A thin lipid bilayer separates the interior of a neuron from the extracellular fluid. Because ion concentrations differ across this membrane and because the membrane is selectively permeable, a voltage difference exists. That voltage is the membrane potential.

At rest it typically sits near –70 mV, inside negative. The value is determined by the gradients of potassium, sodium, and other ions together with the relative permeabilities set by open channels. Any change in permeability or in the activity of pumps alters the potential. Depolarization and hyperpolarization are simply shifts of this voltage in the positive or negative direction.

All electrical signaling—synaptic potentials, receptor potentials, action potentials—consists of rapid, controlled changes in membrane potential. Measuring it with intracellular electrodes or estimating it with voltage-sensitive dyes remains fundamental to cellular neurophysiology.

Even small sustained shifts in resting potential can dramatically alter the excitability of a circuit.