Slide #23

	Action Potential for Cells in Nodes	Action Potential for Myocardial Cells
Phase
0	Caused by a CALCIUM conductance increase allowing IN-FLOW	Caused by a SODIUM conductance increase allowing IN-FLOW
1	NONE	Initial Repolarization cause by potassium OUT-FLOW
2	NONE	Plateau during which Potassium OUT-FLOW is matched by Calcium IN-FLOW
3	Repolarization caused by Potassium OUT-FLOW.	Repolarization during which Potassium OUT-FLOW is increased and Calcium IN-FLOW is decreased.
4	Slow depolarization resulting from Sodium IN-FLOW	Resting Membrane Potential

Notes:

• Potassium OUT-FLOW in phase 3 of the nodal action potential is the stimulator of the Sodium IN-FLOW in phase 4.
• The heart rate is reduced by reducing the slope of phase 4 or by increasing the length of phase 3.

Current through membrane of SA Node Cells: