When the membrane potential decreases to a threshold value of -60 mV, the voltage sensitive external Na gates open.

• Once the first Na⁺ gates open positive Na ions move inside the axon causing a further decrease in the membrane potential, which will result in more Na⁺ gates opening.

• This sequence of events represents a positive feedback mechanism that leads to an explosive opening of more and more Na⁺ channels"

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission