A restudy of cardiac conduction pathways by techniques for visualization of cholinesterase reaction.

Histochemical reactions which demonstrate cholinesterase reactions in tissues were used for slides of serial frozen sections of hearts of pigs, dogs, and rats to determine whether there are special types of modified muscle cells in continuous pathways from the SA (sinoatrial) to the AV (atrioventricular) node. There were positive reactions for acetylcholinesterase with less reaction for butyryl cholinesterase in ganglion cells and nerve fibers. No continuous pathways of cholinesterase-reacting cardiac muscle fibers from the SA to the AV node were identified although the muscle fibers were in intimate relation with the nerve fibers. No cells of Purkinje type were demonstrated in the atria.