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At least 1,693 records · Page 94Linked to original sources

Adaptive force generation for precision-grip lifting by a spectral timing model of the cerebellum.

We modeled adaptive generation of precision grip forces during object lifting. The model presented adjusts reactive and anticipatory grip forces to a level just above that needed to stabilize lifted objects in the hand. The model obeys principles of cerebellar structure and function by using slip sensations as error signals to adapt phasic motor commands to tonic force generators associated with output synergies controlling grip aperture. The learned phasic commands are weight- and texture-dependent. Simulations of the new circuit model reproduce key aspects of experimental observations of force application. Over learning trials, the onset of grip force buildup comes to lead the load force buildup, and the rate-of-rise of grip force, but not load force, scales inversely with the friction of the object.

Adaptation, Physiological↗

3D-Visualization of particle deposition patterns in the human lung generated by Monte Carlo modeling: methodology and applications.

An advanced stochastic model is described which enables the generation of three-dimensional particle deposition patterns in the human lung. While particle trajectories are represented as a combination of randomly oriented vectors in a coordinate system with the trachea defining the z direction, deposition sites of single particles are determined by using a grid of specific volume elements (voxels). After storage in an array, the spatial coordinates are visualized with an appropriate graphic editor, enabling the combination of respective deposition images with lung outlines and the creation of two-dimensional distributions by sectioning the three-dimensional structures at pre-defined positions.

Aerosols↗

Generations and aging: a longitudinal study.

This study is a longitudinal investigation of the relationship between age and subjective outlook. Over the years, a number of theoretical positions have been introduced to either account for or to minimize age differences in attitudes, values and beliefs. The author has organized these theories of aging into three basic sociological fremeworks or models: the "generations" model, the "age status" model and the "illusion of differences" model. Using a relatively simple methodological desihn, hypotheses derived from these models were tested through secondary analysis of survey data. Strong support was found for the "generations" hypothesis, weak support for the "age status" hypothesis, and no support at all for the "illusion of differences" hypothesis.

Adult↗

Modeling a non-inactivating delayed rectifier cardiac current using voltage clamp data.

This paper describes a new parameter estimation method applicable to experimental voltage-clamp records. The method is based on the Hodgkin-Huxley (HH) representation of a generic non-inactivating delayed rectifier current (IK) which can be assimilated to the delayed rectifier potassium current of cardiac cells. The model involves a single gating variable of activation (chi) of degree (lambda chi). Its parameters include the voltage-dependent steady-state characteristic (chi infinity), time constant tau chi, the degree lambda chi as a positive integer, and the maximal conductance gK. The method is based on linear optimization. It implements a series of least-squares minimization steps to calculate a first estimate of each model parameter, followed by global minimization to obtain final estimates. The required data, in the form of ionic current responses, correspond to standard voltage-clamp protocols. The effects of noise are minimized by avoiding the use of the time derivative of IK in the calculations. Simulated voltage-clamp data using either a HH model or a five-state Markov chain (MC) model served two purposes: (i) to test the performance of the HH parameter estimation method, and (ii) to study the suitability of the HH model to reproduce data generated by models other than HH. A nominal MC model was obtained by fitting its current responses to those of the HH model. Rate constants of the nominal MC model were then modified and voltage-clamp current responses were generated. Excellent results were obtained with HH and nominal MC data. Data sets generated by a 20% change in the rate constants of the nominal MC model showed that the closed-state rate constants have only a limited influence on the HH parameter estimates, whereas changes in the closed-to-open rate constants produce substantial effects. Nevertheless, a given MC data set can be fitted quite closely by a HH model. In the light of these simulation results it is indicated that an hybrid HH-MC representation of IK data would be more flexible than a straight HH model by removing some of the constraints between the rate constants, and less cumbersome than a straight MC model by substantially reducing the number of parameters to be estimated.

Electrophysiology↗

Efficient synthesis of a porphyrin-N-tripod conjugate with covalently linked proximal ligand: toward new-generation active-site models of cytochrome c oxidase.

[formula: see text] A new-generation cytochrome c oxidase active-site model compound (4) featuring both a trisimidazolyl moiety and a proximal base has been designed and efficiently synthesized. During this study, a facile method based on the chemistry of a 4-magnesioimidazole derivative to synthesize 4-imidazolyl-containing tripodal ligands (7) has been developed.

Binding Sites↗

Understanding the antifungal activity of terbinafine analogues using quantitative structure-activity relationship (QSAR) models.

