(see also "Several Insights into muscle contraction mechanism" part 1 and 2)

           It is known that a muscle fiber can be stretched until thick and thin filaments no longer overlap and a gap occurs between the ends of thick and thin filaments. In such greatly stretched muscle, a set of superthin  filaments is seen bridging the gap.



         In 1991,Trombitas,  Baatsen,  Kellermayer and Pollack used monoclonal antibodies to titin and obtained evidence confirming that gap filaments contain titin and identified them as parts of filaments connecting the thick filament ends to the Z lines in sarcomere (Nature and origin of gap filaments in striated muscle).

        Unfortunately, the authors ignored (at that time, at least) the possibility that gap filaments may contain yet another set of filaments (possible made up by nebulin) connecting the so called "free ends" of the thin filaments to the Z line from the opposite half of the sarcomere.

         However, three years earlier (1988), Higuchi, Yoshioka and Maruyama  reported that skinned fibers from frog semitendinosus muscle when stretched in relaxing solution from a sarcomere length of 2.5 micrometer to greater sarcomere lengths (about 3.3 micrometer), spontaneously shortened back to their original length when the stretch ceased.

         When the stretch was done to a sarcomere lengths beyond overlap of the thick and thin filaments, the thin filaments did not re-enter the thick filament array but buckled at the A-I junction.

        If these fibres were subsequently activated (via calcium-induced interactions of thin and thick filaments in the presence of ATP) they contracted and the thin filaments re-entered the thick filament array, taking up their former positions.

        In order to explain how titin manages to guide both thin and thick filaments, Funatsu, Higuchi, Yoshioka, Maruyama et al. proposed in 1993 a complicated model of a three-dimensional network of titin filaments in the I band in which titin filament is divided into two thinner strands, one of them being weakly and randomly associated with thin filaments.



          The incredible performance that, once  the stretching forces are removed, the thin filaments are able to re-enter the thick filament lattice (see a microscop electronic image of a myofibril transversal section, in order to better understand that this task  is, really, a difficult one),



and to regain their previous positions into this lattice could be possible only if thin filaments were guided by threads connecting their so called "free ends" to the Z line from the opposite half of the sarcomere.

         Moreover, F-actin (the main constituent of the thin filament) is a very flexible structure. The first experimental evidence for flexibility was obtained  by S. Fujime (1970) by quasi-elastic scattering measurements. Thermal bending movements of single F-actin filaments were directly observed by dark field optical microscopy, using F-actin decorated with heavy mero-myosine fragments (H. Nagashima and S. Asakura, 1980, J. Mol. Biol. 136, 169). Using their published micrographs, we constructed two videoclips showing the successive profiles of a such filament having 10 micrometer in length (1 and 2).

         The fact that after exaggerate stretches, the thin and thick filaments interdigitate completely only when ATP and calcium ions are added, supports the hypothesis that these threads may have contractile properties, so being able to pull thin filaments in their appropriates old positions between thick filaments.