Membrane potential

Question 1

Explain the Nernst potential.

Answer 1

The diffusion potential level across a membrane that exactly opposes the net diffusion of an ion.
At equilibrium, the chemical gradient will drive just as many ions in one direction as the electrical gradient drives them in the other direction.
The formula for the Nernst potential is at 37 degrees C and the EC is at 0 potential.
That is, the Nernst potential is the potential inside the membrane.

Question 2

Explain the Goldmann equation.

Answer 2

Used when the membrane is permeable for several ions.

Depends on:

• The charge of the ion
• The permeability of the membrane to the ion
• The concentration of the ion inside and outside the membrane

Question 3

Describe the Na/K pump

Answer 3

3Na out:2K in
Electrogenic pump –> net transport of 1+ to extracellular fluid, therefore making internal cellular fluid negatively charged.
(Actually contributes only about 4mV to this resting membrane potential).

Question 4

What is the resting membrane potential?

Answer 4

-90mV inside the cell.
Mainly caused by the leaky K channels (-86mV).
Small contribution from the Na/K pump (-4mV).

Question 5

Explain the leaky K and Na channels.

Answer 5

Na and K ions are allowed to leak through the membrane.
the channels are 100 times more permeable to Na ions.

Nernst potential for K = -94mV
Nernst potential for Na = +61mV

Question 6

What is the result of decreasing the external Na+ concentration?

Answer 6

Decreasing the external Na+ concentration decreases the size of the action potential (decreases gradient of action potential?).
Has little effect on the resting membrane potential, since the permeability of the membrane to Na+ concentration at rest is relatively low.

Question 7

What is the result of increasing the external K+ concentration?

Answer 7

Increasing the external K+ concentration decreases the resting membrane potential.
Lethal

Question 8

What is the result of decreasing extracellular Ca2+ concentration?

Answer 8

A decrease in extracellular Ca2+ concentration increases the excitability of the nerve and muscle cells by decreasing the amount of depolarisation necessary to initiate the changes in the Na+ and K+ conductance that produce the action potential.
Results in tetany.

Question 9

What is the result of an increase in extracellular Ca2+ concentration?

Answer 9

An increase in extracellular Ca2+ concentration ‘stabilises the membrane’ by decreasing excitability.

Question 10

What is the velocity of an action potential?

Answer 10

Myelin decreases ion flow by 5000 times.
Saltatory conduction increases velocity.
Small unmyelinated fibres = 0.25m/s
Large myelinated fibres = 100m/s

Question 11

What are inhibitors of excitation (membrane stabilising factors)?

Answer 11

Procaine and tetracaine (anaesthetics) = Acts on the activation gate of Na+, decreases Na+ permeability.
TEA = K+ channel blocker.
TTX = Na+ channel blocker

Blockage of K+ channel = slowing repolarisation.
Blockage of Na+ channel = reduces excitability, makes it harder to make an action potential, reduces action potential frequency or even stops it from occurring at all.