Electrochemical Potentials
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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.

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Electrochemical Potentials

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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


RT [Na+]out

___

___

ENa =

ln

[Na+]in

zF

R = Gas constant

T = Temp K

z = valence of X

F = Faraday’s constant

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).

For K+ around -90mV

For 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


Resting Membrane 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 Membrane Potential

C. Role of active transport

ENa is +55 mV in human muscle

Vm is -65-70 mV in human 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. 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

- 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 (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 Vrest

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 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

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 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. 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 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.


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