Question

Describe the Nernst and the GHK equations. What do each equation tell us? Using the GHK principles of the basis for membrane potential, briefly discuss a small experiment to show that the peak of the AP is dependent on activation and opening of Na+ channels

Answer 1

Goldman–Hodgkin–Katz (GHK) equation establishes as relation between the resting potential and the concentration of ions on both the sides of the membrane and also the respective membrane permeability for each ion.

Three assumptions were made – a) constant electric field across the membrane; b) Nernst Plancks equation is applicable and c) Ions move independently. The equation is as follows -

IS is the current carried by ion across the membrane

PS is the permeability

zS is the charge of ion

Vm is the transmembrane potential

F is the Faraday constant

R is the gas constant

T is the absolute temperature

[S]i is the intracellular concentration of ion

[S]o is the extracellular concentration of ion

Nernst Equation -

Nernst potential is the resting potential at which electrical and chemical driving forces are equal or balanced.

Experiment –

Hypothesis – Peak action potential is dependent on activation and opening of Na+ channel.

· Use an electronic device voltage-clamp amplifier to stabilize the membrane potential to any desired level.

· Measure the current at this stabilized voltage.

· The current is due to the ions movements hence the current is equivalent to permeability of ions through the membrane.

· Membrane potential is then clamped to different voltages and at each voltage reading changes in Na+ conductances is measured as

· The higher the cell is depolarized, the greater is the Na+ conductance