Muscle cells have become adapted to a variety of different functions during their evolution, so that the details of the contractile process and its control are not always identical with those in vertebrate skeletal muscles. In this chapter we examine the properties of mammalian heart and smooth muscles.

Cardiac muscle

Mammalian heart muscle consists of a large number of branching uninucleate cells connected to each other at their ends by intercalated discs (Fig. 11.1). Electron micrography shows that the intercalated discs consist largely of accumulations of dense material on the insides of the two cell membranes; these apparently serve to fix the cells together and allow the filaments of the contractile apparatus in one cell to pull on those of the next one in the line. Gap junction channels (p. 116) are also present in the intercalated discs and these allow electrical currents to flow from one cell to another.

The contractile apparatus is much the same as in skeletal muscles, with thick myosin and thin actin filaments aligned transversely so that the muscle cells as a whole are cross-striated in appearance. As in a skeletal muscle fibre, the interior of the cell also contains mitochondria, sarcoplasmic reticulum and the transverse tubules of the T system.

The cardiac action potential

Intracellular recordings from heart muscle fibres were first made using isolated bundles of Purkinje fibres from dogs. The Purkinje fibres form a specialized conducting system which serves to carry excitation through the ventricle.