Membrane Potential

Membrane potential is the difference of electrical 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 rules governing the movement of ions, they move from higher to lower conc

Being charges bearing particles, ions also move away from like charges and toward opposite charges 

Another factor that controls ion movement is 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 

2 solutions of different concentrations of sodium chloride are separated by a membrane

If the membrane is equally permeable to both na and cl, both ions will diffuse from higher to lower conc and the 2 solutions will eventually have the same concentration

The electric charges remain the same on both sides and membrane potential is zero

Assume the membrane is permeable only to 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 the influence of two forces: 

Diffusion force drives them in on direction while electrostatic force drives them in the opposite direction

The equilibrium is reached when these 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

Equilibrium potential=the voltage required to maintain this particular conc gradient and can be calculated as a function thereof

A typical resting neuron maintains unequal distributions of 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 the sodium gradient is directed into the cell, its equilibrium potential must be positive to drive sodium out 

Potassium has the reverse conc gradient, hence negative equilibrium potential 

Chloride has the same inward concentration direction as sodium but because its a negative charge it requires a negative environment inside the cell to push it out

The resting membrane potential of a neuron is about -70 mV

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