Anatomy & Physiology

Anatomy & Physiology Study Guide

10.4 Chapter Ten Review Skeletal muscle performs six major functions: • The three types of muscle tissue are skeletal muscle, cardiac muscle, and smooth muscle. • Skeletal muscles attach to bones directly or indirectly. Their functions are to (1) produce skeletal movement, (2) maintain posture and body position, (3) support soft tissues, (4) guard entrances and exits, (5) maintain body temperature, and (6) store nutrient reserves. Skeletal muscle fibers have distinctive features: • A skeletal muscle fiber has a sarcolemma, or plasma membrane; sarcoplasm (cytoplasm); and sarcoplasmic reticulum (SR), like the smooth endoplasmic reticulum of other cells. Transverse (T) tubules and myofibrils aid in contraction. Filaments in a myofibril are organized into repeating functional units called sarcomeres. • Myofilaments called thin filaments and thick filaments form myofibrils. • Thin filaments consist of F-actin, nebulin, tropomyosin, and troponin. Tropomyosin molecules cover active sites on the G-actin subunits that form the F-actin strand. Troponin binds to G- actin and tropomyosin and holds the tropomyosin in position. • Thick filaments consist of a bundle of myosin molecules around a titin core. Each myosin molecule has an elongated tail and a globular head, which forms cross-bridges during contraction. In a resting muscle cell, the attachment of myosin heads to active sites on G-actin is prevented by tropomyosin. • The relationship between thick and thin filaments changes as a muscle fiber contracts. Communication between the nervous system and skeletal muscles occurs at the neuromuscular junction: • When muscle cells contract, they create tension and pull on the attached tendons. • The activity of a muscle fiber is controlled by a neuron at a neuromuscular (myoneural) junction (NMJ). • When an action potential arrives at the synaptic terminal, acetylcholine (ACh) is released into the synaptic cleft. The binding of ACh to receptors on the opposing junctional folds leads to the generation of an action potential in the sarcolemma. • Excitation–contraction coupling occurs as the passage of an action potential along a T tubule triggers the release of Ca2+ from the cisternae of the SR at triads. • Acetylcholinesterase (AChE) breaks down ACh and limits the duration of muscle stimulation. • Release of Ca2+ initiates a contraction cycle of attachment, pivoting, detachment, and return. The calcium ions bind to troponin, which changes position and moves tropomyosin away from the active sites of actin. Cross-bridges of myosin heads then bind to actin. Next, each myosin head pivots at its base, pulling the actin filament toward the center of the sarcomere.

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