Terbinafine and its analogues, which are a major class of non-azole antifungal agents, are known to act by inhibition of squalene epoxidase enzyme in fungal cells. We have performed a quantitative structure-activity relationship (QSAR) study on a series of 92 molecules using different types of physicochemical descriptors. Inhibitors were divided into five classes depending upon chemical structure. QSAR models were generated for correlation between antifungal activity against Candida albicans using genetic function approximation (GFA) technique. Equations were evaluated using internal as well as external test set predictions. Models generated for all these classes show that steric properties and conformational rigidity of side chains play an important role for the activity. The present QSAR analysis agrees with the results of the previously reported CoMFA study.

Antifungal Agents↗

Adjustable primitive pattern generator: a novel cerebellar model for reaching movements.

Cerebellum has been assumed as an array of adjustable pattern generators (APGs). In recent years, electrophysiological researches have suggested the existence of modular structures in spinal cord called motor primitives. In our proposed model, each "adjustable primitive pattern generator" (APPG) module in the cerebellum is consisted of a large number of parallel APGs, the output of each module being the weighted sum of the outputs of these APGs. Each spinal field is tuned by a coefficient, representing a descending supraspinal command, which is modulated by ith APPG correspondingly. According to this model, motor control can be interpreted in terms of the modification of these coefficients. Vector summation of force fields implies that the complex nonlinearities in neuronal behavior are eliminated, causing our model to be simple and linear. The force field vectors, derived from motor primitives, depend on the state of movement and its derivative and the time that causes different repertoire of movement. This is physiologically plausible. Our model agrees with virtual trajectory hypothesis, stating that dynamics are not computed explicitly in central nervous system, but the desired trajectory, is fed into the spinal cord. We think that the dysmetria and the ataxia seen in some cerebellar diseases may be the result of local disruption of some APPGs. Accordingly, determining the exact location of related motor primitives in human spinal cord and stimulating them by functional neurostimulation may provide a good management for these clinical signs. Surely, experimental researches and clinical trials are needed to validate our hypothesis.

Animals↗

An analytical model for surface EMG generation in volume conductors with smooth conductivity variations.

A nonspace invariant model of volume conductor for surface electromyography (EMG) signal generation is analytically investigated. The volume conductor comprises planar layers representing the muscle and subcutaneous tissues. The muscle tissue is homogeneous and anisotropic while the subcutaneous layer is inhomogeneous and isotropic. The inhomogeneity is modeled as a smooth variation in conductivity along the muscle fiber direction. This may reflect a practical situation of tissues with different conductivity properties in different locations or of transitions between tissues with different properties. The problem is studied with the regular perturbation theory, through a series expansion of the electric potential. This leads to a set of Poisson's problems, for which the source term in an equation and the boundary conditions are determined by the solution of the previous equations. This set of problems can be solved iteratively. The solution is obtained in the two-dimensional Fourier domain, with spatial angular frequencies corresponding to the longitudinal and perpendicular direction with respect to the muscle fibers, in planes parallel to the detection surface. The series expansion is truncated for the practical implementation. Representative simulations are presented. The proposed model constitutes a new approach for surface EMG signal simulation with applications related to the validation of methods for information extraction from this signal.

Action Potentials↗

Ventrolateral medullary respiratory network and a model of cough motor pattern generation.

The primary hypothesis of this study was that the cough motor pattern is produced, at least in part, by the medullary respiratory neuronal network in response to inputs from "cough" and pulmonary stretch receptor relay neurons in the nucleus tractus solitarii. Computer simulations of a distributed network model with proposed connections from the nucleus tractus solitarii to ventrolateral medullary respiratory neurons produced coughlike inspiratory and expiratory motor patterns. Predicted responses of various "types" of neurons (I-DRIVER, I-AUG, I-DEC, E-AUG, and E-DEC) derived from the simulations were tested in vivo. Parallel and sequential responses of functionally characterized respiratory-modulated neurons were monitored during fictive cough in decerebrate, paralyzed, ventilated cats. Coughlike patterns in phrenic and lumbar nerves were elicited by mechanical stimulation of the intrathoracic trachea. Altered discharge patterns were measured in most types of respiratory neurons during fictive cough. The results supported many of the specific predictions of our cough generation model and suggested several revisions. The two main conclusions were as follows: 1) The Bötzinger/rostral ventral respiratory group neurons implicated in the generation of the eupneic pattern of breathing also participate in the configuration of the cough motor pattern. 2) This altered activity of Bötzinger/rostral ventral respiratory group neurons is transmitted to phrenic, intercostal, and abdominal motoneurons via the same bulbospinal neurons that provide descending drive during eupnea.

Animals↗

Three-dimensional soft tissue prediction using finite elements. Part I: Implementation of a new procedure.

