Skeletal Muscle Structure and Function

• Human body contains over 400 skeletal muscles, accounting for 40-50% of total body weight.
• Muscle, when shortened, pulls on bony levers in order to cause movement.
• Skeletal muscle functions include:
• Force production for locomotion
• Postural support and breathing
• Heat production during cold stress
• Major site of glucose disposal

Skeletal Muscle Structure

• Features of skeletal muscle:
• Striated
• Multi-nucleated
• Sarcomeres are functional units

Myofilaments

• Consist of actin and myosin, accounting for approximately 85% of the myofibrillar complex.
• Other proteins serve structural functions or affect protein filament interaction during muscle action.
• Examples including: tropomyosin, troponin, titin, α-actinin, β-actinin, M protein, and C protein

Muscle Cell Layers

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

Fascia Layers

• Skeletal muscles contain wrappings of fibrous connective tissue known as fascia.

• Epimysium: the outer layer

• 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 bone periosteum.

• Tension developed in a muscle fiber is transmitted directly to the tendon and bone.

Axial vs Appendicular Skeleton

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

Composition

• Water accounts for 75% of the muscle (sarcoplasm).
• Protein accounts for 20% of the muscle (e.g., actin, myosin).
• Salts and other substances account for 5% of the muscle.
• Myofilaments consist of actin and myosin, which account for ~85% of the myofibrillar complex.

Sliding Filament Model

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

Excitation-Contraction Coupling

• Muscle action potential depolarizes transverse tubules at the sarcomere's A-I junction.
• Sac-like vesicles within the terminal axon release 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, producing myosin crossbridge movement.
• Muscle shortening occurs after ATP binds to the myosin crossbridge, breaking the actin-myosin bond and allowing crossbridge dissociation from actin and sliding of thick and thin filaments.
• Ca2+ removal restores troponin-tropomyosin inhibitory action. Actin and myosin remain in the dissociated relaxed state with ATP present..
• When muscle stimulation ceases, Ca2+ moves back into the sarcoplasmic reticulum lateral sacs through active transport via ATP hydrolysis.
• Crossbridge activation continues when Ca2+ concentration remains high to inhibit troponin-tropomyosin action.

Excitation-Contraction Coupling Steps

• Action Potential (AP) travels down axon terminal
• Acetylcholine (Ach) is released.
• Ach binds to receptors on sarcolemma.
• AP travels to T-tubules.
• Sarcoplasmic reticulum (SR) releases Ca2+.
• Ca2+ binds to Troponin.
• Tropomyosin shifts.
• Myosin binds to active site to form crossbridge.
• Power stroke & sliding of filaments occurs.

Explore the structure and function of skeletal muscles, including their role in locomotion, posture, breathing, and heat production. Learn about skeletal muscle features such as striated, multi-nucleated with sarcomeres. Understand myofilaments and muscle cell layers.