Key Points

Heart muscle cells contract in response to an action potential on their membrane. Cells in the SA node region of the atrium depolarize spontaneously and act as the heart's pacemaker.

Action potentials spread electronically from cell to cell over the entire heart. Slow conduction through the AVnode, the only excitation pathway from the atria to the ventricles, allows atrial contraction to precede ventricular contraction.

Rapidly conducting Purkinje fibers cause rapid activation of the ventricle.

• Cardiac muscle has a long plateau in the action potential to maintain a prolonged period of activation.

• The plateau (phase 2) is maintained by opening of slow voltage-gated calcium channels and closure of potassium channels.

• The plateau is preceded by rapid depolarization (phase 0) because of opening of voltage-gated sodium channels, and followed by repolarization (phase 3) because of the spontaneous closure of calcium channels and a reopening of potassium channels.

Essential Medical Physiology, Third Edition l75

Copyright © 2004, Elsevier Science (USA) All rights reserved.

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