Cardiac Cycle - Regulation of The Cardiac Cycle

Regulation of The Cardiac Cycle

Cardiac muscle has automaticity, which means that it is self-exciting. The muscle contractions are myogenic, or generated by the muscle cell itself. This is in contrast with skeletal muscle, which requires nervous stimuli (either conscious or reflex) for excitation. The heart's rhythmic contractions occur spontaneously, although the rate of contraction can be changed by nervous or hormonal influences, exercise and emotions. For example, the sympathetic nerve accelerates heart rate and the vagus nerve decelerates heart rate.

The rhythmic sequence of contractions is coordinated by the sinoatrial (SA) and atrioventricular (AV) nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation that initiates atrial contraction by creating an action potential. Once the wave reaches the AV node, situated in the lower right atrium, it is delayed there before being conducted through the bundles of His and back up the Purkinje fibers, leading to a contraction of the ventricles. The delay at the AV node allows enough time for all of the blood in the atria to fill their respective ventricles. In the event of severe pathology, the AV node can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the pacemaker cells in the SA node and hence is overridden.