**Question:** Pacemaker of the heart is:

A. Sinoatrial (SA) node
B. Atrioventricular (AV) node
C. Purkinje fibers
D. Myocardium

**Core Concept:**
The heart has a natural pacemaker called the sinoatrial (SA) node, which is responsible for initiating the electrical impulse responsible for the heart's contractions. The SA node is located in the right atrium, near the superior vena cava and coronary sinus ostia.

**Why the Correct Answer is Right:**
The SA node is the primary pacemaker of the heart due to its properties like automaticity, sensitivity to calcium ions, and refractory period. Automaticity refers to the ability to generate electrical impulses without external stimulation. The SA node's sensitivity to calcium ions allows it to maintain a regular pace during normal conditions and in response to various stimuli. The refractory period prevents the SA node from generating impulses immediately after a contraction, ensuring proper coordination between atrial and ventricular contractions.

**Why Each Wrong Option is Incorrect:**

A. The atrioventricular (AV) node is the second pacemaker responsible for transmitting the electrical impulse from the atria to the ventricles. Although the AV node plays a role in the heart's rate and rhythm, it is not the primary pacemaker.

B. Purkinje fibers are responsible for conducting the electrical impulse from the atria to the ventricles, ensuring proper contraction coordination. However, Purkinje fibers are not the primary pacemaker itself.

C. Myocardium is the heart muscle tissue that contracts and pumps blood. While myocardium contributes to the heart's overall function, it is not the primary pacemaker.

**Clinical Pearl:**
The heart's pacemaker function is crucial for maintaining normal heart rate and rhythm. Any disturbance in the normal functioning of the pacemaker can lead to arrhythmias, affecting the heart's ability to pump blood efficiently and causing symptoms like palpitations, fatigue, chest pain, and syncope.