Cardiac Muscle

Cardiac muscle cells are relatively small and usually have a single, centrally placed nucleus. Cardiac muscle is a unique tissue forming the wall of the heart. Like skeletal muscle fibres, cardiac muscle cells contain an orderly arrangement of myofibrils and have cross-striations, but significant differences exist in their structures and functions. The most obvious structural difference is that cardiac muscle cells are branched and each cardiac cell contacts several others at specialized sites called intercalated discs. These cellular connections contain gap junctions that allow the movement of ions and small molecules and the rapid passage of action potentials from cell to cell, resulting in their simultaneous contraction. Because the myofibrils are also attached to the intercalated discs, the cells "pull together" quite efficiently.

Cardiac muscle and skeletal muscle have the following important functional differences:

1. Cardiac muscle tissue contracts without neural stimulation, a property called automaticity. The timing of contractions is normally determined by specialized cardiac muscle cells called pacemaker cells.
2. Cardiac muscle cell contractions last roughly 10 times longer than those of skeletal muscle fibres.
3. The properties of cardiac muscle cell membranes differ from those of skeletal muscle fibres. As a result, cardiac muscle tissue cannot undergo tetanus (sustained contraction). This property is important because a heart in tetany could not pump blood.
4. An action potential not only triggers the release of calcium from the sarcoplasmic reticulum but also increases the permeability of the cell membrane to extracellular calcium ions.
5. Cardiac muscle cells rely on aerobic metabolism for the energy needed to continue contracting. The sarcoplasm contains large numbers of mitochondria and abundant reserves of myoglobin (to store oxygen).