The title says it - I wonder why an increase in extracellular Sodium (Na+) concentration increases action potential amplitude?
What I understand:
I understand that an influx of positively charged Na+ ions into the cell leads to depolarization of the membrane potential (which in a positive feedback loop leads to more ion channels being permeable to Sodium, and hence further depolarization).
I understand that according to the Goldman equation, an increase of extracellular Na+ leads to a less negative resting membrane potential.
What I don't understand:
If the action potential generation happens within a specific time window, and the influx rate of Na+ does not change due to a change of the extracellular concentration, the change in membrane potential during an action potential should be the same (as with a lower level of Na+).
Is the higher action potential amplitude hence due to the less-negative starting point?