Structural determinants of slow conduction in the canine sinus node.

Introduction: To elucidate the role of tissue structure as a determinant of the unique conduction properties of the sinus node, we compared the spatial distribution of intercellular connections at gap junctions in the sinus node to the more rapidly conducting crista terminalis and left ventricle, which have been studied previously.
Methods and Results: Samples of four canine sinus nodes were prepared for electron microscopy. The total number and spatial orientation of neighboring myocytes connected by ultrastructurally identified intercalated disks and gap junctions to nine randomly selected index cells were determined by sequentially examining subserial sections. Sinus node cells were sparsely interconnected compared to the extent of interconnections observed previously in other tissues. A typical sinus node cell was connected to only 4.8 +/- 0.7 neighbors compared with 11.3 +/- 2.2 cells in the left ventricle and 6.4 +/- 1.7 cells in the crista terminalis. Sinus node interconnections occurred at small intercalated disks that usually connected cells in partial side-to-side and end-to-end juxtaposition. In contrast, left ventricular myocytes are interconnected at large intercalated disks that adjoin many cells in pure side-to-side and end-to-end orientations. Crista terminalis myocytes are connected primarily in end-to-end fashion. The aggregate gap junction profile length per unit myocyte area was 26.5 times greater in the left ventricle and 5.0 times greater in the crista terminalis than in the sinus node.
Conclusion: Sinus node myocytes exhibit small, sparsely distributed gap junctions that interconnect cells in complex patterns of lateral and terminal apposition. These structural features are consistent with the unique conduction properties of the sinus node.