Histology of Muscle Tissue

Muscle Tissue

• Muscle tissue is composed of cells that optimize the universal cell property of contractility.
• There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal Muscle

• Skeletal muscle consists of long, cylindrical, multinucleated cells with diameters of 10-100 μm and cross-striations.
• Contraction is quick, forceful, and usually under voluntary control.
• Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to skeletal muscle fibers.
• Muscle satellite cells are a small population of reserve progenitor cells that remain adjacent to most fibers of differentiated skeletal muscle.

Organization of Skeletal Muscle

• The entire muscle is surrounded by the epimysium, an external sheath of dense irregular connective tissue.
• The perimysium is a thin connective tissue layer that surrounds each bundle of muscle fibers (fascicle).
• The endomysium is a very thin, delicate layer of reticular fibers and scattered fibroblasts that surrounds the external lamina of individual muscle fibers.
• Collagens in these connective tissue layers serve to transmit the mechanical forces generated by the contracting muscle cells.

Ultrastructure of Skeletal Muscle Fibers

• Longitudinally sectioned skeletal muscle fibers show striations of alternating light and dark bands.
• The sarcoplasm is highly organized, containing primarily long cylindrical filament bundles (myofibrils) that run parallel to the long axis of the fiber.
• Myofibrils consist of an end-to-end repetitive arrangement of sarcomeres.
• The dark bands on the myofibrils are called A bands, and the light bands are called I bands.
• Mitochondria and sarcoplasmic reticulum are found between the myofibrils.

Sarcomeres

• The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively.
• The thick myosin filaments occupy the A band at the middle region of the sarcomere.
• Myosin is a large complex with two identical heavy chains and two pairs of light chains.
• The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).
• The thin, helical actin filaments are each 1.0-μm long and 8-nm wide and run between the thick filaments.

Sarcoplasmic Reticulum and Transverse Tubule System

• The membranous smooth ER, called sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils.
• The sarcolemma has tubular infoldings called transverse or T-tubules.
• The triad complex (T-tubule with two terminal cisternae of sarcoplasmic reticulum) allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca2+ ions into the cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres.

This quiz covers the ultrastructure of skeletal, smooth, and cardiac muscles, including their morphologic and functional characteristics.