Muscle is highly organized across multiple length scales. Consequently, small changes in the arrangement of myofilaments can influence macroscopic mechanical function. Two leg muscles of a cockroach have identical innervation, mass, twitch responses, length-tension curves and force-velocity relationships. However, during running, one muscle is dissipative (a 'brake'), while the other dissipates and produces significant positive mechanical work (bifunctional). Using time-resolved X-ray diffraction in intact, contracting muscle, we simultaneously measured the myofilament lattice spacing, packing structure and macroscopic force production of these muscles to test whether structural differences in the myofilament lattice might correspond to the muscles' different mechanical functions. While the packing patterns are the same, one muscle has 1 nm smaller lattice spacing at rest. Under isometric stimulation, the difference in lattice spacing disappeared, consistent with the two muscles' identical steady-state behavior. During periodic contractions, one muscle undergoes a 1 nm greater change in lattice spacing, which correlates with force. This is the first identified structural feature in the myofilament lattice of these two muscles that shares their whole-muscle dynamic differences and quasi-static similarities.
Small magnetic fields are found to greatly enhance the reversible room temperature conductometric responses of n and p- type porous silicon (PS) interfaces, treated with nanostructured island sites containing paramagnetic Co(II) and Fe(II). At concentrations sufficiently low so as to avoid cross talk between the nanostructured island sites, the response to NO concentrations demonstrates the significant effect which the Co(II) and Fe(II) have on the decorated extrinsic semiconductor majority charge carriers as they direct a dominant electron transduction process for reversible electron transduction and chemical sensing (Inverse Hard and Soft acid/base principle) in the absence of significant chemical bond formation. Co(II) and Fe(II) oxide sites enhance response and provide a means for small magnetic fields to interact with and enhance the sensor interface response. For p-type systems, the interaction is with small virtually constant thermal electron populations lying above the Fermi energy at 0 K. The electron removal rate increases with magnetic field strength. At the highest magnetic fields and NO analyte concentrations the available electron population is depleted, and the response to the analyte decreases at higher concentrations. At lower magnetic fields (<1000 G), the response faithfully follows concentration. For n-type systems, the magnetic field interaction increases resistance. This increase in response may be attributed to the interaction with donor levels ∼0.025 eV below the conduction band. A substantial enhancement of sensor response relative to that for the Co(II) and Fe(II) treated PS interfaces is observed, with the introduction of a small magnetic field greatly increasing an already enhanced conductometric response.
Shortening deactivation (SD) is a delayed decrease in force following rapid muscle shortening, while stretch activation (SA) is a delayed rise in force following muscle lengthening. Together, SD and SA enable power production at constant calcium concentrations in insect indirect flight muscles (IFM), and assist calcium in modulating force levels in vertebrate cardiac muscle. However, the molecular mechanisms behind SA and SD remain unknown. Specifically, the SD mechanism has not been investigated, likely because muscle physiologists have presumed that it is the same as SA. But, no experiments have been conducted to test this. Therefore, we performed fiber mechanics with highly SA Drosophila and Lethocerus IFM, and minimally SA Drosophila jump muscle (TDT), to define SD characteristics and compare them to SA. We found that all fiber types exhibited SD. Lethocerus presented the greatest SD tension decrease of 20 mN/mm 2 . This was $1.5-fold greater than Drosophila TDT, and $6.5-fold greater than Drosophila IFM. However, isometric tension in Drosophila TDT (69 mN/mm 2 ) was higher than Lethocerus (37 mN/mm 2 ) and Drosophila IFM (3 mN/mm 2 ). Thus, when normalized to isometric tension, Drosophila IFM exhibited a 75% SD tension decrease, while Lethocerus and Drosophila TDT decreased by 38% and 18%, respectively. A similar order was found when comparing normalized SA tension: 127% for Drosophila IFM, 40% for Lethocerus IFM, and 6% for Drosophila TDT. The IFM in each species showed the same percent change in tension when comparing SD to SA, but surprisingly, this was not the case in the TDT where SD tension was $3.5-fold greater than SA tension. With an understanding of the SD characteristics of each fiber type, we can now perform experiments, such as protein isoform exchanges between Drosophila muscle types, to elucidate the mechanism behind SD. 1991-PosRestoring Real-Space Images of the Structure of Muscle and Other Biological Specimens from Conventional X-Ray Diffraction Patterns Hiroyuki Iwamoto. Dept Res/Utilization, SPring-8 JASRI, Sayo-gun, Japan. The X-ray diffraction pattern of an object is the Fourier transform of its electron density distribution. Applying an inverse Fourier transformation to the diffraction pattern should in principle result in the restoration of the real-space structure of the object at different magnifications. However, the phase information is lost when the diffraction pattern is recorded on a detector, and the real-space structure is not restored without phase information. This is called the phase problem and has been the greatest obstacle in implementing X-ray diffraction techniques in wide fields. Recently, however, computer-based techniques have been developed to restore the realspace images of objects by recover or preserve phase information. These are collectively called coherent diffractive imaging (CDI). These techniques have been successfully implemented for high-contrast materials such as metal nanoparticles, but they have been less successful for low-contrast biological specim...
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