23.8 : Conduction System of the Heart

Autorhythmicity is a term that refers to the heart's inherent ability to generate electrical signals and instigate muscle contractions. This self-regulating conduction system within the heart consists of two key components: the pacemaker cells and specialized conducting cells.

The pacemaker cells are located in two primary nodes: the sinoatrial (SA) node and the atrioventricular (AV) node. The SA node pacemaker cells can autonomously depolarize, triggering an action potential that leads to the contraction of the atria. This electrical potential then travels to the AV node via specific pathways, pausing briefly to ensure complete atrial contraction before proceeding.

From the AV node, this electrical impulse is transmitted through the AV bundle or the bundle of His, which bifurcates into the left and right bundle branches. Eventually, it reaches the Purkinje fibers, initiating the contraction of the ventricles.

The SA node inherently produces cardiac signals at a rate of roughly 100 contractions per minute. However, the autonomic nervous system (ANS) and various hormones can adjust both the timing and strength of each heartbeat. In a state of rest, the parasympathetic division of the ANS reduces the SA node pacing to approximately 75 contractions per minute. Conversely, the sympathetic division can increase this rate. So, autorhythmicity plays a crucial role in regulating our heartbeat, adapting to the body's needs in different states of activity or rest.