Following one's heart: cardiac rhythms gate central initiation of sympathetic reflexes

Central nervous processing of environmental stimuli requires integration of sensory information with ongoing autonomic control of cardiovascular function. Rhythmic feedback of cardiac and baroreceptor activity contributes dynamically to homeostatic autonomic control. We examined how the processing of brief somatosensory stimuli is altered across the cardiac cycle to evoke differential changes in bodily state. Using functional magnetic resonance imaging of brain and noninvasive beat-to-beat cardiovascular monitoring, we show that stimuli presented before and during early cardiac systole elicited differential changes in neural activity within amygdala, anterior insula and pons, and engendered different effects on blood pressure. Stimulation delivered during early systole inhibited blood pressure increases. Individual differences in heart rate variability predicted magnitude of differential cardiac timing responses within periaqueductal gray, amygdala and insula. Our findings highlight integration of somatosensory and phasic baroreceptor information at cortical, limbic and brainstem levels, with relevance to mechanisms underlying pain control, hypertension and anxiety

Testing Gravity in the Outer Solar System: Results from Trans-Neptunian Objects

The inverse square law of gravity is poorly probed by experimental tests at distances of ~ 10 AUs. Recent analysis of the trajectory of the Pioneer 10 and 11 spacecraft have shown an unmodeled acceleration directed toward the Sun which was not explained by any obvious spacecraft systematics, and occurred when at distances greater than 20 AUs from the Sun. If this acceleration represents a departure from Newtonian gravity or is indicative of an additional mass distribution in the outer solar system, it should be detectable in the orbits of Trans-Neptunian Objects (TNOs). To place limits on deviations from Newtonian gravity, we have selected a well observed sample of TNOs found orbiting between 20 and 100 AU from the Sun. By examining their orbits with modified orbital fitting software, we place tight limits on the perturbations of gravity that could exist in this region of the solar system.Comment: 20 pages, 4 figures, 2 tables, uses AASTex v5.x macro

The Effect of Columnar Disorder on the Superconducting Transition of a Type-II Superconductor in Zero Applied Magnetic Field

We investigate the effect of random columnar disorder on the superconducting phase transition of a type-II superconductor in zero applied magnetic field using numerical simulations of three dimensional XY and vortex loop models. We consider both an unscreened model, in which the bare magnetic penetration length is approximated as infinite, and a strongly screened model, in which the magnetic penetration length is of order the vortex core radius. We consider both equilibrium and dynamic critical exponents. We show that, as in the disorder free case, the equilibrium transitions of the unscreened and strongly screened models lie in the same universality class, however scaling is now anisotropic. We find for the correlation length exponent $\nu=1.2\pm 0.1$, and for the anisotropy exponent $\zeta=1.3\pm 0.1$. We find different dynamic critical exponents for the unscreened and strongly screened models.Comment: 30 pages 12 ps figure

Force generation in small ensembles of Brownian motors

The motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces that can exceed 150 pN. The molecular details of the motor mechanism are still largely unknown, while other features have been identified, such as the ring-shaped protein structure of the PilT motor. The surprisingly high forces generated by the PilT system motivate a model investigation of the generation of large forces in molecular motors. We propose a simple model, involving a small ensemble of motor subunits interacting through the deformations on a circular backbone with finite stiffness. The model describes the motor subunits in terms of diffusing particles in an asymmetric, time-dependent binding potential (flashing ratchet potential), roughly corresponding to the ATP hydrolysis cycle. We compute force-velocity relations in a subset of the parameter space and explore how the maximum force (stall force) is determined by stiffness, binding strength, ensemble size, and degree of asymmetry. We identify two qualitatively different regimes of operation depending on the relation between ensemble size and asymmetry. In the transition between these two regimes, the stall force depends nonlinearly on the number of motor subunits. Compared to its constituents without interactions, we find higher efficiency and qualitatively different force-velocity relations. The model captures several of the qualitative features obtained in experiments on pilus retraction forces, such as roughly constant velocity at low applied forces and insensitivity in the stall force to changes in the ATP concentration.Comment: RevTex 9 pages, 4 figures. Revised version, new subsections in Sec. III, removed typo

Soil carbon and nutrient accumulation under forest plantations in southern Rwanda

