Voltage-gated ion channels have domains called voltage sensors that are extremely sensitive to changes in the membrane potential: changes above a certain threshold value exert sufficient electrical force on these domains to encourage the channel to switch from its closed to its open conformation.

A change in the membrane potential does not affect how wide the channel is open, but instead alters the probability that it will open. Thus, in a large patch of membrane containing many molecules of the channel protein, one might find that on average 10% of them are open at any instant when the membrane is at one potential, whereas 90% are open after this potential changes.

When one type of voltage-gated ion channel opens, the membrane potential of the cell can change. This in turn can activate or inactivate other voltage-gated ion channels.

Such circuits, which couple the opening of ion channels to changes in membrane potential to the opening of additional ion channels, are fundamental to all electrical signaling in cells.