Functional morphology of the mammalian sinuatrial node.

The primary pacemaker area is located at the site with lowest percentage of myofilaments and the highest rate of diastolic depolarization in rabbit, guinea-pig, cat and pig. All investigated sinuatrial nodes contained large amounts, 45% or more, of collagen. There was, however, substantially more collagen in the sinuatrial nodes of the cat and the pig than in the rabbit and the guinea-pig. This had, however, no consequences for the sinuatrial conduction time and the regularity of the beat-to-beat cycle length in the different species, because the rabbit and cat had comparable sinuatrial conduction times, although their nodal collagen content was very different and the beat-to-beat cycle length showed a comparable variability in the different species. We conclude that extensive proliferation of collagen and fibroblasts does not necessarily impair intercellular coupling in the sinuatrial node.