Electron microscopic study of the sarcolemma of Purkinje cells of the goat heart.

Three portions were characterized in the sarcolemma of Purkinje cells of the goat heart. These portions, i.e., intercalated disk, internal and surface sarcolemmal portion, show specific and distinctive characteristics. The surface density of both sarcoplasmic reticulum dyads and intercellular junctions were estimated in these three sarcolemmal portions. Dyads occupy 8, 1.6 and 0.3%, respectively, of the area of internal, intercalated disk, and surface sarcolemmal portions. Fasciae and maculae adherentes are 17 and 5% of the intercalated disk and the internal sarcolemma, respectively. The nexal area in the intercalated disk is twice that in the internal sarcolemma. These results are discussed in relation to the excitation-contraction coupling phenomenon and to conduction velocity.