Question

QUESTION ABOUT ACTION POTENTIALS

Answer 1

I understand that Na+ channels open in response to depolarization of the membrane beyond threshold. However, my textbook does not really elaborate on how the membrane reaches the threshold in the first place. How does it do this in general?

This is what I think: some sort of stimulus is applied, which causes some sodium channels to open, which in turn causes Na+ ions to flow inwards. This can generate an AP if the magnitude of current that this stimulus triggered makes the membrane potential reach, or go beyond threshold. When the membrane potential reaches threshold, the remaining neighbouring Na+ channels open and this is what accounts for the rapid depolarization during the AP.

Answer 2

Very nice.

The only thing I would add is that "some sort of stimulus" is a neurotransmitter - a small molecule like dopamine, epinephrine, etc. which binds to the channel protein. This induces a conformational change in the channel protein - pushes it from a closed state to an open state through which ions can flow...