29) Skeletal muscle fibers are formed from embryonic cells called A) fascicles. B) myoblasts. C) myofibrils. D) sarcomeres. E) myomeres. 30) The repeating unit of a skeletal muscle fiber is the A) sarcomere. B) myofilament. C) myofibril. D) sarcolemma. E) sarcoplasmic reticulum. 31) Muscle fibers differ from "typical cells" in that muscle fibers A) have large gaps in the cell membrane. B) lack a plasma membrane. C) have many nuclei. D) lack mitochondria. E) are very small. 32) Which of the following best describes the term sarcoplasmic reticulum? A) thin filaments are anchored here B) repeating unit of striated myofibrils C) largely made of myosin molecules D) protein that accounts for elasticity of resting muscle E) storage and release site for calcium ions 33) The region of the sarcomere containing the thick filaments is the B) I band. A) M line. C) H band. 34) Cross-bridges are portions of A) myosin molecules. B) troponin molecules. C) actin molecules. D) calcium ions. E) tropomyosin molecules. 35) The area in the center of the A band that contains no thin filaments is the A) M line. B) zone of overlap. C) H band. D) Z line. E) I band. 36) At rest, the tropomyosin molecule is held in place by A) actin molecules. B) myosin molecules. C) calcium ions. D) ATP molecules. E) troponin molecules. D) A band. 5 E) Z line. 29) 30) 31) 32) 33) 34) 35) 36) 37) Each thin filament consists of A) six molecules coiled into a helical structure. B) a rod-shaped structure with heads" projecting from each end. C) chains of myosin molecules D) a double strand of myosin molecules. E) two actin protein strands coiled helically around each other. 38) Which of the following best describes the term titin? A) largely made of myosin molecules B) repeating unit of striated myofibrils C) protein that accounts for elasticity of resting muscle D) storage site for calcium ions E) thin filaments are anchored here 39) The region of the sarcomere that always contains thin filaments is the A) M line. B) Z line. C) H band. D) I band. 40) At rest, active sites on the actin are blocked by A) troponin molecules. B) myosin molecules. C) tropomyosin molecules. D) ATP molecules. E) calcium ions. 41) Which statement about the microscopic anatomy of skeletal muscle fibers is true? A) Each fiber has many nuclei. B) Tubular extensions of the sarcolemma penetrate the fiber transversely. C) Cross striations result from the lateral alignment of thick and thin filaments. D) Muscle fibers are continuous from tendon to tendon. E) All of the answers are correct. 2) When a skeletal muscle fiber contracts, A) the zones of overlap get larger. B) the Z lines get closer together. C) the H bands and I bands get smaller. D) the width of the A band remains constant. E) All of the answers are correct. E) A band. 37) 38) (39) 40) 41) 42)