The morphological basis for impulse conduction in the fowl heart

The structures involved in initiating and conducting the cardiac impulse to the contractile myofilaments are the plasma membranes of the cells in normal andbspecialised myocardium, their intercellular junctions and the sarcoplasmic reticulum. These have been studied by a variety of light and electronmicroscopic methods. The stability of intercellular junctions has been investigated by divalent cation chelation and the size of dissociated cells has been measured. Extracellular labelling techniques have been used to confirm the absence of both a transverse tubular system and a subunit pattern at the nexus of intercellular junctions.

The impulse conducting system of the fowl consists of (1) a sinoatrial node, lying between the right venous valve and the right superior vena cava ; 2) an atrioventricular node, lying in the caudal end of the interatrial sepjnrm; (3) an atrioventricular bundle continuing from the atrioventricular node; (4) an interatrial node lying between the atrioventricular node and the left interatrial septum; (5) a right Purkinje ring, which issues from the caudal end of the atrioventricular node in the fibrous ring; (6) an extensive subendocardial and periarterial Purkinje network in the atria and ventricles.

In addition to this well described system in birds, peculiar muscle fibres resembling those of nodal tissue are present at the bases of the valve cusps of the aorta and pulmonary artery in the fowl.

The ultrastructure of normal atrial and ventricular fowl myocardium differs from normal mammalian myocardium in the arrangement of cells in a fibre. The cells of fowl myocardium are much narrower than those in mammalian hearts but are similar in diameter to those of amphibians and reptiles. No transverse tubular system is present in the normal myocardium of the fowl but an extensive sarcoplasmic reticulum makes many contacts with the plasma membranw. The membrame specialisations present at intercellular junctions are similar to those in amphibians and reptiles in that only a few small nexuses are present compared with the large number and size of nexuses in the myocardium of mammals.

The cells of the SA node of the fowl are intermediate in structure between normal and Purkimje cells. The AV node is composed of cells which show a range of ultrastructure varying from true Purkinje cells to normal cells.

The Purkinje cells of the fowl are similar to those of mammals.

The structure of the impulse conducting system of the fowl is compared with its structure in other birds, mammals and cold blooded vertebrated.

The structure of the impulse conducting system of the fowl is compared with its structure in other birds, mammals and cold blooded vertebrated.