The ultrastructure of alligator conductive tissue: an electron microscopic study of the sino-atrial node.

A combined electrophysiological and morphological study was made to distinguish between the conductive and contractile elements of the alligator mississippiensis heart. Microelectrophysiological recordings revealed a frequent occurrence of cells in the sino-atrial node that showed spontaneous diastolic depolarization, while light and electronmicroscopic studies indicated that the corresponding area is characterized by cells with a high glycogen content, poorly developed sarcoplasmic reticulum and a relative absence of contractile material. The findings provide evidence that the alligator heart is a non-avian and non-mammalian heart that possesses a specialized stimulus-generating and conductive system.