Basic mechanism of membrane potential in animals
Three major factors cause the membrane potential- Differential permeability of the membrane to diffusion of ions, The Na+ – K+ electrogenic pump and Trapping of negatively charged anions inside the cell.
Differential permeability of the membrane to diffusion of ions
During resting state the cell membrane is 50 to100 times more permeable to K+ ions out side the cell membrane through the leak channels than Na+ ions due to their concentration gradient. This contributes accumulation of positive charges immediately outside the membrane.
The resting membrane is almost completely impermeable /very slightly permeable to Na+ ions.
The Na+ – K+ electrogenic pump
It generates positive membrane potential out side the cell by actively pumping 3 Na+ ions out of the cell for every 2 K+ ions pumped into the cell against their concentration gradient.
Trapping of negatively charged anions inside the cell
Many intracellular anions are large molecules proteins, organic SO4 and PO4 are trapped within the cell and are attracted to the inner surface of the cell membrane generate negative electrical potential inside the cell.
These three factors are the primary cause of membrane potential. The magnitude of this potential produced by diffusion of ions can be predicted by the Nernst equation.