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At least 1,063 records · Page 59Linked to original sources
PubMed1980

A quantitative structural model of the testis of fertile males with normal sperm counts.

The morphology of testicular biopsies of 8 healthy fertile men with proven normal sperm counts was studied using stereological techniques. Quantitative estimations were made of different cellular compartments in the testicular biopsies. The experimental errors of observation, homogeneity of the section, accuracy of the measurements and errors caused during the preparation of the sections were studied. A stereological model of the human testis comprising all compartments was designed. The present observations on the stereological parameters were compared with the results in literature; generally a good agreement was found. This stereological model enabled the authors to establish the quantitative correlations that exist between the separate compartments of the human testis. The tubular length density was positively correlated with the tubular surface density. The tubular surface density showed a negative correlation with the volume density of the germinal cell nuclei. The tubular volume density showed a negative correlation with the volume density of the Leydig cells. The intra-tubular volume density correlated negatively with the volume density of the Leydig cells and with the volume density of the remaining extra-tubular tissue. The intra-tubular tissue density correlated negatively with the volume density of the intra-tubular space.

Biopsy↗
PubMed2006

Structural models for twin interfaces in Pd thin films.

In a Pd thin film grown on a 6H-SiC (0001) substrate, several twin interfaces have been observed, classified as (1) {111}Sigma = 3 twin interfaces and (2) {221}Sigma = 3 twin interfaces. The first ones are coherent twin interfaces and in some cases exhibit steps of one or more atomic layers in height. The second ones are perpendicular to the first ones. They usually appear with a rigid-body displacement, but in some cases no displacement was observed. Models for the interfaces are proposed.

Journal Article↗
PubMed2002

A biorobotic structural model of the mammalian muscle spindle primary afferent response.

A biorobotic model of the mammalian muscle spindle Ia response was implemented in precision hardware. We derived engineering specifications from displacement, receptor potential, and Ia data in the muscle spindle literature, allowing reproduction of muscle spindle behavior directly in the robot's hardware; a linear actuator replicated intrafusal contractile behavior, a cantilever-based transducer reproduced sensory membrane depolarization, and a voltage-controlled oscillator encoded strain into a frequency signal. Aspects of muscle spindle behavior not intrinsic to the physical design were added in control software using an adaptation of Schaafsma's mathematical model. We tuned the response to biological ramp and hold metrics including peak, mean, dynamic index, time domain response, and sensory region displacement. The model was validated against biological Ia response to ramp and hold, sinusoidal and fusimotor inputs. The response with dynamic or static gamma motorneuron input was excellent across all studies. The passive spindle response matched well in five of the nine measures. Potential applications include basic science muscle spindle research and applied research in prosthetics and robotics.

Algorithms↗
PubMed1985

Structural models for human spinal motion segments based on a poroelastic view of the intervertebral disk.

Analytical and finite element models (FEMs) were used to quantify poroelastic material properties for a human intervertebral disk. An axisymmetric FEM based on a poroelastic view of disk constituents was developed for a representative human spinal motion segment (SMS). Creep and steady-state response predicted by FEMs agreed with experimental observations, i.e., long-time creep occurs with flow in the SMS, whereas for rapid steady-state loading an "undrained," nearly incompressible response is evident. A relatively low value was determined for discal permeability. Transient and long-term creep FE analyses included the study of deformation, pore fluid flow, stress, and pore fluid pressure. Relative fluid motion associated with transient creep is related to nuclear nutrition and the overall mechanical response in the normal disk. Degeneration of the disk may be associated with an increase in permeability.

Biomechanical Phenomena↗
PubMed1992

Temporal structure model of binaural masking level difference.

It is shown that a simple cross-correlation model is not adequate to explain both binaural masking level difference (MLD) and spatial selective attention. The reason is that for a low-intensity signal in NoS(pi) condition the maximal activity in the binaural analyzer as a function of interaural delay in single spectral channel is independent of signal intensity. On the other hand, if detection ability is associated with the isolation of tonically firing units, MLD is simply explained as the increase in firing synchronization as a function of the signal's interaural phase difference (IPD). Quantitatively results are presented based on numerical solutions of the model.

Attention↗
PubMed2006

Structural modeling and analysis of signaling pathways based on Petri nets.

