ca in the sa and av nodes The inward Ca2+ is the current source for the upstroke depolarization ( Phase 0) There is no nat channels Their upstroke is slower than those in the cardiac muscle cells The speed of the conducted action potential is much slower that that of any other cardiac tissue This feature in the av node leads to an electrical delay between atrial contraction and ventricular contractionICa in the SA and AV nodes • The inward Ca2+ is the current source for the upstroke/depolarization (Phase 0). • There is no Na+ channels. • Their upstroke is slower than those in the cardiac muscle cells. • The speed of the conducted action potential is much slower that that of any other cardiac tissue. • This feature in the AV node leads to an electrical delay between atrial contraction and ventricular contraction. 20