Which statement related to calcium in skeletal muscles is true? In a muscle fiber at rest, calcium is higher in the cytoplasm than in the sarcoplasmic reticulum. • The DHP receptor is found in the sarcoplasmic reticulum membrane. Calcium triggers the interaction of actin and myosin and the sliding of filaments. DHP and ryanodine receptors are not physically connected. The DHP receptor is a calcium channel. 5. Which feature creates the striated appearance of skeletal muscles? Number of actin filaments in a myofibril Pattern of overlap of actin and myosin filaments in a sarcomere • Number of myofibrils in a muscle cell Arrangement of muscle fibers in a muscle Number of actin filaments surrounding a myosin filament 6. Which statement about muscle contractile proteins is true? Three chains of actin monomers twisted together form an actin filament. O A molecule of myosin consists of two long linear polypeptide chains, each with a globular head. A myosin filament consists of a single myosin molecule with its heads projecting at the ends of the filament. The polypeptides that make up actin have globular heads. Tropomyosin twists around the myosin molecule. 7. Which statement about muscle contraction is true? . When a neuron fires, the muscle fibers with which it synapses contract in sequence. The cell membrane of a muscle cell can generate and conduct action potentials. The sarcoplasmic reticulum spreads the incoming action potential from a neuron into the deep interior of a muscle fiber. A neuromuscular junction is the area of contact between the axon of a neuron and the sarcoplasmic reticulum of a skeletal muscle cell. A single neuron can excite only one muscle fiber at a time. 8. Which statement about the mechanism of muscle contraction is true? Calcium released into the cytoplasm binds to tropomyosin, changing the protein’s conformation. When a muscle is relaxed, troponin blocks the sites on the actin filament where myosin can bind. Removal of calcium from the cytoplasm returns tropomyosin to the conformation responsible for blocking myosin binding sites on actin. Changes in tropomyosin’s conformation trigger changes in troponin’s conformation, exposing the sites on the actin filament where myosin can bind. When calcium levels in the cytoplasm increase, the muscle relaxes. 9. Which of the following characterizes cardiac muscle? • Similar to smooth muscle, its cells have numerous nuclei. Similar to smooth muscle, its cells do not have a striated appearance. Its cells are found in the walls of arteries. Unlike smooth muscle, its cells lack gap junctions. Its individual cells are connected by specialized junctions. 10. Smooth muscle uses a sliding filament contractile mechanism. appears striated when viewed under the light microscope. is not under the control of the nervous system. is responsible for the beating of the heart. cannot produce enough force to move body fluids.

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