Excitable cells 3: Ionic basis of APs

Question 1

What is the equilibrium potential and how is it calculated?

Answer 1

Equilibrium potential
- If membrane is selectively permeable for an ion the membrane potential will reach a value where the concentration and electrical gradients balance each other

• Calculated with Nernst Equation

Question 2

Nernst Equation? Explain each term

Equation at room temp?

Answer 2

EA = RT/zF . ln([A]o/[A]i)

EA – Equilibrium potential for ion A (mV)
R – gas constant (8.3 Joule.K-1.mol-1)
T – Temperature in Kelvin
F – Faraday’s constant (96500 Coulombs.mol-1)
z – valency of ion (e.g. +1 for monovalent cations)

EA = 58/z . log10([A]o/[A]i)

Question 3

Which ion EA is resting potential closest to and why?

Answer 3

Resting membrane potential is close to EK (equilibrium potential for K), but actually slightly positive to it

The reason – membrane is not entirely selective for K only

Question 4

Main discerning factor for membrane potential?

Answer 4

Permeability of membrane to ions

Question 5

Why does membrane potential drastically change during APs?

Answer 5

due to voltage sensitive ion channels opening and closing

Question 6

What is a voltage clamp and how does it work? Why is it better than

Answer 6

Technique is a way of controlling membrane potential and measuring movement of charged ions across the membrane as electric current

Used to measure very rapid current events with high resolution

A feedback amplifier (FBA) supplies current to rapidly return membrane potential to a chosen level (command potential) -> keeps membrane potential constant
The supplied current compensates for (and is therefore a measure of) membrane currents

Question 7

Diagram showing relationship between Na channel inactivation, deactivation, activation and Open+Closed+Inactivated states

Reference absolute refractory period

Answer 7

Depolarisation ————————>

       activation     inactivation
          ------>            --------> Closed             Open            Inactivated