Vibrational state distribution of 2-Na^+ ions created in ultracold collisions

The vibrational distribution P(v) of 2-Na^+ ions created in ultracold collisions in a magneto-optical trap has been deter- mined. Only two vibrational states with v = 2 and 3 are popu- lated and we find P(2)=0.29±0.02 and P(3)=0.71±0.02. The results provide conclusive evidence that the ionization mech- anism is photo-associative autoionization,and not photo- associative photoionization and will form a fundamental test for the theoretical description of the process

Large-scale Models Reveal the Two-component Mechanics of Striated Muscle

This paper provides a comprehensive explanation of striated muscle mechanics and contraction on the basis of filament rotations. Helical proteins, particularly the coiled-coils of tropomyosin, myosin and α-actinin, shorten their H-bonds cooperatively and produce torque and filament rotations when the Coulombic net-charge repulsion of their highly charged side-chains is diminished by interaction with ions. The classical “two-component model” of active muscle differentiated a “contractile component” which stretches the “series elastic component” during force production. The contractile components are the helically shaped thin filaments of muscle that shorten the sarcomeres by clockwise drilling into the myosin cross-bridges with torque decrease (= force-deficit). Muscle stretch means drawing out the thin filament helices off the cross-bridges under passive counterclockwise rotation with torque increase (= stretch activation). Since each thin filament is anchored by four elastic α-actinin Z-filaments (provided with force-regulating sites for Ca2+ binding), the thin filament rotations change the torsional twist of the four Z-filaments as the “series elastic components”. Large scale models simulate the changes of structure and force in the Z-band by the different Z-filament twisting stages A, B, C, D, E, F and G. Stage D corresponds to the isometric state. The basic phenomena of muscle physiology, i. e. latency relaxation, Fenn-effect, the force-velocity relation, the length-tension relation, unexplained energy, shortening heat, the Huxley-Simmons phases, etc. are explained and interpreted with the help of the model experiments

Risk Perception in Flight Choice Behaviour

The role of safety is often not included in flight choice behaviour. The perception of safety however does influence the choice of flight. To that end the factors that influence the safety perception are identified and this safety perception is captured as a summarising variable in the flight choice model. From these models combined the willingness-to-pay is determined to improve attributes affecting the safety perception.Civil Engineering and GeosciencesTransport & PlanningTransport, Infrastructure and Logistic

The influence of velocity of length change on tension development in skeletal muscle: Model calculations and experimental results

Force responses obtained during constant velocity length changes on skeletal muscle tissue are simulated by means of two cross-bridge models proposed by Huxley and Simmons (1971, Nature 233, 533–538) and by Julian et al. (1974, Biophys. J. 14 546–562). An implicit method was used for the numerical approximation in the simulations. The simulated force transients due to constant velocity length changes are found to be in qualitative agreement with re-investigated experimental results obtained from the whole sartorius muscle of the frog. A non-linear tension transient is observed, dependent both on amplitude and on velocity of release revealing an inflexion which gives the transient a shoulder shape. When velocity is increased the inflexion occurs earlier and at a lower tension value. A non-linear transient is observed during stretches performed at moderate velocities. Force responses are found to deviate concavely downwards from a linear time course. Simulations, however, predict a rather linear tension transient for comparable velocities. Implications of the experimental findings are discussed for both models

Elastic properties of relaxed, activated, and rigor muscle fibers measured with microsecond resolution.

Tension responses due to small and rapid length changes (completed within 40 microseconds) were obtained from skinned single-fiber segments (4- to 7-mm length) of the iliofibularis muscle of the frog incubated in relaxing, rigor, and activating solution. The fibers were skinned by freeze-drying. The first 500 microseconds of the responses for all three conditions could be described with a linear model, in which the fiber is regarded as a rod composed of infinitesimally small identical segments, containing an undamped elastic element, two damped elastic elements and a mass in series. An additional damped elastic element was needed to describe tension responses of activated fibers up to the first 5 ms. Consequently phase 1 and phase 2 of activated fibers can be described with four apparent elastic constants and three time constants. The results indicate that fully activated fibers and fibers in rigor have similar elastic properties within the first 500 microseconds of tension responses. This points either to an equal number of attached cross-bridges in rigor and activated fibers or to a different number of attached cross-bridges in rigor and activated fibers and nonlinear characteristics in rigor cross-bridges. Mass-shift measurements obtained from equatorial x-ray diffraction patterns support the latter possibility

Weakly attached cross-bridges in relaxed frog muscle fibers.

Tension responses due to small, rapid length changes (completed within 40 microseconds) were obtained from skinned single frog muscle fiber segments (4-10 mm length) incubated in relaxing and rigor solutions at various ionic strengths. The first 2 ms of these responses can be described with a linear model in which the fiber is regarded as a rod, composed of infinitesimally small, identical segments, containing one undamped elastic element and two or three damped elastic elements and a mass in series. Rigor stiffness changed less than 10% in a limited range, 40-160 mM, of ionic strength conditions. Equatorial x-ray diffraction patterns show a similar finding for the filament spacing and intensity ratio I(11)/I(10). Relaxed fibers became stiffer under low ionic strength conditions. This stiffness increment can be correlated with a decreasing filament spacing and (an increased number of) weakly attached cross-bridges. Under low ionic strength conditions an additional recovery (1 ms time constant) became noticeable which might reflect characteristics of weakly attached cross-bridges