The surface area of sheep cardiac Purkinje fibres

1. Measurements combining the techniques of point counting and line integration were performed on light and electron micrographs of Purkinje fibres from the sheep's heart. The measurements were aimed at determining membrane areas of importance for the cellular electrophysiology of this tissue.2. The mean volume fractions of the cells occupied by various constituents were: myofibrils, 0.234; mitochondria, 0.103; and nuclei, 0.009. The mean volume fraction of the fibres occupied by the interspaces between the tightly packed cells was 0.0023.3. The mean fractions of intercellular surface area occupied by junctional specializations were: nexus, 0.17; desmosome, 0.023; and fascia adherens, 0.014.4. The mean surface to volume ratio of the Purkinje cells and fibres was 0.46 mu(-1) which is 11.5 times the value of the surface to volume ratio of a long right circular cylinder 100 mu in diameter.5. There are two reasons for the increment in the surface to volume ratio of the fibre (when compared to that of a long right circular cylinder 100 mu in diameter): the multicellular composition of the fibres and the extensive folding of the surface of the cells.6. After correction for the intercellular nexal area the surface to volume ratio of a long cylindrical fibre 100 mu in diameter was 0.39 mu(-1), or about 10 times the value for a long right circular cylinder 100 mu in diameter. The surface to volume ratio of the tissue interspaces in the same fibre was 170 mu(-1).7. It was concluded that the total sarcolemmal area in this tissue is great enough so that the specific membrane capacitance could be about 1 muF/cm(2) and the specific membrane resistance 20,000 Omega cm(2).