Skeletal Muscle: Structure and Function

• Human body consists of over 400 skeletal muscles.
• Skeletal muscles account for approximately 40-50% of total body weight.
• Muscle contraction pulls on bony levers, leading to movement.
• Functions of skeletal muscle include force production for locomotion, postural support, breathing, heat production during cold stress, and glucose disposal.

Skeletal Muscle Structure

• Skeletal muscles are striated and multi-nucleated.
• Sarcomeres are the functional units of skeletal muscles.
• Each muscle is composed of thousands of cylindrical cells called fibers, which lie parallel to each other.

Muscle Composition

• Water makes up 75% of muscle (sarcoplasm).
• Protein accounts for 20% (e.g., actin, myosin).
• Salts and other substances constitute 5%.
• Myofilaments consist of actin and myosin, making up around 85% of the myofibrillar complex.
• Other proteins serve structural functions or affect protein filament interaction during muscle action and are tropomyosin, troponin, titin, a-actinin, B-actinin, M protein, and C protein.

Muscle Cell Layers

• A single multinucleated muscle fiber contains myofibrils that run parallel to the fiber's long axis.
• Myofibrils contain smaller subunits called myofilaments, which are parallel to the myofibril.
• Muscle > fascicle > muscle fiber > myofibril > myofilaments
• Muscle fiber = muscle cell = myocyte = myofiber.

Fascia Structure

• Skeletal muscles contain wrappings of fibrous connective tissue (fascia).
• Epimysium is the outer layer of connective tissue.
• Perimysium surrounds each fasciculus, or group of fibers.
• Endomysium surrounds individual fibers.
• All connective tissue is continuous with the tendon.
• Tendons intermesh with collagenous fibers of the bone periosteum.
• Tension developed in a muscle fiber is transmitted directly to the tendon and bone.

Sliding Filament Model

• Myofilaments consisting of actin and myosin slide past each other toward the center of the sarcomere.
• Sarcomere shortens as the Z-line moves toward the center.
• Filaments overlap more, causing the I-band and H-zone to disappear.

Axial vs Appendicular Skeleton

• Axial skeleton consists of the skull, vertebral column (C1-coccyx), ribs, and sternum.
• Appendicular skeleton consists of shoulder girdle, bones of the arms, wrists, hands, and the pelvic girdle; bones of the legs, ankles, and feet.

Excitation-Contraction Coupling

• Muscle action potential depolarizes transverse tubules at the sarcomere's A-I junction.
• Sac-like vesicles within terminal axon release Acetylcholine (ACh), which diffuses across the synaptic cleft and attaches to specialized ACh receptors on the sarcolemma.
• T-tubule system depolarization causes Ca2+ release from sarcoplasmic reticulum lateral sacs.
• Ca2+ binds to troponin-tropomyosin in actin filaments, which releases inhibition of actin combining with myosin.
• Actin joins myosin ATPase to split ATP with energy release during muscle action, which releases energy that produces myosin crossbridge movement.
• Muscle shortening occurs after ATP binds to the myosin crossbridge, which breaks the actin-myosin bond and allows crossbridge dissociation from actin and sliding of thick and thin filaments.
• Ca2+ removal restores troponin-tropomyosin inhibitory action, and with ATP present, actin and myosin remain in the dissociated relaxed state.
• Crossbridge activation continues when Ca2+ concentration remains high (from membrane depolarization) to inhibit troponin-tropomyosin action.
• When muscle stimulation ceases, Ca2+ moves back into the sarcoplasmic reticulum lateral sacs through active transport via ATP hydrolysis.

Excitation-Contraction Coupling Summary

• Action potential (AP) travels down the axon terminal and releases Acetylcholine (Ach).
• Ach binds to receptors on the sarcolemma.
• AP travels to T-tubules.
• Calcium (Ca^2+) is released from the sarcoplasmic reticulum (SR) into the sarcoplasm.
• Ca^2+ binds to Troponin, causing Tropomyosin to shift.
• Myosin binds to the active site to form a crossbridge.
• Results in power stroke.
• Sliding of filaments occurs.

Explore the structure and function of skeletal muscles, including their role in movement, posture, and heat production. Learn about the composition of muscle fibers, the arrangement of sarcomeres, and the importance of proteins like actin and myosin in muscle contraction. Understand how muscles contribute to overall body weight and glucose disposal.