In electrophysiology, E is known as what?

• A. Equilibrium potential
• B. Membrane potential
• C. Resting potential
• D. Action potential

Answer: (A)

Equilibrium potential is what E represents in electrophysiology. It’s the voltage at which the electrochemical forces for a specific ion balance, so there is no net current for that ion. This value is given by the Nernst equation and depends on the ion’s charge, temperature, and the concentration gradient across the membrane. For example, potassium has an equilibrium potential near -90 mV, while sodium is around +60 mV in many cells. The overall membrane potential you measure is not the equilibrium potential of a single ion; it’s a combined result of multiple ions’ conductances and currents at any moment. Resting potential and action potential describe broader states or events of the cell’s voltage, not the specific balance point for one ion.