Well, because this is the voltage that is needed to activate the voltage gated on channels. And this includes not only the sodium, but also the potassium voltage gated on channels. So as the voltage increases from the resting membrane potential of about negative 70 millivolts to about negative 40 millivolts, the voltage gated sodium channels begin to open, and they open very quickly. And this is what initiates the action potential. And the value of negative 70 millivolts is known as the threshold value. So as soon as we reach this threshold value, that will initiate that action potential.