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As we have seen above, each heartbeat involves precise and accurately timed contraction and relaxation of the heart’s chambers during the cardiac cycle. The five phases of the cardiac cycle are coordinated and timed to split-second accuracy via the cardiac conductive system. This consists of a series of specialised interconnected cardiac muscle fibres which originate in the atria before permeating deep into the ventricles (Figure 3.6a). You may find it useful to think of this system as behaving in a similar way to the cells of the nervous system in that it conducts electrical signals. As these electrical impulses travel through the heart, a coordinated wave of muscular contraction is initiated within the myocardium which corresponds to a single heartbeat. Since the heart is continually beating, this electrical activity within the conductive tissues is continuous and is readily recordable on an ECG.

The cardiac conductive system consists of several distinct regions, as follows: the sinoatrial node (SAN), commonly referred to as the heart’s natural pacemaker because it sets the basic rhythm of the heart. Within the SAN, pacemaker cells spontaneously generate regular electrical impulses called action potentials which then travel rapidly over the atria, initiating atrial systole; the atrioventricular node (AVN), a key region of the conductive system located in the lower portion of the right atrium. Action potentials that have spread over the atria rapidly converge on the AVN and are delayed here for around a tenth of a second, allowing time for blood to pass from the atria into the ventricles. Should the SAN be damaged (e.g. following infarction), the AV node is able to take over the role of pacemaker. The AVN is connected to atrioventricular bundle (AVB), frequently referred to as the bundle of His; this consists of a relatively thick bunch of fibres that extends the length of the interventricular septum before splitting into the right and left bundle branches.

Following the delay at the AVN, the AVB and bundle branches rapidly conduct action potentials into the ventricles to initiate ventricular depolarisation and contraction (systole). The right and left bundle branches split into fine extensions called Purkinje fibres which permeate deeply into the myocardium of the left and right ventricles. These allow rapid propagation of action potentials through the ventricles, ensuring that all the muscle fibres of the ventricles contract in synchrony to enable the most efficient ejection of blood from the heart by ensuring that the pumping chambers contract as a single unit (termed a syncytium).

Figure 3.6a and b The electrical conductive tissues and ECG waves

Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction