Several Insights


Muscle Contraction Mechanism



         It is a known fact that in striated muscle fibers each myofibril is composed by serially connected sarcomeres, in which thin and thick filaments may interdigitate and longitudinally slide against each other when muscle contracts (or when it is forcibly lengthened).








  The ends of thin filaments are described as being free (i.e., not connected to other structures within the sarcomere); on the contrary, the ends of thick filaments are considered not free; they are anchored to the Z lines by longitudinal structures made up of titin.



  The ends of  thin filaments are necessarily anchored to Z lines by some longitudinal structures (i.e., hypothetical ultra-thin N filaments) made up, most probable, of nebulin.






The hypothetical N filament in sarcomere


        In a paper entitled "Is there a continuity of filaments in the sarcomere? " (J. theor. Biol. 1985, 112, 177-181), Dan Eremia proposed (for the first time in literature) the necessary existence of some hypothetical ultra-thin filaments connecting the Z line to the tips of thin filaments in the opposite half of the sarcomere. He named these hypothetical ultra-thin structures "N filaments", considering their existence as the only possible explanation of the manner in which the N2 line changes its place in sarcomere when a non-contracting muscle is forcibly lengthened.

        As far as we know, the very existence of the postulated N filaments has not yet been demonstrated histologically (possibly because they were not looked up to at purpose).

         However, by the inference to the best explanation method, one may a priori posit the existence of something that can't be seen yet, so as to explain something else that we all agree takes place.

        For a large body of knowledge of muscle physiology, the predicted behavior of the postulated N filaments is not only a possible explanation; actually, it is the best explanation.

        According to the knowledge in the domain, nebulin (in skeletal muscle) and nebulette (in some cardiac muscle fibers) may be the best candidates to account for the hypothetical ultra-thin N filaments.



Several drawbacks of the cross-bridges theory of muscle contraction


     It is a known fact that during isotonic contraction of striated muscle (or when external forces are  axially applied on an inactive non-contracting striated muscle fiber) thin filaments slide along the thick filaments.


     According to the cross-bridges theory, the active myofilaments sliding is driven by myosin cross-bridges that extend from the thick filaments and cyclically interact with the thin filaments as adenosine triphosphate (ATP) is hydrolyzed.

        It is well know that, from the chemical point of view, the result of the acto-myosin interactions is  the ATP hydrolysis, accompanied by a chemical energy liberation (used, among other, for initiation of some chemical reaction, for promoting intracellular transport of molecules and ions etc). It is also true that acto-myosin interaction are able to produce (by itself) force and motion. Actually, a lot of biological processes take advantage of this force.

       Naturally, we do not put in doubt the very existence of acto-myosin molecular motors in muscles. However, we appreciate that the striated muscle contraction (a very fast and highly coordinated process) cannot rely exclusively on the mechanical side of the acto-myosin interaction. We consider that, in striated muscle, the contribution of acto-myosin motors may be important only in rigor states (and, possibly, in delta states of the muscle, i.e., when muscle is "super-precipitated" with ATP and syneresis of acto-myosin occurs).

       In 2008, Toshio Mitsui and Hiroyuki Ohshima revealed an inconsistency, from a thermodynamic point of view, either in the power stroke model or in the swinging lever model. This inconsistency makes the authors suppose that a molecular complex somewhat similar to a polaron is formed between myosin and actin molecules in muscle.

        Moreover, in our opinion, if the cross-bridges theory has to remain (at any price) the official theory of muscle contraction, it must incorporate at least three new features, in order to better explain some aspects of the striated muscle contraction:

     1) the thin filaments have to behave as rigid rods or (alternatively) they should be perfectly stretched (otherwise, the result of the cross-bridge power stroke will be the bend of thin filaments, without any sarcomere shortening)

        2) after an overstretch of myofibril (i.e. until thick and thin filaments no longer overlap and a gap occurs between the ends of thick and thin filaments),

thin filaments must be guided  by some threads, in order to be able to reoccupy their proper positions inside the thick filament lattice

        During active contraction, when double overlap of thin filaments from opposite ends of the sarcomere starts to occur,

the so-called "free" ends of thin filaments must be pulled (probably by something like elastic or contractile threads) toward the Z lines in the opposite edge of the sarcomere. Otherwise, thin filaments extremities would be pushed away by the forces produced by myosin heads emerging from the thick filaments in the opposite half of the sarcomere

        3) the lengths of the two overlapping regions of thin and thick filament in the sarcomere must stay permanently rigorously equals (otherwise, any initial  imbalance would result in an asymmetrical and ineffective contraction).


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