Cardiac muscle has the ability to depolarize and contract independent of the nervous system.
This intrinsic conduction system of the heart consists of specialized cells (SA node, AV node, bundle of His, and Purkinje fibers) that initiate and distribute electrical impulses to the rest of the cardiac muscle to control heart rate and myocardial contraction.
The heart also receives extrinsic regulation by autonomic nerves (sympathetic and parasympathetic). Sympathetic nerves arise from T1–T4 spinal cord levels, course to the heart within cardiac splanchnic nerves, and increase heart rate and force of contractility. Parasympathetic nerves arise from the medulla oblongata, course to the heart within the vagus nerve, and decrease heart rate.
The heart's intrinsic conducting system consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart. In this way the heart depolarizes and contracts in an orderly, sequential manner from atria to ventricles. The sequence of excitation is as follows (Figure 4-5):
Sinoatrial (SA) node. Located in the superior region of the crista terminalis, where the RA meets the SVC.
Function. Serves as the “pacemaker of the heart” and initiates the heartbeat; is altered by autonomic nerves (sympathetic stimulation speeds it up, parasympathetic (vagal) stimulation slows it down). The wave of depolarization spreads down the walls of the atria, stimulating contraction of the myocardium, and eventually reaches and stimulates the AV node.
Vascular supply. Supplied by the artery to the SA node, which arises from the RCA.
Atrioventricular (AV) node. Located in the interatrial septum directly above the opening of the coronary sinus.
Function. This node receives impulses from the SA node and passes them to the AV bundle (of His). The AV node also receives autonomic branches, primarily from sympathetics.
Vascular supply. Supplied by the PDA, which arises from the RCA.
AV bundle (of His). Arises at the AV node and descends through the fibrous skeleton of the heart where it divides into the left and right bundles (of His), corresponding to the left and right ventricles, respectively.
Function. Transmits impulses from the AV node, down the interventricular septum to the ventricles. The left and right bundles give rise to Purkinje fibers, which ultimately distribute the impulse to the ventricular muscle.
The atrioventricular conduction system receives blood supply from the atrioventricular branch of the right coronary artery and the septal perforating branch of the left anterior descending coronary artery. Similarly, the right bundle branch and the posterior division of the left bundle branch each obtain blood flow from both the left anterior descending and right coronary arteries. The posteromedial papillary muscle receives blood supply from one coronary artery, usually the right coronary artery.