Nernst & Goldman Equation

How to find the membrane charge with different ions?

1) Where is the ion most concentrated?
- inside or outside of neuron
2) Due to chemical diffusion where will the ion flow?
- into or out of neuron
3) Given the ion's charge, how will this flux alter the distribution of charges across the mebrane?
- inside more positive or negative

Example: Sodium
1) most concentrated outside
2) will flow in neuron
3) it is positive so inside will become more positive

What is the Nernst Equation?

Allows us to calculate voltage at equilibrium ("Nernst Potential")
- deal with one type of ion at a time

- Ex is the resting potential for an ion
- RT / zF is calculated for us & is 58
- X is the ion concentration in extracellular fluid (ECF) and cytoplasm
- ECF top, cytoplasm bottom
- concentration measured in mM (millimoles)

- 58 for sodium
- negative 58 for chloride because of -ive charge or +ive depending on how equation is used (if fraction flipped its +ive)
- 29 for calcium because denominator in constant is 2 (has a +2 charge)

Equilibrium Potentials of different ions

Sodium = 67 mv
Potassium = -84 mV
Chloride = -67 mV
Calcium = 116 mV

How to find resting membrane potential?

More than one ion flows across the membrane in real neurons so what is the resting voltage of a neuron?
- found using a voltage amplifier & oscilloscope

What two factors determine the resting potential of a neuron?

1) Membrane is primarily permeable to K+ via leak channels
- at rest K+ is mainly flowing
- resting potential closest to Nernst potential for K+

2) K+ IS more concentrated inside vs. outside
- concentration gradient drives it out but the electrical potential difference drives it back in the cell

Goldman Equation

Used to resting membrane potential for a neuron
- potassium & Na are out / in but Cl is in/ out

P = permeability of ion, which is proportional to how many ion channels are open for each ion

At rest, Pk : PNa : PCl = 1.0 : 0.04 : 0.45

- this causes the Vm (membrane potential to be -65 mV)