Development of the conduction system
- Cells 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 is responsible for the pacemaking component of the cardiac conduction pathway.
- The atrioventricular node and bundle of His are formed at the base of the outflow tract shortly after differentiation of the sinoatrial node.
- Cells in the atrioventricular junction differentiate to form the atrioventricular node which serves to regulate conduction between the atria and ventricles, and normally receives impulses from the sinoatrial node via pathways within the crista terminalis.
- The bundle of His is formed by a cluster of cells nearby 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, making it one of the few organs that must be functional very soon after formation. These initial contractions are typically asynchronous.
- Spontaneous depolarization is initiated within the myocardial cells of the heart itself 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 to the atrioventicular node and then through the Bundle of His.
- The terminal portion of the bundle of His sends one branch to each ventricle which each extensively branches into multiple Purkinji fibers that spread electrical depolarization to the myocardial cells within each ventricle.
- These specialized conduction pathways ensures a coordinated and efficient contractions of the right and left atria and ventricles.
- The sinoatrial and atrioventricular node as well as the bundle of His receive sympathetic and parasympathetic nervous input throughout gestation and after birth, so to complete the development of cardiac conduction system. These autonomic nervous system inputs serve to modify, but not to initiate the heart rate.
Types of tissue involved in the conduction pathway
- Staining of myocardial tissue for antibodies for two specific proteins can be employed for differentiation between tissues that are primary contractile and those that primarily function in electrical impulse conduction: e.g., Connexin40 (constituent of rapidly conducting cellular connections) and atrial natiuretic factor.
- The myocardium of the primary heart tube stains negative for connexin40 and atrial natriuretic factor and is thus primarily considered to consist of slow conducting tissue.
- Parts of the developing cardiac chambers (secondary myocardium), including walls of the atrial appendages and the apical components of both ventricles stain positive for both connexin40 and atrial natriuretic factor. These tissues are referred to as the chamber tissue and are functionally contractile, conducting cardiac impulses rapidly.
- The primary atrial septum grows from the roof of the common atrium, derived from myocardial cells that migrate into growing heart from the dorsal mesocardium (mediastinal mesocardium). This tissue is distinct from both the primary myocardium and chamber myocardium, as it stains positive for atrial natriuretic factor but negative for connexin40.
- The cranial portion of this primary atrial septum forms the primordium for formation of the sinoatrial node, while the caudal portion, continuous with atrioventricular canal, provides the primordium for formation of the atrioventricular node.
- The myocardium that surrounds the opening of the pulmonary vein into the left atrium originates from mediastinal myocardium and like the primary atrial septual tissue, stains positive for atrial natriuretic factor and negative for connexin40.
- The pulmonary veins are later enclosed in working myocardium, referred to as myocardial sleeve, via transformation of surrounding mesenchymal cells.