The purpose of this paper is to discuss how to model and analyze signaling pathways by using Petri net. Firstly, we propose a modeling method based on Petri net by paying attention to the molecular interactions and mechanisms. Then, we introduce a new notion "activation transduction component" in order to describe an enzymic activation process of reactions in signaling pathways and shows its correspondence to a so-called elementary T-invariant in the Petri net models. Further, we design an algorithm to effectively find basic enzymic activation processes by obtaining a series of elementary T-invariants in the Petri net models. Finally, we demonstrate how our method is practically used in modeling and analyzing signaling pathway mediated by thrombopoietin as an example.

Algorithms↗
PubMed1995

Anatomy and electrophysiology of fast central synapses lead to a structural model for long-term potentiation.

Detailed knowledge of the anatomy of central synapses is essential to the interpretation of the vast quantity of electrophysiological findings that have been published in recent years. When their function is considered, it is not surprising that, in both anatomy and electrophysiology, fast central synapses show important differences to the neuromuscular junction. This review concentrates on the detailed anatomy of the common excitatory synapses that impinge on dendritic spines, but also refers to other glutamatergic and GABAergic synapses. This information is brought together with present knowledge of the electrophysiology of fast neurotransmission in the brain. Various types of evidence are outlined, explaining why it is now widely accepted that release of transmitter from a single vesicle virtually saturates the small number of receptors available on the postsynaptic membrane of central synapses. Finally, the anatomic literature suggests that a particular type of spine synapse, which electron microscopy reveals to have a perforated active zone, may represent a synapse with high efficacy. This suggestion is shown to be completely compatible with the electrophysiological data, and a model is presented that shows that all the apparently conflicting data in the field of long-term potentiation could be compatible. This stresses the need for cooperative collaboration between laboratories that have apparently conflicting findings.

Animals↗
PubMed2004

Structural models of comorbidity among common mental disorders: connections to chronic pain.

Patterns of comorbidity among common mental disorders can be understood from the perspective of a model that regards mood, anxiety and somatization disorders as elements within an internalizing spectrum of disorder, and substance use and antisocial behavior disorders as elements within a separate externalizing spectrum of disorder. In this chapter, we evaluate the possibility of linking this model to literature on chronic pain. Evidence from psychosocial and biological perspectives points towards mechanisms that link chronic pain to internalizing disorders. Such evidence indicates that the internalizing-externalizing model may provide a useful framework for suggesting new directions for research on connections between chronic pain and mood, anxiety, and related disorders and traits.

Antidepressive Agents↗
PubMed2004

Surface structure, model and mechanism of an insect integument adapted to be damaged easily.

BACKGROUND: Several sawfly larvae of the Tenthredinidae (Hymenoptera) are called easy bleeders because their whole body integument, except the head capsule, disrupts very easily at a given spot, under a slight mechanical stress at this spot. The exuding haemolymph droplet acts as a feeding deterrent towards invertebrate predators. The present study aimed to describe the cuticle surface, to consider it from a mechanistic point of view, and to discuss potential consequences of the integument surface in the predator-prey relationships. RESULTS: The integument surface of sawfly larvae was investigated by light microscopy (LM) and scanning electron microscopy (SEM) which revealed that the cuticle of easy bleeders was densely covered by what we call "spider-like" microstructures. Such microstructures were not detected in non-easy bleeders. A model by finite elements of the cuticle layer was developed to get an insight into the potential function of the microstructures during easy bleeding. Cuticle parameters (i.e., size of the microstructures and thickness of the epi-versus procuticle) were measured on integument sections and used in the model. A shear force applied on the modelled cuticle surface led to higher stress values when microstructures were present, as compared to a plan surface. Furthermore, by measuring the diameter of a water droplet deposited on sawfly larvae, the integument of several sawfly species was determined as hydrophobic (e.g., more than Teflon(R)), which was related to the sawfly larvae's ability to bleed easily. CONCLUSION: Easy bleeders show spider-like microstructures on their cuticle surface. It is suggested that these microstructures may facilitate integument disruption as well as render the integument hydrophobic. This latter property would allow the exuding haemolymph to be maintained as a droplet at the integument surface.

Journal Article↗