Where does the action potential begin?
At the beginning of the axon, where it first leaves the soma, there is a concentration of voltage-gated sodium channels. This area is called the initial segment, or axon hillock. The propagation down the axon begins with these channels opening. When these channels open, positive charge enters the cell and spreads passively down the axon. This positive charge trips the next voltage-gated sodium channels on the axon, causing a sodium influx there, which then causes the next channels to open, and so on. Why doesnt the signal go backward? Remember the absolute refractory period. The positive charge will, in fact, passively spread in both directions in the axon. When it reaches the segment that just fired, however, those voltage-gated sodium channels will still be inactivated, and thus unable to fire an AP. How can one ensure that the propagation will make it? Axons can be leaky, and concentration of voltage-gated sodium channels can vary. A situation could arise in which the positive charg
