Functional and morphological organization of the cat sinoatrial node.

The feline sinoatrial node has a unifocal impulse generation as previously described for rodents. Its main component is collagen. The primary pacemaker consists of at most 2000 cells, but appears to function normally with less than 500 cells. Primary pacemaker cells are found in the area where empty cells are predominant. A negative correlation between myofilament density and diastolic depolarization rate, known to exist in the rabbit and guinea-pig, is absent in the cat. Gap junctions are seen in the center and in the periphery of the nodal region, but they are extremely rare. The electrophysiological characteristics of the primary pacemaker of the cat are quite similar to those of the rabbit, although the nodal morphology is very different. Abrupt transitions from one cell type into another are observed in the feline sinoatrial node. From this morphological point of view the feline sinoatrial node resembles the canine and human sinoatrial nodes more than the lapine sinoatrial node.