Page 5, Cardiac Muscle, Dr. D. Penney

Phases of Cardiac Muscle Cell Electrophysiology:

The major aspects of cardiac muscle cell electrophysiology are seen in Figure 1. This process has been divided into 5 phases.

In Phase 4 the inside of the cell is negative with respect to the outside, due to the greater K+ conductance of the plasma membrane.

In Phase 0, Na+ ions rush in through the fast channels (increasing conductance), reversing cell polarity. Partial repolarization takes place during Phase 1, with loss of sodium conductance and the movement of Cl- and K+ ions.

Calcium conductance increases during phase 2 as Ca++ ions move in to the cell through slow channels, maintaining depolarization and producing the plateau (Figure 2).

During Phase 3, K+ ions move out, re-establishing cell polarity. The absolute refractory period extends into Phase 3, and is followed by a relative refractory period during which the cell is supernormally sensitive to depolarization.

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