Regulation of Cardiac Activity

Unlike skeletal muscle, which contracts only when it receives a nerve impulse originating from the brain or spinal cord, the heart is described as myogenic: the electrical impulse that initiates every single heartbeat arises spontaneously within specialised cardiac muscle tissue itself, without needing any external nervous stimulus to start it. This is why an isolated heart, or even isolated cardiac muscle tissue, can be shown to continue beating rhythmically in a suitable nutrient solution even after every nerve connecting it to the rest of the body has been severed.

The impulse for each heartbeat originates in a small patch of specialised, self-exciting cardiac muscle called the sino-atrial node (SA node), embedded in the wall of the right atrium near the point where the great veins enter it. The SA node generates spontaneous electrical impulses at a faster intrinsic rate than any other part of the heart's conduction system, and because it is the fastest, it is the SA node's own rate — roughly seventy-two to seventy-five impulses per minute at rest in a healthy adult — that sets the pace for the entire heart; for this reason, the SA node is called the pacemaker of the heart, and it is this natural pacemaker that an artificial pacemaker device is designed to replace or support when it fails.

From the SA node, the electrical impulse spreads rapidly outward across the muscle of both atria, triggering the coordinated atrial contraction (atrial systole) described in the earlier section on the cardiac cycle. The spreading impulse then reaches a second specialised node, the atrio-ventricular node (AV node), situated in the wall of the right atrium close to the atrio-ventricular septum, near the boundary with the ventricles. The AV node briefly delays the impulse for a fraction of a second before passing it on — a deliberate, functionally important delay, since it allows the atria to finish contracting and completely empty their blood into the ventricles before the ventricles themselves begin to contract, ensuring the two events do not overlap and interfere with each other. From the AV node, the impulse passes into a bundle of specialised conducting fibres called the atrio-ventricular bundle, or bundle of His, which runs down through the inter-ventricular septum and then divides into finer branches known as Purkinje fibres, spreading rapidly through the walls of both ventricles. Because the bundle of His and the Purkinje fibres conduct the impulse considerably faster than ordinary cardiac muscle would on its own, they ensure that the impulse reaches essentially the entire ventricular muscle mass almost simultaneously, so that both ventricles contract together, as a single coordinated unit, rather than as an uncoordinated, spreading wave that would pump blood far less efficiently. …