Medical Physiology - Physiological Anatomy of Skeletal Muscle

Medical Physiology - Physiological Anatomy of Skeletal Muscle
ZApproximately 40% of body mass comprises skeletal muscle, with an additional 10% consisting of smooth and cardiac muscle. Numerous concepts of contraction are applicable to all three muscle types. Skeletal Muscle Fiber In the majority of muscles, the fibers span the full length of the muscle. Each fiber is innervated by a single nerve terminal. Myofibrils consist of actin and myosin filaments. Each muscle fiber comprises hundreds to thousands of myofibrils, with each myofibril consisting of approximately 1500 myosin filaments and 3000 actin filaments arranged laterally. These filaments are substantial polymerized protein molecules that facilitate muscle contraction. The thick filaments consist of myosin, whereas the thin filaments comprise actin; observe the subsequent characteristics:

• light and dark bands.
The myosin and actin filaments partially interdigitate, resulting in the myofibrils exhibiting alternating bright and dark bands. The light bands consist solely of actin filaments and are referred to as I bands. The dark regions known as A bands comprise myosin filaments along with the terminal portions of actin filaments. The A band length corresponds to the length of the myosin filament
. • Cross-bridges. The diminutive extensions from the myosin filament sides are termed cross-bridges. They extend from the surfaces of the myosin filament over its whole length, with the exception of the core. Myosin cross-bridges engage with actin filaments, resulting in contraction.
• Z the disc. The termini of the actin filaments are affixed to Z discs.The Z disc traverses the myofibril, connecting and aligning the myofibrils throughout the muscle fiber.

The complete muscle fiber exhibits alternating light and dark bands, resulting in a striated appearance in skeletal and cardiac muscle.

• Sarcomere. The segment of a myofibril situated between two consecutive Z discs is referred to as a sarcomere. At rest, actin filaments exhibit ideal interdigitation with myosin filaments in skeletal muscle, while in cardiac muscle, the interdigitation is somewhat less than optimal.