Porth's Essentials of Pathophysiology, 4e

18

Cell and Tissue Function

U N I T 1

Membrane Potentials

U N D E R S T A N D I N G

Electrochemical potentials are present across the membranes of virtually all cells in the body. Some cells, such as nerve and muscle cells, are capable of generating rapidly changing electrical impulses and transmitting these impulses along their membranes. Generation of membrane potentials relies on (1) diffusion of current-carrying ions, (2) development of an electrochemical equilibrium, and (3) establishment of a resting membrane potential and triggering an action potential.

Diffusion potentials. A diffusion potential is a potential difference generated across a membrane when a current-carrying ion, such as the potassium (K + ) ion, diffuses down its concentration gradient. Two conditions are necessary for this to occur: (1) the membrane must be selectively permeable to a particular ion, and (2) the concentration of the diffusible ion must be greater on one side of the membrane than the other. The magnitude of the diffusion potential, measured in millivolts (mV), depends on the size of the con- centration gradient. The sign (+ or −) or polarity of the potential depends on the diffusing ion. It is negative on the inside when a positively charged ion such as K + diffuses from the inside to the outside of the mem- brane, carrying its charge with it. 1 Equilibrium potentials. An equi- librium potential is the membrane potential that exactly balances and opposes the net diffusion of an ion down its concentration gradient. As a cation diffuses down its concentration gradient, it carries its positive charge across the membrane, thereby gener- ating an electrical force that will even- tually retard and stop its diffusion. An electrochemical equilibrium is one in which the chemical forces driving diffusion and the repelling electrical forces are exactly balanced so that no further diffusion occurs. The equilib- rium potential (EMF, electromotive force) can be calculated by inserting the inside and outside ion concentra- tions into the Nernst Equation. 2

Volts

0

Outside cell

Inside cell

K +

K + –permeable membrane

K +

K +

K +

K +

Concentration gradient for K +

Electrical (ionic) potential

Equilibrium potential

Diffusion (chemical) gradient

Nernst equation EMF (mV) = –61 × log 10 (ion concentration inside/ ion concentration outside)