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PubMed2006

Gravity replacement during running in simulated microgravity.

INTRODUCTION: During treadmill exercise on the International Space Station (ISS), a restoring load from a subject load device (SLD) is applied through a shoulder-and-waist harness to pull the exercising crewmember toward the treadmill surface. The capacity of this arrangement to provide 1-g-like reaction forces may be critical for effective use of the treadmill as a countermeasure to musculoskeletal changes during prolonged spaceflight. This study in simulated microgravity evaluated the comfort and function during running of the ISS harness used with a new SLD in a system that allows more even distribution of the load between the waist and shoulders. METHODS: Using a zero-gravity locomotion simulator, 12 subjects completed three 5-min running trials at a constant speed (3.35 m x s(-1)) using three SLD loads [50%, 75%, and 100% of bodyweight (BW)] presented at random and a shoulder-to-waist loading ratio of 50:50. Subjective ratings of discomfort, ground reaction forces (GRFs), and SLD loads were collected. RESULTS: A load of 100% BW resulted in similar GRF profiles (peak and rate of change of force) to those reported for 1-g running over ground and were also comfortably tolerated (mean Borg scale rating 3.9/10). DISCUSSION: With an appropriate harness and SLD system, 1-g-like GRF profiles can be generated at the feet during simulated microgravity running. Such forces can be achieved with a level of discomfort rated better than "somewhat uncomfortable," suggesting that running with 1-g loads could be an effective component of musculoskeletal countermeasures during long-duration spaceflight.

Adult↗
PubMed2004

Finite element analysis of three designs of an implant-supported molar crown.

STATEMENT OF PROBLEM: The optimal method of implant support for a single mandibular molar crown is controversial because commonly used, threaded, root-form implants developed by Branemark were not originally designed to support individual crowns. PURPOSE: The purpose of this study was to develop a finite element model of a single mandibular first molar crown supported by (1) a standard 3.75-mm-diameter implant, (2) a 5-mm, wide-diameter implant, and (3) double standard-diameter implants, and to compare the induced displacements as a result of various loading conditions. MATERIAL AND METHODS: Three-dimensional finite element models were made to simulate the 3 single-molar implant designs. Each model was analyzed with 2 force magnitudes (35 N and 70 N) and with 2 force directions (vertical and 15 degrees to the vertical axis). Displacements in 3-dimensional space as a result of the simulated loading conditions were evaluated along 3 primary axes, mesiodistal, faciolingual, and superior-inferior. RESULTS: Mesiodistal and buccolingual displacements for the crown supported by the 5-mm-diameter implant were reduced by approximately 50% compared with the crown supported by the 3.75-mm implant when the crowns were loaded at the distobuccal cusp tip or the distal marginal ridge. The double-implant design recorded the least mesiodistal displacement with off-center loading of the crown. CONCLUSION: When the crown was loaded off-center, the double-implant design produced substantially less displacement when compared with either of the single-implant designs.

Biomechanical Phenomena↗
PubMed1998

Cell-based model of the Limulus lateral eye.

We present a cell-based model of the Limulus lateral eye that computes the eye's input to the brain in response to any specified scene. Based on the results of extensive physiological studies, the model simulates the optical sampling of visual space by the array of retinal receptors (ommatidia), the transduction of light into receptor potentials, the integration of excitatory and inhibitory signals into generator potentials, and the conversion of generator potentials into trains of optic nerve impulses. By simulating these processes at the cellular level, model ommatidia can reproduce response variability resulting from noise inherent in the stimulus and the eye itself, and they can adapt to changes in light intensity over a wide operating range. Programmed with these realistic properties, the model eye computes the simultaneous activity of its ensemble of optic nerve fibers, allowing us to explore the retinal code that mediates the visually guided behavior of the animal in its natural habitat. We assess the accuracy of model predictions by comparing the response recorded from a single optic nerve fiber to that computed by the model for the corresponding receptor. Correlation coefficients between recorded and computed responses were typically >95% under laboratory conditions. Parametric analyses of the model together with optic nerve recordings show that animal-to-animal variation in the optical and neural properties of the eye do not alter significantly its response to objects having the size and speed of horseshoe crabs. The eye appears robustly designed for encoding behaviorally important visual stimuli. Simulations with the cell-based model provide insights about the design of the Limulus eye and its encoding of the animal's visual world.

