the development of a dual electrical layer on the boundary between the external medium and the contents of a living cell; the outer surface of the cell at rest is positively charged in relation to its content, which has a negative charge.
Constant bioelectric polarization is caused by structural characteristics of biological membranes and by uneven distribution of inorganic ions (primarily K+, Na+, and Cl-) in the cell and in the surrounding medium (electrochemical gradients). The resting potential is the immediate result of polarization. In most living cells the concentration of K+ ions in the protoplasm is 20 to 50 times higher than in the extracellular fluid. The surface membrane of these cells at rest is more permeable to K+ ions than to other cations. Therefore, K+ ions, in diffusing to the exterior from the cell, lead to accumulation of excess positive charges on the external side of the membrane, while on the inside an excess of negative charges develops (seeMEMBRANE THEORY OF EXCITATION). The membrane in its resting state is only slightly permeable to Na+, Ca2+, and Cl– ions, but in the active state there is a selective increase in permeability to one of these ions, leading to a change in polarization (seeBIOELECTRIC POTENTIALS). Thus, the membrane of an excited section of a nerve becomes for a short time permeable to Na+ ions, whose entry into the cell results in depolarization of the membrane.
If depolarization reaches a critical level, an action potential arises. The descending phase of the action potential, in the course of which polarization of the membrane returns to the resting level, is called the repolarization phase of the membrane. Elevation of the resting potential to a level higher than normal results in hyperpolarization of the membrane.
Relative constancy of polarization of a living cell is ensured by the constancy of electrochemical gradients, which in turn is maintained by ionic pumps (seeSODIUM PUMP), which expend energy on the antigradient transfer of ions through the membrane (seeACTIVE ION TRANSPORT).
L. G. MAGAZANIK