Development of the conduction system
Cardiomyocytes form the conduction system of the heart. Molecular genetic studies have revealed the requirement of a functional network of transcription factors to determine whether a myocardial precursor cell turns into either a working cell or a cell belonging to the conduction system. Contrary to previous assumptions, the conduction system itself has no neural crest influence during development. The innervation of the conduction system in adults though is ensured through cardiac ganglia originating from the neural crest. In embryos, the innervation of this system is still missing.
Cells located in the caudal heart tube, derived from the common cardinal vein or right sinus venosus in the sinoatrial region, differentiate to form the sinoatrial node near the left venous valve. This node acts as a pacemaking component of the cardiac conduction pathway. During early development, the entire sinus venosus has a pacemaker activity. In fact, there are precursor cells recognized by specific expression pattern (TBx3, Hcn4) that form the definite sinus node later. The atrioventricular node and bundle of His are formed at the base of the outflow tract shortly after differentiation of the sinoatrial node. Progenitor cells in the atrioventricular junction differentiate to form the atrioventricular node, which serves to regulate conduction between the atria and ventricles. Those cells normally receive impulses from the sinoatrial node via pathways within the crista terminalis. The bundle of His is formed by a cluster of cells near the atrioventricular node, and acts to transfer depolarizing impulses to the ventricular conduction pathways.
Contraction of the heart
The primitive heart tube begins to contract on day 22 of human development, making it one of the few organs that must be functional soon after formation. These initial contractions are typically asynchronous. Spontaneous depolarization is initiated within the myocardial cells of the heart and spreads between adjacent cells via gap junctions. Each heart beat is normally initiated with the spontaneous depolarization of the cells in the sinoatrial node (pacemaker), which has an inherently faster rate of depolarization than any other area of the heart. Depolarization spreads through the crista terminalis and atrial tissue ultimately reaching the atrioventicular node. The atrioventricular node then relays this depolarization through the Bundle of His. The terminal portion of the bundle of His sends one branch to each ventricle, each which extensively branch onto multiple Purkinji fibers that spread electrical depolarization to the myocardial cells within each ventricle. These specialized conduction pathways ensure coordinated and efficient contractions of both atria and ventricles. The sinoatrial node, atrioventricular node, and the bundle of His receive sympathetic and parasympathetic nervous input throughout gestation and after birth. This completes the development of the cardiac conduction system. These autonomic nervous system inputs serve to modify, but not to initiate, the heart rate.