Natural pacemaker of heart is:
**Core Concept**
The natural pacemaker of the heart is responsible for generating electrical impulses that regulate heart rate and rhythm. This process involves the automaticity of cardiac cells, particularly those in the sinoatrial (SA) node, which possesses a unique electrophysiological property.
**Why the Correct Answer is Right**
The sinoatrial (SA) node, located in the right atrium, acts as the natural pacemaker of the heart. It has the highest rate of automaticity among all cardiac tissues, generating electrical impulses at a rate of approximately 60-100 beats per minute (bpm) at rest. This is due to the presence of a high density of ion channels, particularly the hyperpolarization-activated inward channel (Ih), which allows for the influx of sodium and potassium ions, leading to the depolarization of the SA node cells. The electrical impulses generated by the SA node are then propagated to the atria and eventually to the ventricles, regulating heart rate and rhythm.
**Why Each Wrong Option is Incorrect**
**Option A:** The atrioventricular (AV) node does not have the highest rate of automaticity and is not responsible for generating electrical impulses that regulate heart rate.
**Option B:** The bundle of His is a specialized conduction pathway that transmits electrical impulses from the AV node to the ventricles, but it does not act as the natural pacemaker of the heart.
**Option C:** The Purkinje fibers are specialized cardiac muscle cells that transmit electrical impulses to the ventricles, but they do not have the automaticity to generate electrical impulses on their own.
**Clinical Pearl / High-Yield Fact**
The SA node's automaticity can be influenced by various factors, including temperature, catecholamines, and electrolyte imbalances, which can affect heart rate and rhythm.
**Correct Answer:** C. Sinoatrial (SA) node.