Tree and soil interactions may result in changes in soil carbon and nutrient contents. Forest plantations made up of monodominant stands of 17 different species, some native and some exotic to Rwanda, as well as a stand with mixed native tree species were investigated. Biosequential sampling was used followed by basic soil chemical analyses. Results revealed that the plantation species composition influenced the soil chemical properties. Total soil C and N, C: N ratio, available P, pH, and cation exchange capacity (CEC) differed significantly between plantation stands of different species (P < 0.001, N = 54). Increases in the levels of soil C, total N, CEC and base saturation (BS) were observed mainly in mixed native species (MNS), Polyscia fulva, Casuarina equisetifolia and Eucalyptus saligna. The pH declined slightly in soil beneath some Eucalyptus species treatments and increased in others. The high nutrient uptake by fast-growing trees and the acidic parent material were involved in the acidification process. The findings suggest that the species used in afforestation maintain soil fertility and protect the environment. It is recommended that afforestation of abandoned and less productive lands in Rwanda should utilize fast growing Eucalyptus species in combination with agroforestry and native species in order to maintain or improve soil chemical properties.Key words: Afforestation, base saturation (BS), cation exchange capacity (CEC), forest plantation, soil carbon, total N, Rwanda

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (A(net)) relative to temperature and light limitations in high-latitude mature Pinus sylvestris, including a high-resolution analysis of the seasonality of mesophyll conductance (g(m)) and its effect on the estimation of carboxylation capacity (VCmax). We used a custom-built gas-exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas-exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over A(net). The seasonality of A(net) was strongly dependent on VCmax and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that VCmax limited A(net) in the spring, whereas daily temperatures and the gradual reduction of light availability limited A(net) in the autumn, despite relatively high VCmax. We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between g(m) and A(net), but not between g(m) and stomatal conductance

An Indirect System Identification Technique for Stable Estimation of Continuous-Time Parameters of the Vestibulo-Ocular Reflex (VOR)

The vestibulo-ocular reflex (VOR) is a well-known dual mode bifurcating system that consists of slow and fast modes associated with nystagmus and saccade, respectively. Estimation of continuous-time parameters of nystagmus and saccade models are known to be sensitive to estimation methodology, noise and sampling rate. The stable and accurate estimation of these parameters are critical for accurate disease modelling, clinical diagnosis, robotic control strategies, mission planning for space exploration and pilot safety, etc. This paper presents a novel indirect system identification method for the estimation of continuous-time parameters of VOR employing standardised least-squares with dual sampling rates in a sparse structure. This approach permits the stable and simultaneous estimation of both nystagmus and saccade data. The efficacy of this approach is demonstrated via simulation of a continuous-time model of VOR with typical parameters found in clinical studies and in the presence of output additive noise

Autumn destabilization of deep porewater CO2 store in a northern peatland driven by turbulent diffusion

The CO2 stored in deep peat porewater is viewed as a fixed component of peatland C cycling. Here, the authors reveal a hitherto unknown hydro-physical process that results in sudden losses from this CO2 store every autumn.The deep porewater of northern peatlands stores large amounts of carbon dioxide (CO2). This store is viewed as a stable feature in the peatland CO2 cycle. Here, we report large and rapid fluctuations in deep porewater CO2 concentration recurring every autumn over four consecutive years in a boreal peatland. Estimates of the vertical diffusion of heat indicate that CO2 diffusion occurs at the turbulent rather than molecular rate. The weakening of porewater thermal stratification in autumn likely increases turbulent diffusion, thus fostering a rapid diffusion of deeper porewater CO2 towards the surface where net losses occur. This phenomenon periodically decreases the peat porewater CO2 store by between 29 and 90 g C m(-2) throughout autumn, which is comparable to the peatland's annual C-sink. Our results establish the need to consider the role of turbulent diffusion in regularly destabilizing the CO2 store in peat porewater

Application and validation of the notch master curve in medium and high strength structural steels

This paper applies and validates the Notch master curve in two ferritic steels with medium (steel S460M) and high (steel S690Q) strength. The Notch master curve is an engineering tool that allows the fracture resistance of notched ferritic steels operating within their corresponding ductile-to-brittle transition zone to be estimated. It combines the Master curve and the Theory of critical distances in order to take into account the temperature and the notch effect respectively, assuming that both effects are independent. The results, derived from 168 fracture tests on notched specimens, demonstrate the capability of the Notch master curve for the prediction of the fracture resistance of medium and high strength ferritic steels operating within their ductile-to-brittle transition zone and containing notches