Abstract
HIGH wingbeat frequencies are required for flight in many insects, and have led to striking specialisations1. The thorax and wings form a resonant system whose oscillation is sustained by alternating contractions of antagonistic muscles. The action of these flight muscles is to produce only a small-amplitude deformation of the thorax, mechanically amplified into wing movements. The muscles are diverse in arrangement1 and ultrastructure2. In several insect orders, they are of a characteristic ‘fibrillar’ type whose very regular architecture is well known3. As recognised largely through the work of Pringle4, certain properties of fibrillar muscle permit more rapid wing-beats because the oscillation does not depend on pacemaking neural input: the muscles receive only slow and irregular stimulation, and the contraction of each is triggered, after a delay, directly by the stretch applied to it while its antagonist shortens. Studies of isolated glycerol- extracted fibrillar muscle fibres show that unusual properties underlying this sensitivity to stretch are intrinsic to the contractile apparatus itself5. ATPase levels and tension are low in calcium-activated fibres at rest length, but are increased by slight extension (0.1–5%). The muscles exhibit both actin-linked and myosin-linked regulation typical of many invertebrate muscles6, and the additional regulatory effects of mechanical conditions are not understood. Stretch has been presumed to modify cross-bridge attachment by some unknown effect on the myosin filamenats4 (possibly transmitted through structures which connect them to the Z-line in certain insect muscles7). I suggest here that many aspects of stretch–activation in fibrillar muscles may be explained by the particular geometry of their myosin and actin filaments.
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WRAY, J. Filament geometry and the activation of inisect flight muscles. Nature 280, 325–326 (1979). https://doi.org/10.1038/280325a0
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DOI: https://doi.org/10.1038/280325a0
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