For 10 points, name this quantity which changes during an action potential.
In Miledi's oocyte expression assay, mRNA is injected into Xenopus, and then this quantity is measured to determine expression levels. The Drosophila Shaker mutation was in a protein that regulates this value. Proteins that respond to changes in this quantity have six alpha helices and an N-terminal "ball" or "inactivation segment." Patch clamps measure this quantity. The Goldman equation gives this quantity as a function of concentration of three ions. This quantity increases during depolarization, decreases during hyperpolarization, and during the refractory period, remains below the resting level of negative 60 millivolts. It determines whether or not potassium channels are open. For 10 points, name this quantity which changes during an action potential.
In Miledi's oocyte expression assay, mRNA is injected into Xenopus, and then this quantity is measured to determine expression levels.
membrane potential [or membrane voltage; prompt on partial answer]