13.01.2 Cardiac Action Potentials

Figure 13.1 Cardiac action potentials (Copyright QUT, Sheila Doggrell)

The His-Purkinje system is responsible for conducting the impulse around the heart causing it to beat. In the His-Purkinje system, there is a more complex action potential; initially there is a resting membrane potential, then a rapid depolarisation that gives a fast response, followed by 3 phases of repolarisation (rapid, slow, and delayed) (Figure 13.1.1). The rapid depolarisation is due to sodium channels opening with Na⁺ moving into the cardiac cells. Calcium channels open letting Ca²⁺ into the cell and then close during the slow phase of repolarisation. During the delayed repolarisation delayed outward K⁺ channels open, and K⁺ moves out of the cardiac cells. Thus, drugs that act at Na⁺, Ca²⁺, and K⁺ channels can modify cardiac action potentials and, consequently, alter the activity of the heart. In particular, the ion channel modulators can alter cardiac arrhythmias.

To understand how the drugs used to treat cardiac arrhythmias work, it is necessary to know about cardiac action potentials and the mechanisms of arrhythmias. Cardiac action potentials are different in the sino-atrial node (pacemaker) and the His-Purkinje conducting system to the ventricles. At the sino-atrial node, there is a slow response made up of a gradual depolarization, and then more definite depolarisation, followed by repolarisation (Figure 13.1.1), and this process is continual. The more definite depolarization is due to the opening of Ca²⁺ channels with Ca²⁺ entry into the cells. With every slow response, a heart beat is initiated.