Electrophysiological and anatomical observations on the heart of the African lungfish

Electrophysiological and histological observations were made on the heart of the African lungfish. Impulse origin and propagation were studied using simultaneously recorded epicardial and pericardial electrograms. The primary pacemaker site in the lungfish was found to be at the sinus venosus at its junction with the left cardinal vein. Under a variety of circumstances, pacemaker function shifted to other sites. In response to stress, probably under vagal influence, the regular and rapid sinus venosus rate was generally superseded by an irregular and slower atrial pacemaker. Heart rate and sinoatrial and atrioventricular conduction times varied with changes in temperature [Q10=3.77, 2.55, and 5.46, respectively]. Although alterations in impulse formation and conduction did occur, the site of impulse formation and the patterns of conduction between heart chambers were usually fixed, implying the existence of an organized conduction system. Nonetheless, extensive histological study failed to disclose either organized nodal structures or specialized conduction pathways.