Origin and Conduction of Heart Beat

The human heart is myogenic, meaning the cardiac muscle cells themselves can spontaneously and rhythmically depolarise to trigger a heartbeat, with no need for an external nerve signal to start each contraction. The cells that do this fastest, and so set the pace for the whole heart, are called pacemaker cells, located in the sino-atrial (SA) node in the wall of the right atrium. From there the impulse spreads across both atria and reaches a second specialised node on the right atrium's left side, the atrioventricular (AV) node, where it is briefly delayed. Two specialised muscle fibres called the bundle of His then carry the impulse from the AV node down into the interventricular septum, branching into finer Purkinje fibres that spread the signal through both ventricles (see Figure 7.8). At the cellular level, the SA node's pacemaker cells depolarise in two stages: an initial slow depolarisation driven by sodium influx together with reduced potassium efflux, followed by a rapid depolarisation once a threshold potential opens voltage-gated calcium channels, producing a full action potential; the cell then slowly repolarises again through potassium efflux, ready to repeat the cycle. This self-generated rhythm produces the heartbeat proper - the alternating systole (contraction) and diastole (relaxation) of the heart, normally 70-72 beats per minute in an adult - and each beat produces two audible sounds through a stethoscope: the first sound ('lu …