The ultrastructure of a cardiac muscle cell is marked by a prevalence of tightly packed myofibrils #1, #2, #3 in peripheral parts of the cell (their dark and pale bands are strictly ordered and match one another). The myofibrils are separated by rows of mitochondria #1, #2, #3. The central zone of the cardiac muscle cell contains 1-2 nuclei, numerous polyribosomes and mitochondria to provide for myofilament production.

There are specialized plasma membrane portions near the ends of neighboring cardiac muscle cells (where cell processes are close). These membrane portions constitute complex structures called intercalated discs. There, a transverse #1, #2 and a longitudinal part may be found. The transverse parts are formed by desmosomes #1, #2 and adhesive strips (fasciae adherentes). The strips are the sites for fine (actin) strand attachment from the terminal sarcomeres of myofibrils. The longitudinal portions are situated between the lateral surfaces of cardiac muscle cell surfaces. These portions are connected generally by communicating or gap junctions (nexuses), which provide for rapid propagation of an action potential between the muscle cells.