Temperature gradients drive functional heterogeneity within muscle
George, Nicole Tasnin
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During locomotion, muscles respond to an animal's varying need for speed, endurance, strength, and agility. Therefore, in addition to operating as motors, muscles also act as brakes, springs, and struts. Interestingly, if the physical, morphological, and neurological parameters determining muscle performance vary regionally, a muscle may actually concurrently operate with multiple functions. In this study, I investigated the functional consequences of an intramuscular temperature gradient, arising from the inevitable heat exchange between metabolic heat production and surface cooling. In Chapter 1, I defined the determinants of muscle function and highlight the diverse roles muscles perform. I then discussed an emerging field, which shows that the factors determining muscle performance can regionally vary. In Chapter 2 (George and Daniel, 2011), I characterized the temperature gradient throughout a muscle in the hawkmoth, <italic>Manduca sexta</italic>. I recorded multi-site temperature measurements during tethered flight and conducted isometric contraction tests to determine the effect of temperature on contractile dynamics. We found that the significant temperature gradient throughout the muscle will cause the warm region to contract with rapid individual twitches, while the cooler region will contract in unfused tetany. In Chapter 3 (George et al., 2012), I determined how this temperature gradient affects regional mechanical power output. Work-loop methods, where muscle is cyclically lengthened and stimulated, allowed us to measure mechanical work. We found that the warm region of muscle will produce positive power, thereby functioning as a motor. As temperature decreases, power output decreases, transitioning to negative values. Thus, the cooler region of muscle may serve a completely separate role, including that of a brake and/or spring. In Chapter 4, I investigated if a temperature gradient additionally creates a locked-spring lattice, capable of storing and releasing energy, in the cool region of muscle. We used X-ray fiber diffraction to visualize the molecular dynamics of a contraction. A restrained lattice combined with reduced cross-bridge cycling in cool muscle indicates that cross-bridges are less able to detach from their binding sites. Thus, a temperature gradient likely forms a regional locked-spring lattice, whereby energy can be stored in the axial and radial extensions of cross-bridges.
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