Electrochemical Potentials

A. Factors responsible1. ion concentration gradients on either side of the membrane

- maintained by active transport

Electrochemical Potentials

A. Factors responsible2. selectively permeable ion channels

B. Gradients not just chemical, but electrical too1. electromotive force can counterbalance diffusion gradient2. electrochemical 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).

RT [Na+]out

[Na+]in

ENa = zF

ln___ ___

R = Gas constantT = Temp Kz = valence of XF = Faraday’s constant

For K+ around -90mVFor Na+ around +60mV

Resting Membrane Potential

A. Vrest

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 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 Membrane Potential

C. Role of active transportENa is +55 mV in human muscleVm is -65-70 mV in human muscle

Resting Membrane Potential

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. voltage-gated Na+ and K+ channels

Action PotentialsB. Properties

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

Action PotentialsB. Properties

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

Action PotentialsB. Properties

3. depolarization to threshold

- rapid depolarization- results in reverse of polarity

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

Action PotentialsB. Properties

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

Action PotentialsB. Properties

4. overshoot: period of positivity in ICF5. repolarization

a. return to Vrest

b. after-hyperpolarization

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. localizationa. voltage-gated

Gated Ion ChannelsA. Voltage-gated Na+ channels

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

Gated Ion ChannelsA. Voltage-gated Na+ channels

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

Gated Ion ChannelsA. Voltage-gated Na+ channels

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

Gated Ion ChannelsA. Voltage-gated Na+ channels

4. generation of AP dependent only on Na+

repolarization is required before another AP can occurK+ efflux

Gated Ion ChannelsA. Voltage-gated Na+ channels

5. 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

Gated Ion ChannelsB. Voltage-gated K+ channels

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

Gated Ion ChannelsB. 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.