Q.(a) Describe the mechanism by which the human heartbeat is initiated and controlled. OR

The heartbeat is myogenic — it originates within the heart itself at the sino-atrial (SA) node and spreads through the atria, AV node, Bundle of His, and Purkinje fibres to contract the ventricles in a coordinated sequence; its rate is then fine-tuned externally by the autonomic nervous system and hormones.

The human heartbeat is initiated and conducted through the following mechanism:

1. Impulse origin at the SA node — the sino-atrial (SA) node, a small patch of specialised, self-excitable cardiac muscle located in the wall of the right atrium, spontaneously and rhythmically generates electrical impulses on its own, without requiring a signal from the nervous system to start each beat. Because of this, the SA node is called the heart's natural "pacemaker", and the heart is described as myogenic (self-originating its own beat).

2. Atrial contraction — the impulse generated at the SA node spreads rapidly across the muscle of both atria, causing them to contract together (atrial systole) and push blood into the ventricles.

3. Delay and relay at the AV node — the impulse reaches the atrio-ventricular (AV) node, located in the interatrial septum near the junction of the atria and ventricles. The AV node introduces a brief delay before passing the impulse onward, which allows the atria to finish emptying their blood into the ventricles before the ventricles begin to contract.

4. Conduction through the Bundle of His and Purkinje fibres — from the AV node, the impulse travels down a specialised conducting pathway, the Bundle of His, which divides into left and right branches running through the interventricular septum, and then into a network of fine Purkinje fibres that spread rapidly throughout the ventricular walls.

5. Ventricular contraction — the impulse reaching the Purkinje fibres triggers a rapid, coordinated contraction of both ventricles from the apex (bottom) upward (ventricular systole), efficiently ejecting blood into the aorta and pulmonary artery.

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