ANATOMY

Detailed explanation-1: -The SA node starts the sequence by causing the atrial muscles to contract. That’s why doctors sometimes call it the anatomical pacemaker. Next, the signal travels to the AV node, through the bundle of HIS, down the bundle branches, and through the Purkinje fibers, causing the ventricles to contract.

Detailed explanation-2: -The SA (sinoatrial) node generates an electrical signal that causes the upper heart chambers (atria) to contract. The signal then passes through the AV (atrioventricular) node to the lower heart chambers (ventricles), causing them to contract, or pump. The SA node is considered the pacemaker of the heart.

Detailed explanation-3: -The autonomic nervous system tightly controls input into the sinus node. The autonomic fibers regulate the firing of the sinus node to initiate the start of subsequent cardiac cycles and thus, influence the heart rate.

Detailed explanation-4: -The sinus node sends electrical signals that typically start each heartbeat. These electrical signals move across the atria, causing the heart muscles to squeeze (contract) and pump blood into the ventricles.

Detailed explanation-5: -A-V Valves Open; Semilunar Valves Closed This is the first phase of the cardiac cycle. Electrical depolarization of the atria (corresponding to the P wave of the ECG) starts this phase of atrial muscle contraction.