BACKGROUND AND AIM: The prediction of soft tissue esthetics is important for achieving an optimal esthetic outcome in orthodontic treatment planning. Applicable procedures have so far been restricted to two-dimensional profile predictions that have not proven to be very reliable. The goal of this investigation was therefore to develop a novel finite element-based procedure that allows a three-dimensional, easily visualized, quantitative analysis and prediction of soft tissue behavior for the clinician. The procedure to be developed should be easy to handle and not entail any additional radiation exposure for the patient. MATERIAL AND METHODS: Using a three-dimensional scanner, the facial surfaces of 20 probands were digitalized and individual FEM models were generated. RESULTS: After reduction of data redundancy via several conversion steps, a patient-specific simulation model was prepared consisting of 20,000 to 40,000 individual elements to which specific physical properties could be assigned. The average time required for generating a virtual model was 50 minutes. Problems occurring during model generation were rare (mainly shadowing phenomena and movement artifacts). CONCLUSION: The procedure outlined herein makes the reliable generation of patient-specific simulation models possible for facial soft tissue prediction in orthodontics.

Adult↗

Artificial Intelligence for Natural Products Discovery and Development.

Natural products (NPs) remain a cornerstone of modern drug discovery, offering stereochemical complexity and diverse bioactivities that precisely modulate therapeutic targets, refined through billions of years of evolution. However, their research has long been hindered by inefficient, empirical workflows, high resource consumption, structural complexity, and the "multicomponent, multi-target" nature of their mechanisms. The exponential growth of genomic, metabolomic, and spectral data has overwhelmed conventional analytical methods, exposing critical bottlenecks in handling high-dimensional, heterogeneous datasets that exceed human interpretive capacity. Artificial intelligence (AI) is emerging as a transformative paradigm to address these challenges, integrating multi-omics and chemical data to shift NP research from fragmented empiricism toward mechanism-driven, precision-oriented development. By leveraging deep learning architectures- including graph neural networks, Transformers, and diffusion-based generative models-AI enables systematic decoding of NP biosynthesis, automated structure elucidation, rational target identification, knowledge extraction from vast unstructured scientific literature, and de novo molecular design. This review comprehensively surveys recent advances in AI applications across the full NP discovery and development pipeline, encompassing genome mining, structure-based and ligand-based virtual screening, multimodal structural characterization, lead optimization, and biosynthetic pathway engineering. We further examine the emerging roles of protein-centric, molecule- centric, and multimodal foundation models, as well as large language models, in bridging genotype-to-chemotype gaps and unlocking unstructured scientific knowledge. Finally, we discuss critical challenges including data scarcity, representational limitations for complex stereochemistry, physical plausibility in generative models, and the urgent need for experimental validation, while outlining future directions toward autonomous experimentation, closed-loop optimization, and human-AI collaborative discovery.

Artificial intelligence↗

Updated risk adjustment mortality model using the complete 1.1 dataset from the American College of Cardiology National Cardiovascular Data Registry (ACC-NCDR).

OBJECTIVES: To revise and update a risk adjustment model for in-hospital mortality following percutaneous coronary intervention (PCI) procedures using all data from the 1.1 version of the American College of Cardiology National Cardiovascular Data Registry (ACC-NCR). BACKGROUND: A model based on data received at the ACC-NCDR from 1998-2000 was previously reported. The revision of this mortality model reflects all of the data submitted using 1.1 data specifications and collected through the second quarter of 2001. The model was applied to selected high-risk subgroups from a sample of data collected during the year 2001 from version 2.0 of the NCDR. METHODS: Data on 173,743 PCI procedures collected at the ACC-NCDR between January 1, 1998 and March 31, 2001 were analyzed. A mortality model was generated as well as separate models for presentation with and without acute myocardial infarction within 24 hours. The model was used to generate predicted mortalities that were compared to observed mortalities in more current high-risk patient subgroups in the NCDR. RESULTS: The same factors that were previously found to be associated with increased risk of PCI mortality were re-verified in the current analysis. Inclusion of the complete 1.1 dataset produced some changes in the regression weights and the constant value. Excellent discrimination was achieved in the revised model (C-Index = 0.89). The model was applied to high-risk patient groups from data collected on 76,249 during the calendar year 2001 using the 2.0 NCDR data elements and definitions. These analyses showed a high level of agreement between observed mortality of each subgroup and the predicted mortality rates generated from the revised 1.1 PCI mortality model. CONCLUSIONS: Risk adjustment models for in-hospital mortality following PCI for all patients and for those with and without recent MI were regenerated using all data collected from the 1.1 data specifications of the ACC-NCDR and validated on high-risk groups from data collected during 2001 under data version 2.0 of the NCDR. These models reflect the most up-to-date analysis of mortality prediction from this large, multi-center national database.

Aged↗

A bifurcating autoregression model for cell lineages with variable generation means.

