Cardiac Muscle

The myocardium of the heart wall is composed of cardiac muscle fibers arranged as interwoven, spiral layers aligned with the long axis of the heart ventricles. Cardiac muscle fibers are striated like skeletal muscle fibers, and the contraction cycle is similar to that of skeletal muscle. There are some key structural differences that distinguish cardiac and skeletal muscle fibers. Review the table below highlighting these differences.

Examine cardiac muscle in the heart myocardium (sample 1, sample 2), and sample 3. Identify fascicles, connective tissue layers, blood vessels, and fibers cut in multiple planes. Identify intercalated discs and distinguish these from striations.

On the slides and in the images below, compare cardiac and skeletal muscle. Note the similarities (striations) and differences (branching fibers, centrally located single nuclei, and intercalated discs) between these two types of striated muscle.

Next, examine the ultrastructural features of cardiac muscle cells in the images below. The structure of the cardiac muscle sarcomere is the same as the skeletal muscle sarcomere. However, the T tubule system and terminal cisternae of the sarcoplasmic reticulum are organized as dyads, rather than the triads that occur in skeletal muscle. Cardiac muscle cells also have prominent step-like junctional complexes, intercalated discs, containing adhesion and gap junctions that are important for coordinating contraction of the cardiac muscle fibers.

Clinical note: The fact that cardiac muscle lacks satellite cells severely limits its capacity for repair after injury, such as injury from transient ischemia during myocardial infarction (heart attack). As shown below, injured cardiac muscle fibers degenerate and are replaced by connective tissue. Cardiac muscle injury and degeneration causes release of various cytoplasmic enzymes, which can be detected in the circulating blood and used as an indicator of heart damage.

Next is smooth muscle.