Ionic basis of AP in SA node:-
SA node in party mode ~
The resident of the SA node house, the K+ was leaving the house when it suddenly realizes that there’s party in there. It starts lining up for the party.
Na+ the “funny” chap now arrives at the party, to increase the excitement.
Ca++ comes through different doors - first through the ‘T’ door , to bring the excitement to its threshold. And then, through the ‘L’ door, to take the excitement to its peak.
(At the peak of excitement, K+ realizes it’s time to leave now.)