Cardiac muscle and the coordination of the heartbeatSpec 7.12(i), 7.12(ii)
In short
Cardiac muscle is myogenic: it contracts and relaxes rhythmically without nervous stimulation. The sinoatrial node sets the pace and sends a wave of excitation across the atria; the atrioventricular node delays it, then the bundle of His and Purkyne fibres carry it to the ventricles, which contract from the apex upwards.
Cardiac muscle is myogenic: it can initiate its own contraction without a nerve impulse, so the heart beats even if its nerves are cut. Nerves only change the rate.
- The sinoatrial node (SAN), a patch of cardiac muscle cells in the wall of the right atrium, is the pacemaker. It generates a wave of depolarisation (excitation) at regular intervals.
- The wave spreads across the walls of both atria, which contract together (atrial systole).
- A layer of non-conducting fibrous tissue between the atria and the ventricles stops the wave passing directly to the ventricles.
- The wave reaches the atrioventricular node (AVN). The AVN delays it for about 0.1 s, giving the atria time to empty and the ventricles time to fill.
- The wave then passes down the bundle of His, which runs through the septum to the apex (bottom) of the heart.
- The bundle divides into Purkyne fibres, which carry the wave up the walls of both ventricles. The ventricles contract from the apex upwards (ventricular systole), forcing blood out through the arteries.
Exam tip:
Explain the delay at the AVN: it makes sure the atria have finished contracting before the ventricles contract, so the ventricles fill fully.
(opens full size in a new tab)Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026