Psychosocial Stress-Induced Analgesia: An Examination of Effects on Heat Pain Threshold and Tolerance and of Neuroendocrine Mediation

Stress-induced analgesia (SIA) is an adaptive response of reduced nociception following demanding acute internal and external stressors. Although a psychobiological understanding of this phenomenon is of importance for stress-related psychiatric and pain conditions, comparably little is known about the psychobiological mechanisms of SIA in humans. The aim of this study was to investigate the effects of acute psychosocial stress on heat pain perception and its possible neuroendocrine mediation by salivary cortisol levels and α-amylase activity in healthy men. Employing an intra-individual assessment of heat pain parameters, acute psychosocial stress did not influence heat pain threshold but significantly, albeit slightly, increased heat pain tolerance. Using linear mixed-model analysis, this effect of psychosocial stress on heat pain tolerance was not mediated by increases of salivary cortisol and state anxiety levels or by the activity of α-amylase. These results show that while psychosocial stress is selectively analgesic for heat pain tolerance, this observed effect is not mediated by stress-induced increases of salivary cortisol and α-amylase activity, as proxies of both the hypothalamus-pituitary-adrenal axis and the autonomic nervous system activation

Micro-Hall Magnetometry Studies of Thermally Assisted and Pure Quantum Tunneling in Single Molecule Magnet Mn12-Acetate

We have studied the crossover between thermally assisted and pure quantum tunneling in single crystals of high spin (S=10) uniaxial single molecule magnet Mn12-acetate using micro-Hall effect magnetometry. Magnetic hysteresis experiments have been used toinvestigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow (~0.1 K) or broad (~1 K) temperature interval depending on the magnitude and direction of the applied field. For low external fields applied parallel to the easy axis, the energy levels that dominate the tunneling shift abruptly with temperature. In the presence of a transverse field and/or large longitudinal field these energy levels change with temperature more gradually. A comparison of our experimental results with model calculations of this crossover suggest that there are additional mechanisms that enhance the tunneling rate of low lying energy levels and broaden the crossover for small transverse fields.Comment: 5 pages, 5 figure

Experimental Investigation of Injection-Coupled High-Frequency Combustion Instabilities

Self-excited high-frequency combustion instabilities were investigated in a 42-injector cryogenic rocket combustor under representative conditions. In previous research it was found that the instabilities are connected to acoustic resonance of the shear-coaxial injectors. In order to gain a better understanding of the flame dynamics during instabilities, an optical access window was realised in the research combustor. This allowed 2D visualisation of supercritical flame response to acoustics under conditions similar to those found in European launcher engines. Through the window, high-speed imaging of the flame was conducted. Dynamic Mode Decomposition was applied to analyse the flame dynamics at specific frequencies, and was able to isolate the flame response to injector or combustion chamber acoustic modes. The flame response at the eigenfrequencies of the oxygen injectors showed symmetric and longitudinal wave-like structures on the dense oxygen core. With the gained understanding of the BKD coupling mechanism it was possible to derive LOX injector geometry changes in order to reduce the risks of injection-coupled instabilities for future cryogenic rocket engines

High Speed Visualization of Flame Response in a LOX/H2 Combustion Chamber during external Excitation

With a model combustion chamber the interaction of an acoustic excitation with a burning LOX/H2 spray was investigated. Pressure oscillations are coupled into the combustion chamber by an additional exhaust nozzle and a siren wheel. Due to the fact that the orientation of the excited acoustic modes can be fixed with this secondary nozzle it is possible to investigate the interaction of acoustics with combustion chamber processes like injection, atomization, vaporization, mixture formation and combustion either with the spray in a pressure node or anti node. Thus the role of pressure or velocity coupling with the processes mentioned above can be studied. For a variety of injection conditions the interaction of the acoustic excitation with the spray flame has been investigated. Only damped acoustic oscillations have been observed, at no operating conditions combustion instabilities were found. High speed visualization of the OH emission of the flame is used to get information on the spatially and temporally resolved evolution of the heat release. From the OH-emission and dynamic pressure recordings a response factor of the combustion process is determined for the case of the excitation of the first tangential mode (1T). The acoustic 1T-resonances of the combustor show non-Lorentzian line profiles. There is indication that the excited modes are not pure but that there is always a superposition of various different eigenmodes in the combustion chamber. This complex dynamics of the acoustic field is not well understood up to now