Meeting Abstract
Insect flight muscles have historically been divided into two distinct classes: synchronous muscles contract once per neural excitation while asynchronous muscles exhibit delayed stretch activation (SA) and shortening deactivation (SD) to produce power when cyclically stretched. However, SA and SD have been observed in synchronous vertebrate skeletal and cardiac muscle. Instead of distinct classes, we hypothesize that insect flight muscles exhibit a continuum of physiological properties. To address this hypothesis, we mounted synchronous Manduca sexta dorsolongitudinal muscles (DLMs) in a dual-mode muscle lever at 35°C. At tetanus, we stretched the muscle under in vivo conditions while measuring force output. Following a brief hold, we returned the muscle to rest length. Unlike prior work in synchronous locust flight muscle, we found significant SA characterized by a delay of 46 ± 2 ms (n = 7) and tension rise of 330 ± 110 mN (n = 7). Therefore, among flying insects, SA is not unique to asynchronous muscle. Temperature dependences on rise time are consistent with SA in asynchronous beetle and bumblebee muscle but a delay of one wingbeat period is significantly longer than that of the beetle. Despite antagonistic muscle arrangement and SA, Manduca sexta is a synchronous flyer because tonic Ca2+ is not present in vivo and SA may be too slow. Furthermore, we saw no evidence of SD, which may be necessary for asynchronous operation. This also suggests that SA and SD require different molecular machinery. In conclusion, we present asynchronous-like properties of synchronous flight muscle. Instead of distinct muscle classes, perhaps the synchronous and asynchronous operation of insect flight muscle is determined by their operating regime and a continuum of physiological properties.
