Cardiac Muscle Tissue

Microscopic Anatomy

Cardiac muscle appears striated due to the presence of sarcomeres, the highly-organized basic functional unit of muscle tissue.

LEARNING OBJECTIVES

Identify the microscopic anatomy of cardiac muscles

KEY TAKEAWAYS

Key Points

Cardiac muscle, composed of the contractile cells of the heart, has a striated appearance due to alternating thick and thin filaments composed of myosin and actin.

Actin and myosin are contractile PROTEIN filaments, with actin making up thin filaments, and myosin contributing to thick filaments. Together, they are considered myofibrils.

Myosin and actin adenosine triphosphate ( ATP ) binding allows for muscle contraction. It is regulated by action potentials and calcium concentrations.

Adherens junctions, GAP junctions, and desmosomes are intercalated discs that connect cardiac muscle cells. Gap junctions specifically allow for the transmission of action potentials within cells.

Key Terms

intercalated discs: Junctions that connect cardiomyocytes together, some of which transmit electrical impulses between cells.

sarcomere: The basic unit of contractile muscle which contains myosin and actin, the two proteins that slide past one another to cause a muscle contraction.

Cardiac muscle, like skeletal muscle, appears striated due to the organization of muscle tissue into sarcomeres. While similar to skeletal muscle, cardiac muscle is DIFFERENT in a few ways. Cardiac muscles are composed of tubular cardiomyocytes, or cardiac muscle cells. The cardiomyocytes are composed of tubular myofibrils, which are repeating sections of sarcomeres. Intercalated disks transmit electrical action potentials between sarcomeres.

Sarcomere STRUCTURE

A sarcomere is the basic unit of muscle tissue in both cardiac and skeletal muscle. Sarcomeres appear under the microscope as striations, with alternating dark and light bands. Sarcomeres are connected to a plasma membrane, called a sarcolemma, by T-tubules, which speed up the rate of depolarization within the sarcomere.

Individual sarcomeres are composed of long, fibrous proteins that slide past each other when the muscles contract and relax. The two most important proteins within sarcomeres are myosin, which forms a thick, flexible filament, and actin, which forms the thin, more rigid filament. Myosin has a long, fibrous tail and a globular head which binds to actin. The myosin head also binds to ATP, the source of energy for muscle movement. Actin molecules are bound to the Z-disc, which forms the borders of the sarcomere. Together, myosin and actin form myofibrils, the repeating molecular structure of sarcomeres.

Myofibril activity is required for muscle contraction on the molecular level. When ATP binds to myosin, it separates from the actin of the myofibril, which causes a contraction. Muscle contraction is a complex process regulated by calcium influx and the stimulus of electrical impulses.