(see also "Several Insights into muscle contraction mechanism " part 1 )

 

 

An alternative view

on the mechanism of striated muscle contraction

 

 

        Even the very existence of elastic threads connecting the tips of thin filaments to the Z lines from the opposite edge of sarcomere would be accepted, still many sides of muscle contraction could not be satisfactorily explained by the cross-bridges theory of muscle contraction. Definitively, an alternative theory is needed.

        For instance, Gerald Pollack admited that, similar to the mechanism of contraction in many artificial muscles, the filaments in sarcomere contract themselves, very like the condensation of polymers during a phase transition:

http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=17178&DID=219763&action=detailmusclecontraction

      

       We propose here a new theory of the striated (essentially different from the cross-bridges theory).

         This theory is based on the following assumptions:

      a) the threads connecting the tips of thin filaments to the Z lines from the opposite edge of sarcomere are made up of contractile units, serially  connected

       

        b) these units would be able to shorten when they are in the overlap region of thin and thick filaments (if the muscle fiber is in active state)

 

          Hereby, the folding of monomers would be nothing but a "side-effect" of ATP splitting.

 

 

 

See here a critical opinion (written about two decades ago by Tatsuo Iwazumi, who happens to be then a referee of one of our papers focused on this new theory).

       c) the N filament active shortening would be accomplished by progressive and sequential folding of its monomers:

 

         If these assumptions prove to be true, a large body of knowledge of muscle physiology could be more readily assimilated into this framework of ideas.

         For instance, the yet unexplained mechanical behavior during stretch of an active contracting muscle  may be easily explained. It is known, for example, that when an actively contracting muscle is forcibly lengthened, the mechanical tension developed by muscle increases significantly during the stretch (Edman et al. 1978). This enhancement of mechanical performance by stretch during tetanic contractions of vertebrate skeletal muscle fibers has not yet received a satisfactory explanation.

        According to many authors, the cross-bridge theory cannot explain the nature of the shortening heat during muscle contraction.

      The cross-bridge model also fails to correctly predict force transients during muscle lengthening.

       There are many other experimental data that cross-bridge theory cannot explain. One of them is the strange behavior of a sarcomere with broken thin filaments (K.Trombitas, A.Tigyi-Sebes -Cross-bridge interaction with oppositely polarized actin filaments in double-overlap zones of insect flight muscle - 1984,  Nature, 309, p. 168), behavior which  can be easily explained if one accepts that N filaments do exist and have contractile properties.

        Borejdo and Oplatka showed that a skinned glycerinated striated muscle fiber may shorten even if its thick filaments were removed; all it needs is globular segments S1 of myosin, calcium ions and ATP (Oplatka, A., I. Gadasi, J.Borejdo, The contraction of ghost myofibrils and glycerinated muscle fibers irrigated with heavy meromyosin subfragment-1, Biochem.Biophys.Res. Commun. 1974, 58, 905-912) http://linkinghub.elsevier.com/retrieve/pii/S0006291X74802299

 

     

       Obviously, cross-bridge theory cannot explain the contraction of a cross-bridge depleted muscle. However, this extraordinary fact can easily be explained if one accepts that N filaments do really exist and have contractile properties.

 

 

Acknowledgments

        The author is grateful to dr. Adrian Iftime, who generously permitted the use of some of his animated illustrations in this paper. He also thanks him for stimulating discussions.