When Is a Fly in the Ointment a Solution and not a Problem?

Abstract

See related article, pages 1212–1218 You never know from where and when the next clue to any particular scientific problem will arise. In the case of cardiac function, it may have been in 1948 from Professor J.W.S. Pringle, who was trying to figure out how flies manage to fly upside down.1 Having mounted a truncated fly wing apparatus on a gyroscopic base, he serendipitously noted that when inertial and damping conditions were just right, the truncated wings oscillated at more than 100 s−1 independent of neuronal innervation (Figure, A). This, he surmised, was attributable to matching an intrinsic resonant property of insect flight muscle to the elastic and inertial properties of the wing and exoskeletal structure, producing a resonant system generating oscillatory power. He called this intrinsic property of insect flight muscle “stretch–activation” because it is a recurrent stretching in the face of persistent Ca2+ levels, not Ca2+ pulses, that activate the actomyosin interaction in these insect flight muscles. This resonant system resembles a parent pushing a child on a swing. The parent must push at the correct time; that is, the addition of energy to the system must be matched to the intrinsic resonance of the system. A, Graph of what Pringle called “free oscillation” of the truncated wing apparatus. The work loop is counter-clockwise, so that as the muscle oscillates, tension is greater during shortening than at the same length during the stretch part of the cycle. The muscle is performing work on the apparatus during shortening. Adapted from Pringle1 with permission of the publisher. B, Abstract tracing of an isometrically contracting rabbit slow muscle fiber after an imposed stretch showing the immediate spike in tension, decay, and second rise in tension. The resonant system in flight muscle can also be …