We learn in Physiology that during the depolarization phase in the membrane, more sodium channels tend to open as the membrane potential increases i.e., less negative. This is a kind of positive feedback which leads to the action potential and explains the threshold of depolarization that is required for action potential.
So in short, as the membrane potential becomes less negative sodium channels tend to open.
On the other hand there is famous graph from Bertram Katzung, Susan Masters, Anthony Trevor-Basic and Clinical Pharmacology, Edition 12. which shows the dependence of the sodium channel function based on the membrane potential preceding the stimulus.
The fraction of sodium channels available for opening in response to a stimulus is determined by the membrane potential immediately preceding the stimulus. The decrease in the fraction available when the resting potential is depolarized in the absence of a drug (control curve) results from the voltage-dependent closure of h gates in the channels.
Both these cases look pretty contraindicating to me. But I attempted to explain this by the three stages of the Na+ channel.
Case 1 - may be addressing the transition from closed to open channels, i.e., opening of the m gate. (I'm just hypothesizing here so correct me if wrong)
Case 2 - as mentioned in the book, talks of closing of the h gate leading to inactivated gates at higher membrane potentials.
Do case 1 and case 2 happen one after another? Are the voltage ranges for each different? Is my explanation for case 1 correct?