Structure of the cardiac muscle cell

1. 1. An electron microscopic study of the papillary muscles of rat and dog hearts is reported. The ultrastructural features of the sarcolemma, intercalated disc, myocardial nuclei and mitochondria, sarcoplasmic reticulum, myofibrils, and other cytoplasmic components are described. 2. 2. The observations of other investigators and their interpretations are discussed in relation to these findings. 3. 3. The intercalated disc is defined as a distinctive complex consisting of modified cell membranes with an interposed intercellular space. The concept of a morphologic syncytium is discarded. 4. 4. The sarcoplasmic reticulum is interpreted as a system of anastomosing, membrane-limited channels which traverse the interior of the myocardial cell and come into close relationship with other intracellular structures. 5. 5. The myofibril is conceived as an ordered integrated system composed of thick and thin myofilaments which interdigitate at A band levels. The structural similarity to skeletal muscle is discussed.