Animals↗
PubMed2002

A hybrid Monte Carlo method for crystal structure determination from powder diffraction data.

A hybrid Monte Carlo algorithm for crystal structure determination from powder diffraction data is presented. The algorithm combines the key components of molecular dynamics and Monte Carlo simulations to achieve efficient sampling of phase space, allowing the crystal structure of capsaicin to be determined from powder diffraction data more effectively than by a simulated-annealing approach. The implementation of the algorithm, the choice of the simulation parameters and the performance of the algorithm are discussed.

Journal Article↗
PubMed1999

Molecular dynamics simulations of the ErbB-2 transmembrane domain within an explicit membrane environment: comparison with vacuum simulations.

Two 500-ps molecular dynamics simulations performed on the single transmembrane domain of the ErbB-2 tyrosine kinase receptor immersed in a fully solvated dilauroylphosphatidyl-ethanolamine bilayer (DLPE) are compared to vacuum simulations. One membrane simulation shows that the initial alpha helix undergoes a local pi helix conversion in the peptide part embedded in the membrane core similar to that found in simulation vacuum. Lipid/water/peptide interaction analysis shows that in the helix core, the intramolecular peptide interactions are largely dominant compared to the interactions with water and lipids whereas the helix extremities are much more sensitive to these interactions at the membrane interfaces. Our results suggest that simulations in a lipid environment are required to understand the dynamics of transmembrane helices, but can be reasonably supplemented by in vacuo simulations to explore rapidly its conformational space and to describe the internal deformation of the hydrophobic core.

Amino Acid Sequence↗
PubMed2003

Experimental verification and simulation of negative index of refraction using Snell's law.

We report the results of a Snell's law experiment on a negative index of refraction material in free space from 12.6 to 13.2 GHz. Numerical simulations using Maxwell's equations solvers show good agreement with the experimental results, confirming the existence of negative index of refraction materials. The index of refraction is a function of frequency. At 12.6 GHz we measure and compute the real part of the index of refraction to be -1.05. The measurements and simulations of the electromagnetic field profiles were performed at distances of 14lambda and 28lambda from the sample; the fields were also computed at 100lambda.

Journal Article↗
PubMed2002

[Improvement on simulation algorithm of excitation propagation in heart modeling].

It is important to simulate the excitation propagation process of cardiac bio-electricity in the research of ECG forward problem. Traditional methods describe them with wave simulation algorithm such as LFX simulation algorithm and vector propagation algorithm etc, these methods have some problems to certain extent, due to the presence of discreteness of space and time and asymmetry of the myocardium. This paper discussed the simulation algorithm in 2-dimension space under the circumstance of layered and non-layered structure of myocardium. By calculating the theoretic values of simulating time based on Huygen's principle, we found that there were errors in LFX algorithm and no errors in vector propagation algorithm under the circumstance of non-layered structure of the myocardium, no mater what myocardium is isotropic or anisotropic. However, there exist errors from both algorithms when the myocardium has the layered structure. An improved algorithm is proposed and the simulations have been performed to examine the efficacy of the new algorithm, and the errors are reduced obviously. By increasing the number of myocardial blocks in the model, we also analyzed its influence on the error of simulation algorithm.

Algorithms↗
PubMed1999

Volume kinetics of Ringer solution after surgery for hip fracture.

