Cardiac Muscle

Fibers of cardiac muscle have features somewhat intermediate between those of skeletal and visceral muscle.

• Examine muscle in the heart myocardium (slide 64 and 65).
• Identify fascicles, connective tissue layers, blood microvessels, and fibers cut in both directions (Figs. 10-16 through 10-18).
• Be sure to identify intercalated disks and distinguish these from striations.

Study electron micrographs of cardiac muscle (Figs. 10-17 and 10-18).

• Note the similarities and differences with skeletal muscle, particularly the intercalated disks.
• Note also the secretory granules with atrial natriuretic factor that are present in some atrial myocytes.
• Compare and contrast the structural features of cardiac muscle with the other two muscle types and indicate the functional significance of these features.

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 on the next page, injured cardiac myofibers 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.

Sketch the cells/fibers of the three types of muscle to the same scale.

Now on to nervous tissue.