Neural control of aerodynamic power during flight in Drosophila indirect flight muscles

Insect Indirect Flight Muscles (IFMs) generate the highest mechanical power measured in the animal kingdom. With each wingbeat, IFM contraction is activated by stretch but is not directly coupled to the neural drive. Yet during flight, power must be dynamically regulated in order for the fly to perform different aerial maneuvers. Currently, there is no satisfactory explanation for this well-regulated power in a muscle that is only loosely controlled by the nervous system. To understand this system, these experiments focus on the relationships between the rate of Drosophila IFM neural stimulation and power, and between stimulation rate and calcium levels. A moving visual pattern elicited power modulations in a tethered fly while simultaneously monitoring wing kinematics and IFM spike rate. Steady-state power levels were directly proportional to different spike rates. Yet changes in power were asymmetrical: power rose quickly with spike rate increases, but decayed more slowly with corresponding spike rate decreases. Also, pair-wise recording from combinations of fibers did not provide evidence of selective motor unit recruitment. The nervous system thus retains control of power, but involves non-linear mechanisms.; To assess calcium regulation in IFMs, two techniques were used: (1) calcium-sensitive fluorescent dyes were injected into single IFM fibers; (2) flies were genetically-engineered to express cameleon---a chimeric protein that couples calcium concentration with Fluorescent Resonance Energy Transfer. During flight, high spike rates corresponded with high levels of calcium accumulating in the muscle. Calcium entered the sarcoplasm rapidly with muscle spikes, but decayed more slowly, suggesting calcium is a molecular correlate for flight power. Additionally, calcium responses to individual spikes exhibited a spike-dependent amplitude reduction at flight onset. The relationships between spike rate and calcium levels were further explored by electrically stimulating the muscles in quiescent flies via the giant fiber system. Calcium concentration is directly proportional to stimulation frequency in the range of spike rates observed during flight. Although the calcium dependence curve for stretch activation is a steep sigmoidal function, these results suggest that the nervous system uses spike rate to vary calcium within the central linear range of the activation curve, thereby regulating power output.