TU, M.S.*; DANIEL, T.L.: Cardiac-like behavior in an insect flight muscle.
Despite the well-known length-dependence of muscle force, few studies have actually located the operating lengths of locomotor muscles on their isometric force-length relationships. A handful of studies in frogs, fish, birds and rabbits, suggest that force may be maximized by muscle shortening across the plateau region of the force-length relationship. Data from scallop adductor muscle and vertebrate cardiac muscle, however, show that these muscles operate on the ascending limbs of their force-length relationships, where force is well below peak values. Working on the dorsolongitudinal (Dl1) wing depressor muscles in the hawkmoth, Manduca sexta, we have recently completed the first study that combines measurements of both in vivo operating length and the isometric force-length relationship for a synchronous insect flight muscle. Length changes of the Dl1s were approximately sinusoidal at 23 ± 1.5 Hz. The strain amplitude of 9% ± 2% was considerably greater than that reported for any asynchronous flight muscle. Like vertebrate cardiac muscle, the Dl1s operate entirely on the ascending limb of the force length curve, and the ascending limb for both muscle types is exceptionally steep compared to vertebrate skeletal muscle. In cardiac muscle, contraction on the ascending limb of the force-length curve plays a crucial regulatory role, allowing the muscle to generate greater force as ventricular filling increases. This characteristic could provide a similar advantage for skeletal muscles in locomotor systems, allowing muscles to generate higher force at greater limb excursions. We suggest that such intrinsic, non-neuronal regulation of muscle force may be a widespread, though to our knowledge, undocumented characteristic of locomotor muscle systems. (NSF grant IBN-9511681)
