The globular part which protrudes out of the myosin tail (fig. 5.1. 8.) is called the head the myosin head has sites for attachments with (i) the acti

myosin, actin. tropornyosin and troponin. Together detailsl 1. Myosin Each thick filaments is 1.6 gm long and consist of about 200 molecules, called myosin molecule Each myosin molecule consistsof (fig.5.1. 8.) a tail and a "head' The tail is made up of two charts interwined with each other so that a double helix (cf. DNA molecule) is formed at Its one end, the tail turns up, becomes expanded and globular. The globular part which protrudes out of the myosin tail (fig. 5.1. 8.) is called the head the myosin head has sites for attachments with (i) the actin filament, as well as with the (ii) atp rnolecufe (fig. 5.1.8.) The rnyosin molecule is made up of 6 polypeptide chains Two or these chains produce the double heix of the tail and are called heavy chain. The other polypeptide chains, called the light chains are found to consitute the head N.B. (i) The central part of the rnyosin filament which is present in the H zone does not have any rnyosin head. (i) Note, two terms have been used, the myosin filament and the myosin molecule Myosin filament is the Thick fllament and is made up of about 200 myosin mole cules.2. Actin The actin or thin filament is a double helix (fig.1.7] Each strand of thedouble helix is made up of protien molecules called F actin F actin in turn is a polymer of smaller sized proteins called G actin Each F actin molecule of thin filament contains {fig. 5. 1. 7) a site, called, active site where myosin head is attached The thin filament also contains another protein called tropomyosin. The tropomyosin is, in the relaxed state of the muscle. situated in such a way, that the active sites mentioned above, reman covered by the tropomyosin A 3rd kind of protein, called tropon in remains attached with one of the ends of the tropomyosin molecule (fig. 5.1.7) There are three verities troponin. viz, (a) trepanning I (b) proponent, and (c) troponin C. The actual mechanism of contraction is as follows: 1. Before the actual contraction, an action potential (AP) develops on the sarcolemma (the cell membrane of the muscle fiber) .2. This AP proceeds along the sarcolemma as usual but on the way it meets the T tubules. Once the AP meets the T tubules, it (the AP) also proceeds via the T tubules and en ters within the muscle fiber >-= muscle ceil) This means, now that an AP passes along the T tubules and the is called, T tubule potential'. 3.The T tubule potential (recall, the T tubules come in 'dose contact with the L tubules at the region of the triads) when passing though the zone of the triad, causes sudden release of large amount of Ca++ ions from the L tubules i NB. L tubules store Ca++ ions) 4 These released Ca++ ions combine with troponin C (NB troponin C can combine with ionic calcium very avidly .5. Once the Ca++ ions combine with the troponin C. something happens What happens is most probably as follows normally, at relaxed state, the tropomyosin is so positioned, that tropomyosin cover she active sites of the F actin molecule But when troponin C combines with Ca++ ions. (by some yet undermined way) there develops some conformational change of the tropornyosin molecule, causing the tropomyosin molecule to move away a little, so that the
active sites of the actin F becomes uncovered It is reminded strongly, mat the myosin filament has a strong tendency to combine with the actin. and the attachment occurs between the myosin heads and the active sites of actin F When the muscle is relaxed, the tropomyosin is so placed, that these active sites become covered by the tropornyosin and therefore. the myosin head cannot combine with the actin. 6 .Therefore, now, because of the natural tendency to get attached, the myosin head attaches itself with me actin (at its active sites as shown in fig. 5.2.1. The myosin head, as stated before construes a cross bridge (= a bridge that bridges the gap between the thick and the thin filaments) However, these cross bridges cannot get attached with the thin filament, so long there is no elevated Ca++ level (i.e. the Ca++ level within the muscle fiber remains normal or less than 0 1 u mol/1). When the Ca++ level within the fiber becomes >5u mol per 1. the at attachment between the myosin head and actin develops fully and now it can be said that the cross bridges are My formed, further, when threaten (thin) and the myosin (thick) filaments join together via the cross bridges the ' combined complex is
called act myosin. 7. Now. the myosin head twists a littlie within the groove of the active site of the actin F (fig. 9.1. 4.) This causes movement of the actin. The thin filaments move towards the direction of the H zone. it is reminded that many cross bridges move simultaneously and all towards same directions a result, the thin filament moves vigorously towards the h zone, the cross bridges and the thin filament may be visualized as oars and a river boat respective so that simultaneous move mint of all the cross bridges behave like a power stroke'. 8. Now the head of the rnyosin molecules get detached from the thin filament (i.e. the cross bridges are disrupted). 9. The head of myosin reattaches itself to a new active site of the thin filament twists within the groove further movement of the Bun filament The process is repealed, the thin filament thus slides and proceeds towards the H zone, ultimately when the muscle contracts fully. (I) the H zone becomes nearly obliterated, (ii) the macromere length greatly reduced, but (iii) the Length of the a band remains unaltered Fig. 9 .1. 1. To illustrate the cross stations Because of the fact that the thin filaments appear to slide during the contraction, the above mentioned mechanism of muscle contraction Is also called sliding theory or ratchet theory Finally the Ca++ ions are pumped out of the cymosely of the cell and retimed to the L tubules and the muscle now relaxes Fig. 9.1.2. To illulistrate {schematically) the relaxed and contracted states of muscle. Role of atp On the myosin head, there are sites for attachment with atp modules flog. 5.2.0 There is no doubt that ATP has two crucial roles, although some uncertainties persist regarding finer details Roles of ATP (to some extent speculative) are given below On the head of myosin, atp molecules can attach (tig.5.2.1.). Myosin has some (but not very powerful) Atlases activity, i.e.. myosin can split ATP Into ADP and Pi f=inorganic phosphate) and this splitting causes release of energy The ADP & PI thus formed may remain still within the myosin head for some time On the other hand actornyosin, i.e., actin myosin complex (i.e. when the cross bridges establish connection with the thin (frankest) has powerful Atlases activity To start worth. presume, that the actual contraction is yet to begin, the myosin heads are yet to get