And so these potassium molecules sorry, these potassium mines will move spontaneously in this direction, while at the same time, because we have many more sodium on the outside than the inside, these sodium ions will move spontaneously into the cell. And as they move along their electrochemical gradient, they will cause an increase in the voltage difference across the cell membrane. So remember, for a neuron, the resting membrane potential is around negative 70 millivolts. And so as a result of this, the voltage will actually begin to increase. Now, if the voltage increases to about negative 40 millivolts, this voltage is known as the threshold voltage. Why?