Biomedical subjects

J A Reyes

Publications and source records attributed to J A Reyes.

12 recordsLinked to original sources

PubMed2005

Geometrical analysis of the electro-optical effect in nematic droplets.

Using the boundary-layer approach, we study the radial-axial transition in a nematic sphere in the presence of an external field. We calculate analytically an asymptotic expression for the nematic configuration subject to an external low frequency field. Then, we consider an incident plane wave crossing a nematic droplet immersed in an isotropic matrix under the presence of a low frequency field. We calculate the ray trajectories within the optical limit for various values of the external field and find the ray deviation as a function of the incident position parametrized by the magnitude of the field.

Journal Article↗

PubMed2004

Electrically controlled total internal reflection in nematic hybrid cells.

In this work, we show theoretically how the trajectories of an optical beam propagating in a planar-homeotropic hybrid nematic crystal cell can be modified by applying a low frequency electric field perpendicular to the cell. We use a previously developed formalism for describing the propagation of a polarized beam through the cell. We include a low frequency electric energy for the calculation of the equilibrium orientational configurations of the director's field. The presence of the electric field gives rise to trajectories showing a nontrivial dependence of the beam's range and penetration length with the intensity of the applied electric field.

Journal Article↗

PubMed2003

Absorption effects in liquid crystal waveguides.

An analytical and numerical study of the propagation of optical fields through a nematic hybrid slab is developed. We take into account explicitly the absorption of radiation by the liquid crystal by introducing a complex dielectric tensor. For a low intensity beam we first derive the eikonal equation and from it we calculate the ray trajectories in the optical limit. We show that in the presence of absorption, there are no caustics within the slab. Then we consider the WKB limit and calculate the field transverse magnetic modes, their number and their cutoff frequencies. We show that for both limits the agreement between our analytical and numerical results for the propagation constants is excellent, while there are larger differences in the analytically and numerically calculated field amplitudes. These differences show that absorption effects are important for this quantity and have their origin in the fact that the chosen parameter values in our exact numerical calculations, strictly speaking, do not lie within the limits of validity of the WKB approximation. Although a more precise comparison between these approaches requires the use of different sets of values of the relevant parameters, our analysis shows the effects and complications arising from the inclusion of absorption. Finally, we discuss the scope and limitations of our approach.

Journal Article↗

PubMed2003

Standard and embedded solitons in nematic optical fibers.

A model for a non-Kerr cylindrical nematic fiber is presented. We use the multiple scales method to show the possibility of constructing different kinds of wave packets of transverse magnetic modes propagating through the fiber. This procedure allows us to generate different hierarchies of nonlinear partial differential equations which describe the propagation of optical pulses along the fiber. We go beyond the usual weakly nonlinear limit of a Kerr medium and derive a complex modified Korteweg-de Vries equation (CM KdV) which governs the dynamics for the amplitude of the wave packet. In this derivation the dispersion, self-focussing, and diffraction in the nematic fiber are taken into account. It is shown that this CM KdV equation has two-parameter families of bright and dark complex solitons. We show analytically that under certain conditions, the bright solitons are actually double-embedded solitons. We explain why these solitons do not radiate at all, even though their wave numbers are contained in the linear spectrum of the system. We study (numerically and analytically) the stability of these solitons. Our results show that these embedded solitons are stable solutions, which is an interesting property since in most systems the embedded solitons are weakly unstable solutions. Finally, we close the paper by making comments on the advantages as well as the limitations of our approach, and on further generalizations of the model and method presented.

Journal Article↗

PubMed2003

Twist defects in helical sonic structures.

We analyze theoretically both the acoustic wave propagation in periodic media made of anisotropic materials whose stiffness tensor is uniformly rotating along a given axis x(3) and the defect mode produced by twisting about x(3) one part of the helical structure with respect to the other. Within the Bragg band of the periodic structure, the twist defect gives rise to a resonant mode that is a superposition of two standing waves: one localized with exp(-gamma|x(3)|) dependence centered at the defect and the other extended over the whole sample. The ratio between the amplitudes of the localized and nonlocalized waves depends sharply on both the twist angle and the elastic anisotropy, and can assume huge values. The defect mode and the resonance frequency omega(0) are defined by fully analytical and very simple expressions. Finally, we discuss how around omega(0), a finite sample acts as a frequency filter for circularly polarized shear waves, whose bandwidth can be changed by many orders of magnitude by varying the sample thickness, the twist angle, or the elastic anisotropy.

