Potentials

The membrane potential is the charge at the inner-membrane surface of a cell and takes into account the activities of all movements of ions into and out of the cell. The membrane potential might remain constant over a period of time ... its 'resting' potential. At other times it might be changing over a certain period of time ... an 'action' potential.

An equilibrium potential is a specific membrane potential that will counterbalance the driving force of an ion's concentration gradient thus stopping that ionic current.

Membrane Potentials

Resting Potential

Voltage-gated channels are closed at the resting potential but other activities are moving ions across the membrane affecting the charge at the inner-membrane surface.

• There are potassium ion channels that are not voltage-gated and allow the efflux of potassium ions. This constant loss of positive charge moves the membrane potential in the negative direction. In addition ...
• Sodium-potassium pumps simultaneously move 3 sodium ions (3Na⁺) out of the cell and 2 potassium ions (2K⁺) into the cell. This net loss of a positive charge also moves the membrane potential in the negative direction. However ...
• Sodium-calcium exchangers move 3 sodium ions (3Na⁺) into the cell in exchange for one calcium ion (Ca²⁺). The net gain of a positive charge moves the membrane potential in the positive direction via this mechanism.

The combined activities of these three mechanisms results is a relatively stable membrane potential of ~-90mV; this is the resting potential of cardiac contractile cells.

Action Potential

If positive ions move into the cell (influx) the negativity of the resting potential will become less negative. Change of the membrane potential in this direction is called depolarization. If positive ions move out of the cell (efflux) the membrane potential will become more negative; this is called repolarization. An unstable membrane potential that is depolarizing then repolarizing is called an action potential.

What determines whether an ion will move into or out of a cell, if a channel is open, and cause these changes?

Equilibrium Potentials

Ion movement is determined by two forces; its concentration gradient and the electrical gradient. The internal and external concentrations of the various ions are relatively constant:

• Potassium (K+): 150 inside/ 4 outside
• Sodium (Na+): 20 inside/ 145 outside
• Calcium (Ca++): 0.0001 inside/ 2.5 outside

As described in Currents & Gradients while the concentration gradient creates a current of ions moving in one direction an electrical gradient builds that increasingly opposes that current. A membrane potential (electrical gradient) will eventually be reached where there no longer be a current. This membrane potential at this point is called the equilibrium potential and, for the concentration gradients listed above, are as follows:

• The equilibrium potential for potassium is ~-96mV. This force is sufficient to counterbalance the efflux of potassium down their concentration gradient of 150mM at the inner surface compared to only 4mM at the outer surface.
• The equilibrium potential for sodium is ~+60mV. This force is sufficient to counterbalance the influx of sodium down their concentration gradient of 145mM at the outer surface compared to 20mM at the inner surface.
• The equilibrium potential for calcium is ~+137mV. This force is sufficient to counterbalance the influx of calcium down their concentration gradient of 2.5mM at the outer surface compared to only 0.0001mM at the inner surface.

Updated: 5/2/2015

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