After the initial sodium channel opens, what causes the other sodium channels to open?

More sodium ions diffuse into the cell than potassium ions diffuse out of it. the inactivation gates of the voltage-gated sodium ion channels begin to open and the diffusion of sodium ions decreases. After the passage of the action potential, thesodium-potassium pump reestablishes the resting membrane potential. i hope it helps 

The truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey