ANSWER: (B) Lies on top of actinREF: Guyton 12th ed page 72-76SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION:In resting muscle, the ends of the actin filaments extending from two successive Z discs barely begin to overlap one another. Troponin I is tightly bound to actin and tropomyosin covers the sites where myosin heads bind to actin i.e the tropomyosin molecules lie on top of the active sites of the actin strands, Thus, the troponin-tropomyosin complex constitutes a "relaxing protein" that inhibits the interaction between actin and myosin filaments to cause contractionIn contracted state, these actin filaments have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism.Actin filaments slide inward among the myosin filaments by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive. But when an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils. When the Ca2+ released by the action potential binds to troponin C, the binding of troponin I to actin is presumably weakened, and this permits the tropomyosin to move laterally. This movement uncovers binding sites for the myosin heads. ATP is then split and contraction occurs. Seven myosin-binding sites are uncovered for each molecule of troponin that binds a calcium ion. But energy is needed for the contractile process to proceed. This energy comes from high-energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy.Molecular Basis of ContractionThe process by which the shortening of the contractile elements in muscle is brought about is a sliding of the thin filaments over the thick filaments. The width of the A bands is constant, whereas the Z lines move closer together when the muscle contracts and farther apart when it is stretched. The sliding during muscle contraction occurs when the myosin heads bind firmly to actin, bend at the junction of the head with the neck, and then detach. This "power stroke" depends on the simultaneous hydrolysis of ATP. Myosin-II molecules are dimers that have two heads, but only one attaches to actin at a time. The process by which depolarization of the muscle fiber initiates contraction is called excitation-contraction coupling.Steps in contractionDischarge of motor neuron.Release of transmitter (acetylcholine) at motor end-plate.Binding of acetylcholine to nicotinic acetylcholine receptors.Increased Na+ and K+ conductance in end-plate membrane.Generation of end-plate potential.Generation of action potential in muscle fibers.Inward spread of depolarization along T tubules.Release of Ca2+ from terminal cisterns of sarcoplasmic reticulum and diffusion to thick and thin filaments.Binding of Ca2+ to troponin C, uncovering myosin-binding sites on actin.Formation of cross-linkages between actin and myosin and sliding of thin on thick filaments, producing movementSteps in relaxationCa2+ pumped back into sarcoplasmic reticulum.Release of Ca2+ from troponin.Cessation of interaction between actin and myosin.