Cardiac muscle

Cardiac muscle, composed of cardiac muscle cells called Cardiocytes or cardiac myocytes, is found exclu­sively in the heart. It is considered as involuntary, striated muscle.

Cardiac versus skeletal muscle cells

Cardiac muscle has several structural differences from skeletal muscle fibers:

1.

2. Cardiocytes generally have a single, centrally located nucleus, although occasionally a cell may have two or more nuclei. In comparison, skeletal muscle fibers are multinucleated.

3. T tubules are shorter and broader, and they lack a triad. The T tubules encircle the sarcomere at the Z-Iine rather than at the overlap between thin and thick filaments.

4. The SR lacks terminal cisternae.

5. The sarcoplasm of Cardiocytes contains a large num­ber of mitochondria since cardiac muscle is almost exclusively dependent on aerobic metabolism.

6. Cardiocytes connect with each other at specialized junctions called intercalated (intercal = to insert between) discs.

At the intercalated discs, the Sarcolemma of the adjacent Cardiocytes have irregular thickenings that connect the cells to one another. Within these regions are desmosomes that hold the cells together, as well as gap junctions. The gap junctions allow ions, small molecules, and an action potential to move between adjacent cells. Since the cardiocytes are mechanically and functionally connected, they are said to act as a functional syncytium, meaning that they act as a fused mass of cells.

Functional characteristics

Unlike skeletal muscle, cardiac muscle can contract without neural stimulation. Therefore, cardiac muscle is said to be autorhythmic. Such contraction is gener­ally controlled by a specialized group of cardiac cells that act as the pacemaker cells.

As will be discussed in Chapter 13, contraction of cardiac muscle cells lasts approximately 10 times longer than that of skeletal muscle. This is due to the differing mechanism of the action potential. In addi­tion, cardiac muscle cells cannot display wave sum­mation, nor can they produce titanic contractions. These features are important for the pumping function of the heart.