Myosin and actin filaments are kept in place by -
Ans. is 'd' i.e., Titin o The side-by-side relationship between the myosin and actin filaments is difficult to maintain.o This is achieved by a large number of filamentous molecules of a protein called titin.o Titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work.Important muscle proteins1. Myosin Myosin is the protein that constitutes the thick filaments. Myosin of skeletal muscle is myosin-II. Myosin participates in the contractile mechanism and also acts as an ATPase.2. Actin :- Actin is the major protein of thin filament. It is the actin which slides over myosin during contraction.3. Tropomyosin : - It is the other protein of thin filament. It covers the active sites (myosin binding sites) on actin. When Ca+2 concentration of cytoplasm (sarcoplasm) is raised, it uncovers the active sites of actin and allow the contraction So, the 'cross-bridge cycling ' is switched off 'or on by the tropomyosin molecule which slides on the actin molecule to cover or uncover the active sites on it.4. Troponin It is a protein also associated with thin filament. The regulatory action of Ca+2 on tropomysin is mediated by troponin, a Ca+2 binding protein. When the cytoplasmic (sarcoplasmic) Ca+2 concentration rises Ca+2 ions bind to troponin-C. The troponin - Ca+2- complex induces changes in Troponin-I and Troponin-T, which in turn brings about a shift of tropomyosin away from the active sites of actin. When the sarcoplasmic Ca+2 concentration falls, Ca+2 dissociates from troponin-C and tropomyosin slides back on the actin filament to cover the active sites. Thus, the first event during muscle contraction, after cytoplasmic rise in Ca+2 concentration, is binding of troponin C to Ca+2 which trigers the further steps by sliding the tropomysin away from the active sites of actin.o There are additional structural proteins in skeletal muscles.Actinin : It binds actin to Z-lines.Thin : It is the largest known protein and connects Z-Iines to M-linesand is responsible for passive stiffness of muscle, by limiting range of motion of sarcomere in tension.Desntin : It adds structures to Z-lines in part by binding the Z-lines to plasma membrane.
Tropomyosin
Troponin
Actinin
Titin
Skeletal Muscle: Contraction and Excitation