Muscle Structure & Function Quiz

Muscle Structure & Function

• Skeletal muscles make up 40-50% of total body weight
• The human body contains over 400 skeletal muscles
• Muscles contract, pulling on bony levers to cause movement
• Skeletal muscle functions include force production for locomotion, postural support and breathing, heat production during cold stress, and glucose disposal

Axial vs Appendicular Skeleton

• Axial skeleton: skull, vertebral column (C1-coccyx), ribs, and sternum
• Appendicular skeleton: shoulder girdle, arm bones, wrists, hands, pelvic girdle, leg bones, ankles, and feet

Skeletal Muscle Structure

• Skeletal muscles are composed of cylindrical cells called muscle fibers that lie parallel to each other
• Muscle fibers contain myofibrils that consist of smaller subunits, myofilaments, arranged parallel to the myofibril
• Muscle fiber > fascicle > muscle fiber > myofibril > myofilaments
• Muscle fiber = muscle cell = myocyte = myofiber

Skeletal Muscle Composition

• Water makes up 75% of a skeletal muscle (sarcoplasm)
• Protein (e.g., actin and myosin) accounts for 20%
• Salts and other substances make up 5%
• Myofilaments (actin and myosin) compose ~85% of the myofibrillar complex
• Other proteins perform structural functions or affect protein filament interaction during muscle action. These include tropomyosin, troponin, titin, α-actinin, β-actinin, M protein, and C protein

Fascia Layers

• Muscles have layers of fibrous connective tissue (fascia) wrapping them
• Epimysium is the outer layer of fascia
• Perimysium surrounds fascicles (bundles of fibers)
• Endomysium surrounds individual fibers

Fascia

• Connective tissues in muscles are continuous with tendons
• Tendons interconnect muscles to bones
• Tension in a muscle is transmitted directly to the tendon and bone

Excitation-Contraction Coupling

• Action potential (AP) triggers muscle contraction
• Acetylcholine (Ach) released from axon terminal binds to receptors on the sarcolemma
• AP travels to T-tubules, causing calcium (Ca2+) release from sarcoplasmic reticulum (SR) into the sarcoplasm
• Ca2+ binds to troponin, causing tropomyosin to shift, exposing the myosin binding sites on actin
• Myosin binds to active sites on actin, forming crossbridges and initiating a power stroke, causing sliding of filaments, leading to contraction.
• When stimulation ceases, Ca2+ returns to the SR, tropomyosin shifts back to cover the binding sites, and the muscle relaxes.

Review Questions

• Explain the sliding filament model and how the bands and zones change length during muscle contraction
• Describe the layers of muscle fibers and fascia that surround them
• Understand the steps in excitation-contraction coupling