Absolute refractory period Action Potential

Question

I know that in this period a new action potential cannot occur because the membrane is still depolarizing from the preceding AP

also I know that is because Na+ channels are inactivated. but aren't the Na+ channels inactivated when it is depolarizing? isn't it a conflict? Or I have got everything wrong?

Thank you

Answer 1

Hello Ava! Hope you find this helpful. For more inquiries kindly message me.

During Resting Membrane Potential (RMP) the membrane is negatively charged, but outside it is positively charged (take not that these are net charges). This is because in RMP the sodium (Na) channel are inactive for (closed) whereas potassium (K) are open (active). This leads to an influx of 2K and ex-flux of 3Na, thus a net charge of negative inside. Normally a normal RMP is -90mV.

When a impulse excites the membrane an Action Potential (AP) is fired. This will cause the the Na channels to open (activated) which will cause more influx of 3Na and ex-flux of 2 K. This cause the membrane to be more positively charged. At this point the membrane is DEPOLARIZED!

Take note: in RMP Na channels inactive (closed), and in Depolarization Na channels are activated (open).

Another Important feature to note is that the Na channel (each of them) has to 2 gates. 1) Activation gate (outer part of membrane) and 2) inactivation gate (inner pet of membrane). These open or close at a given time and here comes the main point of the answer to your question. During RMP Activation gate is closed and inactivation is open, so no Na enters and membrane is more negative. During and AP both gates are open so Na quickly enters the membrane. And shortly after AP is the inactivation gate is closed (NOTE: but membrane is still activated), this is where it may appear to be as a conflict. So when the texts say that during the refractory period, it is depolarized period, but Na channel is closed, it simply means that the inactivation gate is close though at this point in time the membrane is still depolarized, so it will not accept any incoming AP.

Lastly os the membrane Potential returns to normal, the inactivation gate will open, and the activation gate will close, and cycle repeats.

Hope this helps. Message me for more. Good luck and Happy studying!