Q.Describe the mechanism by which the human heart beat is initiated and controlled.

Step 1. Initiation. The human heart is myogenic - its cardiac muscle cells can spontaneously and rhythmically depolarise without needing an external nerve signal. The cells that do this fastest, and so set the pace for the whole heart, are the pacemaker cells of the sino-atrial (SA) node in the wall of the right atrium; their spontaneous depolarisation (slow sodium-driven depolarisation followed by rapid calcium-channel-driven depolarisation, then potassium-driven repolarisation) is what triggers every heartbeat.

Step 2. Conduction. From the SA node, the excitation wave spreads across both atria and reaches the atrioventricular (AV) node, where it is briefly delayed; it then travels via the bundle of His down into the interventricular septum and out through the Purkinje fibres, which depolarise the contractile muscle of both ventricles - this ordered sequence ensures the atria contract fully before the ventricles do.

Step 3. Neural control. Even though the beat originates in the heart, its rate is continuously adjusted by the autonomic nervous system: sympathetic neurons (and adrenal-medulla epinephrine) release nor-epinephrine, which binds beta-adrenergic receptors to speed the heart up, while parasympathetic fibres of the vagus nerve release acetylcholine, which binds muscarinic receptors on the SA and AV nodes to slow the heart down. …

Separate the answer into initiation (myogenic SA-node pacemaker), conduction pathway (SA -> AV -> bundle of His -> Purkinje), …

• Describing the heartbeat as nerve-triggered from the outset, rather than recognising it is myogenic and only modulated (not started) by nerves. …