PURPOSE: To study the time course of volume changes during and after infusion of Ringer's solution in elderly patients after a standardised trauma. METHODS: The kinetics of 12.5 ml.kg-1 Ringer's solution infused over 30 min were studied in ten patients one day after surgery for hip fracture (mean age, 70 yr) and in an age- and sex-matched control group. Hemodilution, as measured every five minutes for 90 min, was used to calculate the size of the fluid space expanded by the fluid (V) and the elimination rate constant (kr). The baseline fluid balance status in the patients and the controls was compared by bioelectrical impedance analysis. RESULTS: The size of V was 4.1 +/- 0.51 (mean +/- SEM) in the patients and 3.4 +/- 0.21 in the controls (P:NS) while the corresponding results for kr were 85 +/- 12 and 166 +/- 27 ml.min-1, respectively (P < 0.04). Bioelectrical impedance analysis showed that the extracellular fluid space and the total body water volumes did not differ between the two groups. Computer simulations based on the data obtained for V and kr indicate that trauma increases the dilution of the plasma volume and the retention of fluid in response to slow and moderate infusion rates, while these indices of short-term changes in fluid balance remain the same in the two groups during very rapid infusion of Ringer's solution. CONCLUSION: A slower elimination rate increased dilution of plasma and retention of fluid when Ringer's solution was infused in elderly trauma patients.

Aged↗
PubMed2002

Effect of the glial envelope on extracellular K(+) diffusion in olfactory glomeruli.

In many species, including vertebrates and invertebrates, first-order olfactory neuropils are organized into spherical glomeruli, partially enveloped by glial borders. The effect of this characteristic organization on olfactory information processing is poorly understood. The extracellular concentration of potassium ions ([K(+)]) must rise around olfactory receptor axons in specific glomeruli following odor-induced activation. To explore the time course and magnitude of K(+) accumulation and possible effects of such accumulation on neural activity within and among glomeruli, we developed a theoretical model to simulate the diffusion of K(+) in extracellular spaces of the glomeruli of the moth Manduca sexta. K(+) released by activated axons was assumed to diffuse through the extracellular spaces in glomeruli and the glial borders that surround them. The time-dependent diffusion equations were solved in spherical coordinates using a finite-difference method. The results indicate that the glial envelope forms a significant barrier to the spread of K(+) between neighboring glomeruli, thus reducing the likelihood of cross-talk between glomeruli, and may cause elevation of extracellular [K(+)] to levels that influence neural activity within the activated glomerulus for many seconds. Such effects could enhance olfactory discrimination and sensitivity, respectively.

Animals↗
PubMed2000

The use of micro-worlds for human factors research in extended spaceflight.

This article provides an analysis of the utility of micro-worlds for human factors research in extended spaceflight. It outlines potential areas of application of the micro-world research paradigm in the context of spaceflight. While the research literature provides several examples of micro-worlds that simulate different work environments of high complexity, little work has been done with this tool in the operational environment of spaceflight. The author presents a micro-world, called CAMS, that has been especially developed for use in space-related research. The results from a research programme (comprising seven studies using CAMS) are presented. Based on a comparative analysis of the benefits and limitations of micro-worlds compared to other simulation types, the author argues that micro-worlds can be a very effective tool, provided their inherent limitations are taken into consideration.

Astronauts↗
PubMedDate not supplied

[Validity of "lump approximation" for the description of overall brain electrical activity using an simulation model].

Brain rhythmic activity model on the basis of space uniform neuronal network consisting of excitatory and inhibitory elements was examined. Network activity description by "lump approximation" was compared with simulation model computation. Lump approximation parameters dependence on the properties of neurons and synapses as well as conditions of the transition between network activity types which imitate desynchronous (low voltage fast activity) and synchronous (high voltage slow waves) brain activities were obtained.

Animals↗
PubMed2006

A severe vicious cycle in uncontrolled subarachnoid hemorrhage: the effects on cerebral blood flow and hemodynamic responses upon intracranial hypertension.

