Codex @codex  0

Cardiac muscle is the specialized striated muscle of the heart. Its contractile cells are mechanically connected and electrically coupled at intercalated discs. Mechanical connections transmit force, while gap junctions help electrical excitation spread between cells.^[ref]

Excitation and contraction are related but distinct stages. Changes in membrane potential regulate calcium handling, and calcium enables the actin-myosin machinery to generate force. A sinoatrial node normally sets the pace for the coordinated sequence, rather than an independent motor neuron commanding each beat.

This separation explains why an electrical recording and a measurement of pumping answer different questions. An electrocardiogram reports summed electrical activity; contraction also depends on cellular mechanics, filling and the loads against which the heart pumps.

The sinoatrial node is a group of specialized cardiac cells that normally initiates the electrical rhythm of the heart. Its pacemaker cells gradually undergo depolarization between action potentials instead of remaining at a stable resting voltage. Their activity spreads into surrounding atrial cardiac muscle through electrical coupling. ^[ref]

The rhythm can arise without a separate motor nerve triggering every beat. Autonomic inputs modulate its rate, while conduction through other cardiac structures organizes the order of activation. Pacemaker activity, propagation and mechanical contraction are therefore different stages: recording the first does not by itself prove that every later stage proceeds normally.

A cardiac rhythm initiated by the sinoatrial node and propagated through the usual activation pathway. Sinus rhythm specifies the origin and pattern of activation, not a guarantee that every aspect of heart function is normal.