Muscle filament lattices and stretch-activation: the match-mismatch model reassessed.

A mechanism for the observed enhanced stretch-activation phenomenon in insect asynchronous flight muscles has been postulated and developed in terms of the matched helical structures of the actin and myosin filaments in the asynchronous flight muscles of Lethocerus. It was suggested that at different sarcomere lengths with different filament overlaps there would be a changing probability of myosin crossbridge attachment to actin according to whether there was match or mismatch between the myosin and actin arrays. Evidence is provided here that, when Lethocerus structure is considered in detail, the explanation appears to fail. Results on other insect asynchronous flight muscles of different structure (e.g. Apis) also seem to contradict the match-mismatch model. All striated muscle types considered here (fish, frog, Lethocerus, Apis, blowfly) appear to be designed to give constant probability of crossbridge attachment to actin as the filaments move axially, apart from the well-known effects of changing total filament overlap. Alternative stretch-activation mechanisms are considered, especially in terms of the unusual thin filament regulatory system in some insect asynchronous flight muscles.