In subarachnoid hemorrhage (SAH), Cushing postulated that the increase in systemic arterial pressure (SAP) in response to elevation of intracranial pressure (ICP) was beneficial to cerebral perfusion. However, in uncontrolled SAH, the increased SAP may cause more bleeding into the subarachnoid space and further increase the ICP. We created an animal model to simulate SAH by connecting a femoral arterial catheter to the subarachnoid space. The global cerebral blood flow (CBF) was measured with a venous outflow method. The purposes were to observe the CBF change under the simulated SAH, and to evaluate the effects of an adrenergic blocker and a vasodilator. In addition, spectral analysis of the aortic pressure and flow was employed for the analysis of hemodynamic changes at various ICP levels. When the femoral arterial blood was allowed to flow into the subarachnoid space, the ICP was elevated. The Cushing response to increased ICP caused an increase in SAP. A vicious cycle was generated between ICP and SAP. The CBF under the vicious cycle was greatly depressed. The dog developed pulmonary edema (PE) within 5 mins. An alpha-adrenergic blocker (phentolamine) and a vasodilator (nitroprusside) were beneficial to the reduction of SAP and ICP, improvement of CBF, and prevention of PE. Hemodynamic analysis revealed that graded increases in ICP caused increases in SAP, total peripheral resistance, arterial impedance, and pulse reflection with decreases in stroke volume, cardiac output and arterial compliance. The hemodynamic changes may contribute to acute left ventricular failure that leads to pressure and volume loading in the lung circulation, and finally acute PE.

Adrenergic alpha-Antagonists↗
PubMed2002

Simulations of NMR-detected diffusion in suspensions of red cells: the "signatures" in q-space plots of various lattice arrangements.

Coherence effects from pulsed field-gradient spin-echo (PGSE) nuclear magnetic resonance diffusion experiments have been observed and characterized for diffusants in many heterogeneous systems, ranging from porous materials to cell suspensions. The resulting coherence patterns appear in plots of the normalized PGSE signal intensities as a function of the spatial wave vector Q in a so-called q-space plot. The origin of these phenomena and their mathematical and physical underpinnings are now well established. We have conducted a number of studies of diffusion-coherence phenomena in suspensions of red blood cells and have made extensive use of computer simulations of molecular diffusion in virtual lattices of cells to aid in the interpretation and analysis of experimental data. In the current work we extended the canonical model used in these studies to investigate the effect that varying the packing arrangement of cells in the suspension has on the coherence patterns, as seen in q-space plots. We show that changes in the packing arrangement of cells are reflected in the q-space plots and in the results of diffusion tensor analysis and thus we speculate upon the possible clinical importance of these findings.

Anisotropy↗
PubMed2005

Assessment of simulated surgical skills in parabolic microgravity.

BACKGROUND: During spaceflight crew health is paramount in the success of flight missions. The delivery of healthcare during flight requires crew readiness for medical and surgical response. METHODS: There were 20 participants who were evaluated for accurate performance of 4 basic laparoscopic surgical skills (clip applying, cutting, grasping, and suturing) during parabolic weightlessness using an inanimate workstation aboard the NASA KC-135 aircraft. RESULTS: Data indicate that motor skill performance decreased within the parabolic microgravity flight environment. Performance in parabolic microgravity flight included futile effort with an increase in number of tasks attempted and a decrease in tasks completed successfully. CONCLUSIONS: There is a decreased frequency of accurate task completion in parabolic microgravity flight, but it is not an obstacle to implementation of effective training for providing in-flight medical care. The data reveal that individuals perform basic laparoscopic surgical simulation with greater effort in microgravity following simulation training.

Aerospace Medicine↗
PubMed2005

Surface plasmon interference nanolithography.

A new nanophotolithography technique based on the interference of surface plasmon waves is proposed and demonstrated by using computer simulations. The wavelengths of the surface plasmon waves at metal and dielectric interfaces can reach the nanometer scale while their frequencies remain in the optical range. As a result, the resolution of this surface plasmon interference nanolithography (SPIN) can go far beyond the free-space diffraction limit of the light. Simulation results show that one-dimensional and two-dimensional periodical structures of 40-100 nm features can be patterned using interfering surface plasmons launched by 1D gratings. Detailed characteristics of SPIN such as field distribution and contrast are also investigated.

Computer Simulation↗
PubMed2005

Optical measurements reveal nature of intercellular coupling across ventricular wall.

