Geometry of cell and bundle appositions in cardiac muscle: light microscopy.

A strand each of cardiac conduction and working cells of the left ventricle is studied in serial sections with the light microscope to define the geometry of cell appositions that form networks of cardiac muscle cells. Anatomic and thus electrical coupling is very frequent among all cells; it is accomplished within a few hundred micrometers axially regardless at which point of the strand electrical current is assumed to originate. Most individual cardiac myocytes are not only connected in longitudinal direction but also make lateral contacts. Only a few bundles of varying diameters remain unconnected over appreciable distances of greater than 200 micron (so-called unit bundles). Thus abnormal current vectors are averted, at least in normal cardiac tissue, even if excitation were to originate from a point. Plastic thick sections studied with the light microscope were unsuitable to define cell lengths.