Option "C"
During the UPSTROKE of the action potential (ASCENDING LIMB of AP), the depolarizing wave rapidly opens the ACTIVATION GATES of the Na+ channels. This results in movement of Na+ inside the cell (making the AP more POSITIVE near the equilibrium potential of Na+ --> +65 mV).
(This is also the brief period when the AP is positive.)
Remember that since the resting membrane potential is NEGATIVE (-70 mV),
Depolarizing wave, now, will makes it POSITIVE (between 0 and +65 mV mark; approaches Na+ eq. potential, 3 Na+ moves IN and 2 K+ OUT... 3+ IN and 2 + OUT= +ive INside). DEPOLARIZATION= CELL INTERIOR IS LESS NEGATIVE. (Depolarization also closes the INACTIVATION gates but it takes time for the gates to close and they close later during repolarization. The closure of inactivation gates, during repolarization, will block the Na+ entering inside.)
For a wave of AP to propagate, the membrane potential has to come back to its resting state (so that it can be depolarized again; like a cylic thing). REPOLARIZATION now begins by OPENING OF K+ channels (plus the inactivation gates of Na+ are also closing). Gradually the membrane potential will decrease towards resting state.(0 --> -70mV).
because for every 2 K+ that enters inside, 3 Na+ moves out.
(And after REPOLARIZATION, there's a brief period of HYPERPOLARIZATION (Afterpotential) when the AP approaches the eq. potential of K+ --> -85mV. (HYPERPOLARIZATION= MORE NEGATIVE)
This is because the membrane is more permeable to K+ than Na+ at rest. (Eq potential of K+ is near resting membrane potential !!!)
Hope that clears alot!
PS I think I've simplified it to Ground Zero.
