Cardiac Muscle

Cardiac muscle or myocardium exhibits many structural and functional characteristics intermediate between those of skeletal and visceral muscle. Like the former, its contractions are strong and utilise a great deal of energy, and like the latter the contractions are continuous and initiated by inherent mechanisms, although they are modulated by external autonomic and hormonal stimuli.

Cardiac muscle fibres are essentially long cylindrical cells with one or at most two nuclei, centrally located within the cell. The ends of the fibres are split longitudinally into a small number of branches, the ends of which abut onto similar branches of adjacent cells giving the impression of a continuous three-dimensional cytoplasmic network; this was formerly described as a syncytium before the discrete intercellular boundaries were recognised.

Between the muscle fibres, delicate collagenous tissue analogous to the endomysium of skeletal muscle supports the extremely rich capillary network necessary to meet the high metabolic demands of strong continuous activity.

Cardiac muscle fibres have an arrangement of contractile proteins similar to that of skeletal muscle and are consequently striated in a similar manner. However, this is often difficult to see with light microscopy due to the irregular branching shape of the cells and their myofibrils. Cardiac muscle fibres also have a system of T tubules and sarcoplasmic reticulum analogous to that of skeletal muscles. In the case of cardiac muscle, however, there is a slow leak of calcium ions into the cytoplasm from the sarcoplasmic reticulum after recovery from the preceding contraction; this causes a succession of automatic contractions independent of external stimuli. The rate of this inherent rhythm is then modulated by external autonomic and hormonal stimuli.

Between the ends of adjacent cardiac muscle cells are specialised intercellular junctions, called intercalated discs, which not only provide points of anchorage for the myofibrils but also permit extremely rapid spread of contractile stimuli from one cell to another. Thus, adjacent fibres are triggered to contract almost simultaneously, thereby acting as a functional syncytium. In addition, a system of highly modified cardiac muscle cells constitutes the pacemaker regions of the heart and ramifies throughout the organ as the Purkinje system, thus coordinating contraction of the myocardium as a whole in each cardiac cycle; this is illustrated and described in more detail in Chapter 8.

Cardiac muscle cells in certain locations in the heart are responsible for secreting hormones into the bloodstream.