[Specific extranodal sinoatrial muscle cells in the crista terminalis. Cyto-morphometric study and anatomoclinical considerations].

To assess the occurrence, location and structure of specialized pacemaker slender cells, beyond the conventional anatomic boundaries of the sinoatrial node, cyto-morphometric evaluations were made in specimens from 10 human hearts. The morphometric analysis showed significant histocytological peculiarities of extranodal slender cells, which presented 60% to 70% smaller than those of the ordinary atrial muscle; their preferential location was at a mean distance of 12 to 14 mm from the posterior tip of the sinoatrial node, beneath the crista terminalis. These morphologic data were taken into consideration with regard to electrophysiologic recording of shifts in pacemaking, within and around the sinoatrial node. It was also remarked that the presence and action of extranodal specialized cells along the crista terminalis are consistent with clinical and pathologic evidence obtained from human cases of sick sinus syndrome, as well as from animal experiments at sinoatrial node damage.