Umkehrpotenzial

Umkehrpotenzial, German for "reversal potential," refers to a specific voltage at which the net flow of a particular ion across a cell membrane reverses direction. This concept is fundamental in understanding the electrical activity of cells, particularly neurons and muscle cells.

In electrophysiology, ion channels in the cell membrane allow ions like sodium (Na+), potassium (K+), and calcium (Ca2+) to flow across. This flow is driven by both the concentration gradient of the ion and the electrical potential difference across the membrane. The Umkehrpotenzial for an ion is reached when the electrical driving force acting on that ion exactly balances its chemical diffusion force. At this potential, there is no net movement of that ion.

The Umkehrpotenzial for a specific ion can be calculated using the Nernst equation. The Nernst equation takes into account the ion's charge, the extracellular and intracellular concentrations of the ion, and the temperature. For example, the Umkehrpotenzial for potassium (Ek) is typically around -90 mV, while the Umkehrpotenzial for sodium (ENa) is around +60 mV.

Changes in ion permeability across the cell membrane alter the membrane potential. When a cell is permeable to multiple ions, the actual membrane potential will be a weighted average of their respective Umkehrpotenziale, with the weighting determined by the relative conductances (permeabilities) to each ion. This principle is crucial for understanding how electrical signals, such as action potentials, are generated and propagated in excitable cells.