So I understand what you're saying about the absolute refractory period and that the VG-Na+ channels close, but what I'm asking is whether or not ALL of the VG-Na+ channels are inactivated during the absolute refractory period.
Couldn't some VG-Na+ channels not yet be inactivated and can't they fire when a stimulus is applied and produce another AP on top of that AP with a lower stimulus amplitude? Isn't this what happens during temporal summation when you stimulate the neuron BEFORE it can fully repolarize back to its resting membrane potential?
There is a very high probability that all the Na+ channels are closed during the absolute refractory period. The inactivation gates cannot open until a certain level of
repolarization is achieved. So if the cell doesn't repolarize to the potential value where the inactivation gates start opening, they can't open. Until the cell's potential repolarizes to this value where the inactivation gates start opening, it's in absolute refractory period; so, ALL the Na+ channels are very, very likely to be closed during the absolute refractory period.
Also, I can't tell if you're thinking of summation in the sense that it involves action potentials firing. Summation allows the potential to reach threshold to fire an action potential. So you're not firing action potentials rapidly, rather you're using summation to initiate (or inhibit) an action potential. In your example of temporal summation, the frequency of incoming potentials (not action potentials) is high enough that the inward currents sum up to the threshold to produce an action potential. Ex: the first potential causes some depolarization before the ion channels it uses start closing; if the next potential arrives before the membrane potential is repolarized to normal, the inward current causes even more depolarization (since the cell was slightly depolarized from the previous potential), and so on. This continues until the threshold potential for firing an action potential is reached. If the threshold is reached, an action potential is fired; if the threshold is not reached, no action potential is fired.
If the frequency of the temporal summation is slow enough that it depolarizes the cell but not to threshold, yes, some Na+ channels will close until the membrane potential repolarizes to a certain extent. Remember that temporal/spatial summations are
not firing action potentials; they are used to
initiate or inhibit an action potential. They involve neurotransmission and the neurotransmitter can open/close ion channels in the postsynaptic cell. Hope this helps.
PS. Sorry if I repeated things you already knew about summation. I couldn't tell if you were confused as to what summation is or not so I figured I should go the safe way and explain it anyways.
