in g K 3. after-hyperpolarization continued  in g K approaches E K (E K is about -90 mV)

Gated Ion Channels A. Voltage-gated Na + channels 1. localization a. voltage-gated

Gated Ion Channels A. Voltage-gated Na + channels 2. current flow a. Na + ions flow through channel at 6000/sec at emf of -100mV b. number of open channels depends on time and V m

Gated Ion Channels A. Voltage-gated Na + channels 3. opening of channel a. gating molecule with a net charge

Gated Ion Channels A. Voltage-gated Na + channels 3. opening of channel b. change in voltage causes gating molecule to undergo conformational change

Gated Ion Channels A. Voltage-gated Na + channels 4. generation of AP dependent only on Na + repolarization is required before another AP can occur K + efflux

Gated Ion Channels A. Voltage-gated Na + channels 5. positive feedback in upslope a. countered by reduced emf for Na + as V m approaches E Na b. Na + channels close very quickly after opening (independent of V m )

Gated Ion Channels B. Voltage-gated K + channels 1. slower response to voltage changes than Na + channels 2. g K increases at peak of AP

Gated Ion Channels B. Voltage-gated K + channels 3. high g K during falling phase decreases as V m returns to normal channels close as repolarization progresses

Gated Ion Channels B. Voltage-gated K + channels 4. hastens repolarization for generation of more action potentials

Does [Ion] Change During AP? A. Relatively few ions needed to alter V m B. Large axons show negligible change in Na + and K + concentrations after an AP.