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Membrane potential, or membrane voltage, refers to the DIFFERENCE of electric charges across a cell membrane. Most cells have a NEGATIVE transmembrane potential. Because membrane potential is defined RELATIVE to the exterior of the cell, the negative sign means the cell has MORE negative charges on the INSIDE.

There are 2 basic rules governing the movement of ions:

– they move from HIGHER to LOWER concentration, just like any other molecules;

– being CHARGE-bearing particles, ions also move AWAY from LIKE charges, and TOWARD OPPOSITE charges.

In the case of the cell membrane, there is a THIRD factor that controls ion movement: the PERMEABILITY of the membrane to different ions. Permeability is achieved by OPENING or CLOSING passageways for specific ions, called ION CHANNELS. Permeability can change when the cell adopts a DIFFERENT physiological state.

Consider this example: 2 solutions of different concentrations of sodium chloride are separated by a membrane. If the membrane is EQUALLY permeable to BOTH sodium and chloride, both ions will diffuse from higher to lower concentration and the 2 solutions will eventually have the same concentration. Note that the electric charges remain the same on both sides and membrane potential is zero.

Now let’s assume that the membrane is permeable ONLY to the positively-charged sodium ions, letting them flow down the concentration gradient, while BLOCKING the negatively-charged chloride ions from crossing to the other side. This would result in one solution becoming INCREASINGLY positive and the other INCREASINGLY negative. Since opposite charges attract and like charges repel, positive sodium ions are now under influence of TWO forces: DIFFUSION force drives them in one direction, while ELECTROSTATIC force drives them in the OPPOSITE direction. The equilibrium is reached when these 2 forces COMPLETELY counteract, at which point the NET movement of sodium is ZERO. Note that there is NOW a DIFFERENCE of electric charge across the membrane; there is ALSO a CONCENTRATION gradient of sodium. The two gradients are driving sodium in OPPOSITE directions with the EXACT SAME force. The voltage established at this point is called the EQUILIBRIUM potential for sodium. It’s the voltage required to MAINTAIN this particular concentration gradient and can be calculated as a function thereof.

A typical RESTING neuron maintains UNequal distributions of different ions across the cell membrane. These gradients are used to calculate their equilibrium potentials.  The positive and negative signs represent the DIRECTION of membrane potential. Because sodium gradient is directed INTO the cell, its equilibrium potential must be POSITIVE to drive sodium OUT. Potassium has the REVERSE concentration gradient, hence NEGATIVE equilibrium potential. Chloride has the same INWARD concentration direction as sodium, but because it’s a negative charge, it requires a NEGATIVE environment inside the cell to push it OUT.

The resting membrane potential of a neuron is about -70mV. Notice that ONLY chloride has the equilibrium potential near this value. This means chloride is IN equilibrium in resting neurons, while sodium and potassium are NOT. This is because there is an ACTIVE transport to keep sodium and potassium OUT of equilibrium. This is carried out by the sodium-potassium PUMP which constantly brings potassium IN and pumps sodium OUT of the cell. The resulting resting potential, while costly to maintain, is essential to generation of action potentials when the cell is stimulated.