The bifurcating autoregression model for cell lineage data is extended to allow for observations whose means vary from generation to generation. The maximum likelihood estimates of this extended model are found and used to estimate the generation means and overall variance. The model is also used to test for inherited effects as measured by mother-daughter correlation, and for environmental effects as measured by the sister-sister correlation, conditional on inherited effects. Applications to data sets on EMT6 cells and Escherichia coli are given.

Animals↗

An extended transition probability model of the variability of cell generation times.

The transition probability model of variability of cell generation times is extended so that the rate constant for the transition from the A-state to the B-phase of the cell cycle depends on on time which a particular all has already spent in the A-state. A specific time dependence of this rate constant is introduced. It is determined by the value of one constant which is then an additional parameter of the model. The corresponding cell population kinetics are calculated and compared to existing experimental evidence. The model accounts satisfactorily for the generation time distribution function and for the shortening of the G1 phase of binucleate cells. The time dependence of the transition probability is related to the cell kinetics of an hypothetical cell constituent. A possible relationship is proposed between the chemical parameters with the cell and the parameters of the cell population kinetics.

Animals↗

A 3D structural model of memapsin 2 protease generated from theoretical study.

AIM: To build a 3D structural model of memapsin 2 (M2) protease for theoretical study and drug design. METHODS: Structural alignment was performed based on multiple and pairwise sequence alignment of three templates. After the initial model was generated, energy minimization was completed by applying molecular mechanics method. Molecular dynamics (MD) technique was used to do further structural optimization. RESULTS: The 3D structural model of memapsin 2 was constructed. The model is reasonable according to several validation criteria. The active-site motifs of M2 are structurally supported by a beta-sheet rich domain and linked together with this domain through alpha helices. Tyr132 contained in beta-hairpin is a general characteristic of aspartic protease. The Calpha atom superimposing result is a direct verification that M2 is structurally unique but still belongs to the aspartic protease superfamily. CONCLUSION: The 3D-structure model from our study is informative to guide future molecular biology study about M2 and drug design based on database searching.

Amino Acid Sequence↗

Computer aided stress analysis of long bones utilizing computed tomography.

A computer aided analysis method has been developed which utilizes computed tomography (CT) and a finite element (FE) computer program to determine the stress-displacement pattern in a long bone section. The CT data file provides the geometry, the apparent density and the elastic properties for the three-dimensional FE model. A developed pre-processor generates the FE model of a human diaphyseal tibia section which is then analyzed by the SAP IV finite element program. The results obtained are sorted and displayed by a developed post-processor and compared with stresses and deformations from the literature. The model generation method was verified by applying it to a model of simple geometry and boundary conditions, then comparing the results with the analytical solution of the same problem. The convergence behavior of nodal displacements was tested as a function of mesh refinement. This method provides an automatic, versatile, non-invasive and accurate tool of long bone modeling for finite element stress analysis.

Algorithms↗

[Experimental test of a model of memory-representation-generation in learning and recognition].

A highly structured set of stimuli was used in this study. Each stimulus had four binary attributes, whose values were determined so that any two stimuli could be transformed into each other by changing values of one or more attributes. In one experiment, 93 undergraduates rated similarity of paired stimuli. In another experiment, the same subjects learned three stimuli which were presented one after another for 10 seconds each. Later, in the recognition task, they made "old" or "new" judgment and rated the confidence of their judgment for each of the test stimuli. Two groups of subjects served the two experiments in different order. The results showed that (1) the rated similarity between the paired stimuli is a monotonically decreasing function of the number of transformations needed to get the pair equal, (2) the recognition confidence for new stimuli is significantly higher for stimuli generated by relevant transformations from the learned stimuli than for stimuli not so generated. The results support a model of memory-representation-generation (Suto, 1987, 1988), but not "prototype plus transformation model" nor "context model".

Adult↗

Modeling UHMWPE wear debris generation.

It is widely recognized that polyethylene wear debris is one of the main causes of long-term prosthesis loosening. The noxious bioreactivity associated with this debris is determined by its size, shape, and quantity. The aim of this study was to develop a numerical tool that can be used to investigate the primary polyethylene wear mechanisms involved. This model illustrates the formation of varying flow of polyethylene debris with various shapes and sizes caused by elementary mechanical processes. Instead of using the classical continuum mechanics formulation for this purpose, we used a divided materials approach to simulate debris production and release. This approach involves complex nonlinear bulk behaviors, frictional adhesive contact, and characterizes material damage as a loss of adhesion. All the associated models were validated with various benchmark tests. The examples given show the ability of the numerical model to generate debris of various shapes and sizes such as those observed in implant retrieval studies. Most of wear mechanisms such as abrasion, adhesion, and the shearing off of micro-asperities can be described using this approach. Furthermore, it could be applied to study the effects of friction couples, macroscopic geometries, and material processing (e.g. irradiation) on wear.

Biocompatible Materials↗