Membranpotential

Membranpotential, or membrane potential, is the voltage difference across the plasma membrane of a cell. It results from the uneven distribution of ions between the intracellular and extracellular compartments and from the selective permeability of the cell membrane. In most animal cells, the resting membrane potential is negative inside relative to the outside, typically around -70 millivolts, but it can range from about -40 to -90 mV depending on cell type.

The maintenance of the membrane potential depends on ion gradients and membrane permeability. The main permeant ions are potassium, sodium, chloride, and calcium. The cell membrane is more permeable to potassium than to sodium at rest, largely due to leak channels, and the Na+/K+-ATPase pump maintains the gradients by actively transporting Na+ out and K+ in. Because of these factors, the resting potential is close to the equilibrium potential of potassium, with contributions from other ions weighted by their permeabilities. Equilibrium potentials for the ions can be estimated with the Nernst equation; for example, E_K is about -90 mV, E_Na about +60 mV, and E_Cl around -70 mV. The overall resting potential is determined by the Goldman-Hodgkin-Katz equation, which accounts for multiple permeant ions.

During excitation, voltage-gated ion channels alter membrane permeability. An action potential occurs when depolarization opens voltage-gated Na+ channels, driving the membrane toward the Na+ equilibrium potential, followed by K+ channel opening and repolarization. The Na+/K+-ATPase helps restore ion gradients after activity. Measurement methods include microelectrodes and patch-clamp techniques.

Membrane potential is essential for electrical signaling in nerves and muscles, and is sensitive to changes in ionic concentrations and channel activity.