# C. Establishes an equilibrium potential for a particular ion

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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). Eion = RT ln [X]outside zF [X]inside

Goldman Equation 1. quantitative representation of Vm when membrane is permeable to more than one ion species 2. involves permeability constants (P) RT PK [K+]out + PNa [Na+]out + PCl[Cl-]in ___ _____________________________ ENa,K,Cl = ln PK [K+]in + PNa [Na+]in + PCl[Cl-]out F pp 72-73

Resting Potential A. Vrest
1. represents potential difference at non-excited state -30 to -100mV depending on cell type 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

Resting 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 Vrest (due to increased permeability)

Resting Potential C. Role of active transport
ENa is + 63 mV in frog muscle Vm is to -100mV in frog muscle

Action Potentials large, transient change in Vm
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. different types of Na+ and K+ channels than seen in most cells - voltage-gated

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
- or just local response (potential) if it does not reach threshold - rapid depolarization - results in reverse of polarity

Action Potentials B. Properties a. threshold current (-30 to -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 Vrest b. after-hyperpolarization

Action Potentials B. Properties 6. accommodation
a. time-dependent decrease in excitability b. result of a series of subthreshold depolarizations c. threshold increases d. the slower the rate of  depolarization (current intensity), the greater the  in threshold e. change in sensitivity of ion channels

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 gNa
overshoot approaches ENa (ENa is about +60 mV) 2. falling phase:  in gNa and  in gK 3. after-hyperpolarization continued  in gK approaches EK (EK is about -90 mV)

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

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

Gated Ion Channels A. Voltage-gated Na+ channels 1. localization
b. ligand-gated at synapses

Gated Ion Channels A. Voltage-gated Na+ channels 1. localization
Na+ channels occupy only a small fraction of surface area channels/m

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 Vm

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. factors contributing to specificity a. anions at mouth of channel b. size c. ability to dehydrate (shed water of hydration)

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

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

Gated Ion Channels B. Voltage-gated K+ channels
3. high gK during falling phase decreases as Vm 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 Vm B. Large axons show negligible change in Na+ and K+ concentrations after an AP.