10.1: Electrophysiology of Normal Cardiac Rhythm

The normal cardiac rhythm is a synchronized electrical activity that facilitates the regular and coordinated contraction of the heart muscle. This process is essential for efficient blood circulation throughout the body. The fundamental elements involved in establishing and maintaining this rhythm include the unique electrical properties of cardiac muscle cells, the sinoatrial (SA) node's pacemaker function, the specialized conducting system, and the ionic mechanisms underlying each phase of the action potential. Cardiac muscle cells exhibit unique electrical properties such as automaticity, excitability, and conductivity. These properties enable the cells to generate and propagate electrical impulses, ensuring a stable and coordinated heartbeat. The SA node in the right atrium serves as the primary pacemaker, initiating the electrical impulse responsible for each cardiac cycle. This impulse propagates through the specialized conducting system, which includes the atrioventricular (AV) node, the bundle of His, and Purkinje fibers, ensuring a rapid and orderly spread of excitation throughout the heart.

The action potential in cardiac cells comprises several phases (0-4), each characterized by specific ionic mechanisms. Phase 0 involves rapid depolarization due to the opening of voltage-gated sodium (Na⁺) channels, whereas phase 1 represents initial repolarization resulting from the transient outward potassium (K⁺) current. In phase 2, the plateau phase, calcium (Ca²⁺) influx through L-type Ca²⁺ channels balances the outward K⁺ current, maintaining the membrane potential. Phase 3 represents rapid repolarization due to increased K⁺ efflux, and phase 4 is the resting membrane potential maintained by the Na⁺-K⁺ pump and background ion channels. Various factors, including heart disease, drugs, and circulating hormones, can disrupt sinus rhythm. Heart diseases, such as ischemia or myocardial infarction, can impair the electrical conduction system. At the same time, certain drugs and hormones can modulate ion channel function, altering the action potential's trajectory and duration. In conclusion, the heart's electrophysiology involves a complex interplay of specialized structures, ion movements, and membrane potentials. These elements work cohesively to generate and maintain the normal cardiac rhythm, ensuring the efficient circulation of blood throughout the body.

The cardiac rhythm, or heartbeat, results from coordinated contractions controlled by electrical signals.

These signals originate in the SA node, which is made of special noncontractile pacemaker cells. The SA node transmits the signals through the AV node and specialized structures, leading to ventricular contraction and blood circulation.

As the pacemaker impulses reach contractile cardiac muscle cells, their voltage-sensitive membrane channels regulate the ion movement to generate a five-phase action potential.

Phase 0 or depolarization involves rapid Na⁺ ion influx, followed by channel inactivation. This leads to partial repolarization in Phase 1.

In Phase 2, slow Ca²⁺ ion influx creates a plateau. Phase 3, or repolarization, involves Ca²⁺ channel inactivation and K⁺ ion outflow.

The rapid potassium efflux returns the membrane potential to its resting voltage in Phase 4.

The electrical waves produced by the heart are transmitted throughout the body and can be detected by an ECG.