Previously, we showed that intercellular uncoupling through gap junctions is an important mechanism for maintaining transmural heterogeneities of repolarization that are responsible for ventricular arrhythmias in disease states such as heart failure. However, rotational anisotropy between transmural muscle layers also may influence coupling. To determine the effect of rotational anisotropy on transmural coupling, we developed a numerical three-dimensional model of passive cardiac tissue in which rotational anisotropy was varied in a controlled fashion. Simulations of optical mapping demonstrated that spatial averaging produced a voltage decay in space best fit by a single decaying exponential compared with the theoretically predicted decay. As fiber orientation varied by 90 degrees with respect to the transmural surface, the effective transmural space constant (lambda(TM)) changed by only 0.31% in simulations. In contrast, reducing intercellular conductivity by 24% decreased lambda(TM) by 7.7%. In the canine wedge preparation (n = 5), lambda measured by optical mapping of the epicardial and subepicardial surface was similar transverse (lambda(TV) = 0.73 +/- 0.10 mm) and transmural (lambda(TM) = 0.70 +/- 0.08 mm) to subepicardial fibers. We confirmed previous findings that lambda(TM) in subepicardial layers was significantly reduced by 14 +/- 2% compared with deeper layers of myocardium, providing evidence for transmural uncoupling in the epicardial-midmyocardial interface. These data establish the theoretical and experimental basis for measuring intercellular coupling between muscle layers spanning the ventricular wall with optical mapping techniques. Furthermore, this study demonstrates that transmural uncoupling at the epicardial-midmyocardial interface may be attributable to heterogeneous expression of cardiac gap junctions and not rotational anisotropy.

Action Potentials↗
PubMed1995

Morphomechanics of the humero-ulnar joint: I. Joint space width and contact areas as a function of load and flexion angle.

BACKGROUND: Previous studies have shown that the trochlear notch is deeper than necessary for an exact fit with the humerus. However, humero-ulnar joint space width and contact areas have so far not been quantified for variations in the load and angle of flexion. METHODS: Six fresh cadaveric specimens were investigated at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of flexion and at loads of 25 and 500 N, simulating resisted elbow extension. The joint space width and contact were determined, using polyether casting material. RESULTS: At 25 N all joints made contact in the ventral and dorsal aspects of the articular surfaces, whereas in the depth of the trochlear notch the joint space was on average between 0.3 and 2.8 mm wide, with some variation between individuals. At 500 N the joint space width was considerably reduced and the contract areas expanded towards the depth of the notch. The size of the dorsal contact areas was significantly smaller at 30 degrees and that of the ventral ones at 120 degrees, their ventro-dorsal ratio decreasing considerably from 30 degrees to 120 degrees (p < 0.01). CONCLUSION: These results indicate that the size of the contact areas depends to a slight extent on the joint position, but that at all loads and flexion angles a bicentric contact and an important central joint space width emerge because of the concave incongruity of the joint. These data may be used for numerical calculations, analysing the effects of incongruity on the joint stress and on the functional adaptation of the subarticular tissues.

Adult↗
PubMed2002

A k-space method for coupled first-order acoustic propagation equations.

A k-space method for large-scale simulation of ultrasonic pulse propagation is presented. The present method, which solves the coupled first-order differential equations for wave propagation in inhomogeneous media, is derived in a simple form analogous to previous finite-difference methods with staggered spatial and temporal grids. Like k-space methods based on second-order wave equations, the present method is exact for homogeneous media, unconditionally stable for "slow" [c(r) < or = c0] media, and highly accurate for general weakly scattering media. In addition, unlike previous k-space methods, the form of the method allows straightforward inclusion of relaxation absorption and perfectly matched layer (PML) nonreflecting boundary conditions. Numerical examples illustrate the capabilities of the present k-space method. For weakly inhomogeneous media, accurate results are obtained using coarser temporal and spatial steps than possible with comparable finite-difference and pseudospectral methods. The low dispersion of the k-space method allows accurate representation of frequency-dependent attenuation and phase velocity associated with relaxation absorption. A technique for reduction of Gibbs phenomenon artifacts, in which compressibility and exponentially scaled density functions are smoothed by half-band filtering, is introduced. When employed together with this smoothing technique, the k-space method provides high accuracy for media including discontinuities, high-contrast inhomogeneities, and scattering structures smaller than the spatial grid resolution.

Acoustics↗