Muscle cells have become adapted to a variety of different functions during their evolution, so that in other muscle types the details of the contractile process and its control show differences from those in vertebrate skeletal muscles. These final two chapters successively examine the properties of mammalian heart and smooth muscle.

Structure and organization of cardiac cells

Cardiac cells are considerably smaller than skeletal muscle fibres; they are typically up to 10 μm in diameter and 200 μm in length (Figure 12.1). However, adjacent cardiac cells are mechanically and electrically coupled both in a branched and in an end-to-end manner by intercalated disks to give a syncytium through which both electrical activity and mechanical forces are transmitted (Figure 12.1a). Atrial and ventricular myocytes specialized to generate mechanical activity contain contractile elements whose structure is similar to that found in skeletal muscle. Thus they also show thick myosin and thin actin filaments aligned transversely (Figure 12.1b). Cardiac myocytes are accordingly cross-striated in appearance. They similarly contain mitochondria, sarcoplasmic reticulum and transverse tubules. However, the sarcoplasmic reticulum is less developed. In the ventricle, it makes complexes with transverse tubular membrane at dyad rather than triad junctions. In atrial myocytes, the transverse tubular system is considerably less developed, and sarcoplasmic reticulum makes junctions at caveolae in the membrane surface. However, there are additional cardiac cell types with differing specializations that include cells that primarily generate and conduct electrical impulses.