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

C. Establishes an equilibrium potential for a particular ionbased on Donnan equilibrium

Nernst equation1. 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

ENa,K,Cl = RT PK [K+]out + PNa [Na+]out + PCl[Cl-]in

PK [K+]in + PNa [Na+]in + PCl[Cl-]outF_____________________________ln___

Goldman Equation

1. quantitative representation of Vm when membrane is permeable to more than one ion species

2. involves permeability constants (P)

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 membrane1. the the concentration gradient, the greater its contribution to the membrane potential

2. K+ is the key to Vrest (due to increased permeability)

Resting PotentialC. Role of active transport

ENa is + 63 mV in frog muscleVm is + -90 to -100mV in frog muscle

Action Potentials

large, transient change in Vm

depolarization followed by repolarizationpropagated without decrementconsistent in individual axons“all or none”

Action Potentials

A. Depends on1. ion chemical gradients established by active transport through channels2. these electrochemical gradients represent potential energy3. flow of ion currents through “gated” channels

- down electrochemical gradient4. different types of Na+ and K+ channels than seen in most cells

- voltage-gated

Action PotentialsB. Properties

1. only in excitable cells- muscle cells, neurons, some receptors, some secretory cells


2. a cell will normally produce identical action potentials (amplitude)


3. depolarization to threshold

- rapid depolarization- results in reverse of polarity

- or just local response (potential) if it does not reach threshold


a. threshold current (-30 to -55 mV)b. AP regenerative after threshold (self-perpetuating)


4. overshoot: period of positivity in ICF5. repolarization

a. return to Vrest

b. after-hyperpolarization


6. accommodationa. time-dependent decrease in excitability b. result of a series of subthreshold depolarizationsc. threshold increasesd. the slower the rate of depolarization (current intensity), the greater the in thresholde. change in sensitivity of ion channels

Action PotentialsC. Refractory period

1. absolute2. relative

a. strong enough stimulus can elicit another APb. threshold is increased

Action PotentialsD. ∆ Ion conductance

- responsible for current flowing across the membrane

Action PotentialsD. ∆ 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-hyperpolarizationcontinued in gK

approaches EK

(EK is about -90 mV)

Gated Ion ChannelsA. Voltage-gated Na+ channels

1. localization


1. localizationa. voltage-gated


1. localizationb. ligand-gated at synapses


1. localizationNa+ channels occupy only a small fraction of surface area100-500 channels/m


2. current flowa. Na+ ions flow through channel at 6000/sec at emf of -100mVb. number of open channels depends on time and Vm


3. opening of channela. gating molecule with a net charge


3. opening of channelb. change in voltage causes gating molecule to undergo conformational change


4. factors contributing to specificitya. anions at mouth of channelb. sizec. ability to dehydrate (shed water of hydration)


5. generation of AP dependent only on Na+

repolarization is required before another AP can occurK+ efflux


6. positive feedback in upslopea. countered by reduced emf for Na+ as Vm approaches ENa

b. Na+ channels close very quickly after opening (independent of Vm)

Gated Ion ChannelsB. Voltage-gated K+ channels

1. slower response to voltage changes than Na+ channels2. gK increases at peak of AP


3. high gK during falling phasedecreases as Vm returns to normalchannels close as repolarization progresses


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.

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

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