Impulse Propagation Through the Cardiac Junctional Regions of the Axolotl and the Turtle

In isolated preparations from the axolotl and turtle hearts thepropagation of impulses through different cardiac tissues was studied. Intracellular records were taken from the various types of fibers. Notchedaction potentials were recorded in two junctional regions: the atrioventricularring (A-V. R.) and the ventricle-bulbus cordis (V-B. C.).The action potential recorded in either the A-V. R. or the V-B. C.regions has a notch in its depolarization phase. The temporal occurrenceof the notch was determined by the activation of the correspondingneighboring tissues. The two components given by the notch can beseparated out by using different experimental variables.The A-V. R. and the V-B. C. junctional regions are constitutedby groups of horizontal fibers. When these are severed from the otheradjacent cardiac tissues, it can be seen that the junctional fibers presentspontaneous activity and that the action potential recorded from themis characterized by a slow rate of rise, absence of a notch, and a slowdiastolic depolarization.In these junctional regions it was found that the conduction velocitywas lower than that of any of the other cardiac tissues (Table 1). Thesejunctional regions are preferential sites for delays and blockages inpropagation.The morphological studies revealed that the cells of the A-V intermediateregion are characterized by:(a) small cross sectional diameter ofcytoplasmic extensions;(b) isolated and scanty myofibrils;(c) abundantglycogen deposits; and (d) sparsity of junctional contacts.The electrophysiological and ultrastructural characteristics reportedhere, point towards the conclusion that in the axolotl and turtle hearts themechanisms that underlie the delays in propagation in the junctional regions are similar to those of the mammalian in spite of the absenceof specialized conductive tissues in the lower species.