Conduction System of the Heart

Autorhythmicity refers to the innate ability of the heart to initiate electrical signals and trigger muscle contractions.

This intrinsic conduction system comprises the pacemaker and specialized conducting cells.

The pacemaker cells are present in the sinoatrial or the SA node and the atrioventricular or the AV node, while the conducting cells are distributed throughout the myocardium.

The SA node pacemaker cells can spontaneously depolarize to trigger an action potential, leading to atrial contraction.

This cardiac action potential reaches the AV node via internodal pathways, pausing momentarily, allowing atrial contraction.

This impulse progresses through the AV bundles or bundle of His into the left and right bundle branches.

Finally, it reaches the Purkinje fibers, which begin ventricular contraction.

While the SA nodes naturally generate cardiac signals at a rate of about 100 contractions per minute, the ANS and hormones can modify the timing and strength of each heartbeat.

At rest, the parasympathetic division of the ANS slows the pacing of the SA node to around 75 contractions per minute, while the sympathetic division can elevate it.