THORAX ABDOMEN AND PELVIS

Detailed explanation-1: -The sinus node continuously generates electrical impulses, thereby setting the normal rhythm and rate in a healthy heart. Hence, the SA node is referred to as the natural pacemaker of the heart.

Detailed explanation-2: -Phase 0 depolarization is primarily caused by increased gCa through the L-type Ca++ channels that opened toward the end of Phase 4. The “funny” currents, and Ca++ currents through the T-type Ca++ channels, decline during this phase as their respective channels close.

Detailed explanation-3: -SA node pacemaker cells depolarize at a rate of 60 to 100 per minute, while the AV node at 40 to 60 per minute. The pacemaker with the highest rate of depolarization takes over as the primary pacemaker. In healthy individuals, this is the SA node.

Detailed explanation-4: -The conduction system of the heart. Left: Normal excitation originates in the sinoatrial (SA) node then propagates through both atria. The atrial depolarization spreads to the atrioventricular (AV) node, and passes through the bundle of His to the bundle branches/Purkinje fibers.