<prompt> ATPase activity is present inMyosin headActinTroponinTropomyosin  <response>: Ans. a Myosin head; Ref: Ganong, Physiology, 21st ed., 84; Harper 27th ed. Table 48-3. FEATURES OF MYOSIN HEAD Two heavy and four light chains Cross-bridges and hinges ATPase activity It is greater in type IIfast-twitch fibres than in type Islow-twitch fibres. Tropomyosin lies on top of active site of actin so that attraction cannot occur between actin myosin at rest.Troponin C has strong affinity for Ca and it initiates contraction. SKELETAL MUSCLECARDIAC MUSCLESMOOTH MUSCLE1. Striated.1. Striated.1. Nonstriated.2. No syncytium.2. Syncytial.2. Syncytial.3. Small T tubules.3. Large T tubules.3. Generally rudimentary T tubules.4. Sarcoplasmic reticulum well developed and Ca2 pump acts rapidly.4. Sarcoplasmic reticulum present and Ca2 pump acts relatively rapidly.4. Sarcoplasmic reticulum often rudimentary and Ca2 pump acts slowly.5. Plasmalemma lacks many hormone receptors.5. Plasmalemma contains a variety of receptors.5. Plasmalemma contains a variety of receptors.6. Nerve impulse initiates contraction.6. Has intrinsic rhythmicity.6. Contraction initiated by nerve impulses, hormones, etc.7. Extracellular fluid Ca2 not important for contraction.7. Extracellular fluid Ca2 important for contraction.7. Extracellular fluid Ca2 important for contraction.8. Troponin system present.8. Troponin system present.8. Lacks troponin system; uses regulatory head of myosin.9. Caldesmon not involved.9. Caldesmon not involved.9. Caldesmon is important regulatory protein.10.Very rapid cycling of the cross-bridges.10.Relatively rapid cycling of the cross-bridges.10.Slow cycling of the cross-bridges permits slow, prolonged contraction and less utilization of ATP. 