Porth's Essentials of Pathophysiology, 4e

19

Cell Structure and Function

C h a p t e r 1

2 K +

Resting membrane potential (RMP). The RMP, which is neces- sary for electrical excitability, is pres- ent when the cell is not transmitting impulses. Because the resting mem- brane is permeable to K + , it is essen- tially an K + equilibrium potential. This can be explained in terms of the large K + concentration gradient (e.g., 140 mEq/L inside and 4 mEq/L outside), which causes the positively charged K + to diffuse outward, leav- ing the nondiffusible, negatively charged intracellular anions (A – ) behind. This causes the membrane to become polarized, with negative charges aligned along the inside and positive charges along the outside. The Na + /K + membrane pump, which removes three Na + from inside while returning only two K + to the inside, contributes to the maintenance of the RMP. 3 Action potentials. Action poten- tials involve rapid changes in the membrane potential. Each action potential begins with a sudden change from the negative RMP to a positive threshold potential, causing an opening of the membrane chan- nels for Na + (or other ions of the action potential). Opening of the Na + channels allows large amounts of the positively charged Na + ions to diffuse to the interior of the cell, causing the membrane potential to undergo depolarization or a rapid change to positive on the inside and negative on the outside. This is rap- idly followed by closing of Na + chan- nels and opening of the K + channels, which leads to a rapid efflux of K + from the cell and reestablishment of the RMP. 4

K +

Na + /K + pump

A -

K +

Concentration gradient for K +

3 Na +

Outside cell

Inside cell

Na +

Na +

Na +

Na +

Na +

Triggering event opening Na + channels

Outside cell

Inside cell