Q.22 Which one of the following is known as pacemaker of the heart?

(1) Sino-Atrial Node

(2) Atrio-Ventricular Node

(3) Purkinje Fibres

(4) Bundle of His

Pacemaker of the Heart: SA Node MCQ Answer Explained

The pacemaker of the heart is the sino-atrial (SA) node, which generates the primary electrical impulses to initiate each heartbeat. The correct answer to the question is option (1) Sino-Atrial Node.

Correct Answer

Sino-Atrial Node (Option 1). Located in the right atrial wall near the superior vena cava entrance, the SA node spontaneously depolarizes at 60-100 times per minute, setting the heart rate and propagating impulses via gap junctions to atrial myocardium.[ from prior context on biology pathways]

Option Explanations

Sino-Atrial Node (Option 1)

The SA node contains pacemaker cells with unstable resting potentials due to funny current (If) channels and T-type calcium channels, leading to automaticity. It is innervated by sympathetic (accelerates) and parasympathetic (slows) nerves, making it the primary pacemaker.

Atrio-Ventricular Node (Option 2)

The AV node, in the interatrial septum, receives SA impulses after atrial depolarization with a 0.1-second delay for ventricular filling. Its intrinsic rate is slower (40-60 bpm), acting as a secondary pacemaker if SA fails.

Purkinje Fibres (Option 3)

Purkinje fibres form the terminal conduction network in ventricular myocardium, rapidly spreading impulses for synchronized contraction at 20-40 bpm intrinsic rate. They serve as a tertiary (escape) pacemaker but not the primary one.

Bundle of His (Option 4)

The bundle of His (AV bundle) connects AV node to ventricular septum, splitting into left/right bundle branches at 30-40 bpm intrinsic rate. It conducts but lacks the spontaneous depolarization of true pacemakers.

Cardiac Conduction System

Impulses follow: SA node → atrial contraction → AV node (delay) → bundle of His → bundle branches → Purkinje fibres → ventricular contraction.