We introduced cardiac muscle tissue in Chapter 4 and briefly compared its properties with those of other muscle types. Cardiac muscle cells, also called cardiocytes or cardiac myocytes, are relatively small, averaging 10–20 µm in diameter and 50–100 µm in length. A typical cardiac muscle cell (Figure 10-21a,b

DIFFERENCES BETWEEN CARDIAC AND SKELETAL MUSCLE TISSUES

As do skeletal muscle fibers, each cardiac muscle cell contains organized myofibrils, and the presence of many aligned sarcomeres gives it striations. However, cardiac muscle cells are much smaller than skeletal muscle fibers, and significant structural and functional differences exist between the two.

Structural Differences

Important structural differences between skeletal muscle fibers and cardiac muscle cells include the following:

• The T tubules in a cardiac muscle cell are short and broad, and there are no triads. The T tubules encircle the sarcomeres at the Z lines rather than at the zone of overlap.
• The SR of a cardiac muscle cell lacks terminal cisternae, and its tubules contact the cell membrane as well as the T tubules (Figure 10-21c). As in skeletal muscle fibers, the appearance of an action potential triggers calcium release from the SR and the contraction of sarcomeres; it also increases the permeability of the sarcolemma to extracellular calcium ions.
• Cardiac muscle cells are almost totally dependent on aerobic metabolism to obtain the energy needed to continue contracting. The sarcoplasm of a cardiac muscle cell thus contains large numbers of mitochondria and abundant reserves of myoglobin (to store oxygen). Energy reserves are maintained in the form of glycogen and lipid inclusions.
• Each cardiac muscle cell contacts several others at specialized sites known as intercalated discs. Intercalated discs play a vital role in the function of cardiac muscle, as we shall learn next.

Intercalated Discs At an intercalated disc (Figure 10-21a,b), the cell membranes of two adjacent cardiac muscle cells are extensively intertwined and bound together by gap junctions and desmosomes.  These connections help stabilize the relative positions of adjacent cells and maintain the three-dimensional structure of the tissue. The gap junctions allow ions and small molecules to move from one cell to another. This arrangement creates a direct electrical connection between the two muscle cells. An action potential can travel across an intercalated disc, moving quickly from one cardiac muscle cell to another.

Myofibrils in the two interlocking muscle cells are firmly anchored to the membrane at the intercalated disc. Because their myofibrils are essentially locked together, the two muscle cells can "pull together" with maximum efficiency. Because the cardiac muscle cells are mechanically, chemically, and electrically connected to one another, the entire tissue resembles a single, enormous muscle cell. For this reason, cardiac muscle has been called a functional syncytium.

Functional Differences

In Chapter 20, we will examine cardiac muscle physiology in detail; here we will briefly summarize four major functional specialties of cardiac muscle:

1. Cardiac muscle tissue contracts without neural stimulation. This property is called automaticity. The timing of contractions is normally determined by specialized cardiac muscle cells called pacemaker cells.
2. Innervation by the nervous system can alter the pace established by the pacemaker cells and adjust the amount of tension produced during a contraction.
3. Cardiac muscle cell contractions last roughly 10 times longer than do those of skeletal muscle fibers.
4. The properties of cardiac muscle cell membranes differ from those of skeletal muscle fiber membranes. As a result, individual twitches cannot undergo wave summation, and cardiac muscle tissue cannot produce tetanic contractions. This difference is important because a heart in a sustained tetanic contraction could not pump blood.