KEY POINTS
1. Contraction of cardiac muscle cells is triggered by an electrical action potential.
2. The contractile machinery in cardiac muscle is similar to that in skeletal muscle.
3.
Cardiac muscle forms a functional syncytium.4. Cardiac contractions are initiated by action potentials that arise spontaneously in specialized pacemaker cells.
5. A system of specialized cardiac muscle cells initiates and organizes each heartbeat.
6. Cardiac action potentials are extremely long.
7. Membrane calcium channels play a special role in cardiac muscle.
8. The long duration of the cardiac action potential guarantees a period of relaxation (and refilling) between heartbeats.
9. Atrial cells have shorter action potentials than do ventricular cells.
10. Specialized ion channels cause cardiac pacemaker cells to depolarize to threshold and form action potentials.
11. Sympathetic and parasympathetic nerves act on cardiac pacemaker cells to increase or decrease the heart rate.
12. Cells of the atrioventricular node act as auxiliary pacemakers and also protect the ventricles from beating too fast.
13. Sympathetic nerves act on all cardiac cells to cause quicker, more forceful contractions.
14. Parasympathetic effects are opposite to those of sympathetic activation but are generally restricted to the sinoatrial node, atrioventricular node, and atria.
15. Dysfunction in the specialized conducting system leads to abnormalities in cardiac rhythm (arrhythmias).
16. Atrioventricular node block is a common cause of cardiac arrhythmias.
17. Cardiac tachyarrhythmias result either from abnormal action potential formation (by the sinoatrial node or ectopic pacemakers) or from abnormal action potential conduction ("reentry").
18. Common antiarrhythmic drugs affect the ion channels responsible for the cardiac action potential.