FRYE, M.A.*; DANIEL, T.L.: Mechanical encoding properties of the wing hinge stretch receptor in the hawkmoth

Insect flight emerges from complex interactions among patterned motor output, musculoskeletal mechanics, aerodynamic and inertial forces, and neural feedback. In the hawkmoth Manduca sexta, feedback from a wing hinge stretch receptor (SR) is crucial for the visuo-motor control of lift. Using a combination of extracellular recording, optical tracking, and high-speed video, we show that the SR undergoes sinusoidal length changes during tethered flight and fires a burst of spikes near the dorsal stroke reversal. We describe the mechanical encoding properties of the stretch receptor by comparing experimental data to the output of a computational model in an effort to better understand how this specialized sensory organ encodes forces and strains acting on the wing hinge. Using a mechanical actuator, we measured tissue tension, deformation, and SR spiking activity in response to controlled motions of the wing hinge. Step changes in tissue extension evoke an initial rapid increase followed by decay in both tension and SR firing rate. These time dependent responses were fit to models of the tissue and the SR that are comprised of viscous and elastic elements. We compared simulation output to experimental results using sinusoidal motion stimuli matching the in vivo deformations of thewing hinge. For such motions, the SR encodes timing, amplitude, and frequency of experimental wing hinge deformation. Model results closely match tissue tension seen in vitro, however the model does not capture the firing pattern of the stretch receptor. Thus, stretch receptor responses are not determined by simple linear models, and non-linear characteristics or active encoding dynamics are needed to explain the behavior of this mechanoreceptor. Supported by NSF.