Journal Article↗

PubMed2002

Flow dissipation effects in a nonlinear nematic fiber.

Dissipative effects due to the presence of hydrodynamic flow in a cylindrical fiber whose cladding is an initially quiescent incompressible nematic liquid crystal are analyzed. An analytic and iterative solution of the nematodynamic equations coupled to the Maxwell's equations describing the propagation of a narrow wave packet of transverse magnetic modes is provided. We derive a generalized nonlinear Schröedinger equation for the amplitude of this propagating wave packet that takes into account the dissipation in the nematic's reorientation and the hydrodynamical effects. For the solitonlike solution of this equation we find that the penetration length and the real part of the nonlinear refraction index increase by a factor of 1.75, with respect to those values obtained in the absence of hydrodynamical flow. The imaginary part remains unaltered.

Journal Article↗

PubMed2002

Comparative endocrinology of the insulin-like growth factor-binding protein.

Emerging early in chordate evolution, the IGF-regulatory axis diverged from an insulin-like predecessor into a vertebrate regulatory system specializing in cell growth activation and allied anabolic functions. Essential to the divergence of the IGF and insulin systems was an early presence of soluble IGF-binding proteins (IGFBPs), which bind IGF peptides at much higher affinity than that of the insulin receptor but at comparable affinities to that of the IGF receptor. IGFBPs have no homology with IGF receptors. Instead, IGFBPs are a derived group of proteins within a superfamily of cysteine-rich growth factors, whose members are found throughout the animal taxa. While blocking IGF actions through the insulin receptor is a fundamental role, IGFBPs evolved within the vertebrate line into centralized, 'integrators' of the endocrine growth-regulatory apparatus. IGFBPs have substantial influences on the distribution and bioavailability of IGF peptides in the cellular and physiological environments, but they have a variety of other properties. The six principal mammalian IGFBPs exhibit an array of specialized properties that appear to be derived from a complex evolutionary history (including cell membrane association, interaction with proteins that post-translationally modify them, direct IGF-independent effects on cells, and others) and they are regulated by a diversity of 'outside' factors (e.g. other hormones, metabolic status, stress). Thus, IGFBPs are multifunctional integrators having diverse physiological 'agendas'. Much less is known about IGFBPs and their properties in the other vertebrate taxa. Increasingly, however, it is being recognized that they play equally important endocrine roles, in both conserved and non-conserved ways, when compared with those currently defined in mammals. This review highlights selected 'comparative aspects' in current IGFBP research, in an attempt to view this essential group of endocrine regulators from a wider, biological perspective.

Animals↗

PubMed1999

The structure of affect in a non-western culture: evidence for cross-cultural comparability.

We investigated the structure of affect in the Filipino culture and compared our results to those in Western studies. Four samples of students (ns = 397 to 530) rated their mood for today, the past week, or in general, using near-comprehensive sets of Filipino mood adjectives. Results of exploratory and confirmatory factor analyses supported a hierarchical model of affect and the cross-cultural comparability of self-report mood dimensions (i.e., higher-order Positive and Negative Affect, plus specific affects corresponding to hypothesized universal or basic emotions). The results were more consistent with biological theories of affect than a strong social constructivist perspective.

Adolescent↗

PubMed1997

Opioid receptor modulation of mossy fiber synaptogenesis: independence from long-term potentiation.

Long-term potentiation (LTP) at the mossy fiber-CA3 synapse of the rat hippocampus is an NMDA receptor-independent form of synaptic plasticity that is sensitive to opioid receptor antagonists [12]. In the present study, Timm's stain, a zinc detecting histological marker commonly used to infer synaptogenesis in the mossy fiber projection, was used to examine whether synaptogenesis occurs in response to mossy fiber LTP induction in the adult rat in vivo. Seven days following the induction of mossy fiber LTP by non-seizure-inducing high-frequency stimulation of the mossy fibers, a prominent band of Timm's staining appeared bilaterally in the infrapyramidal region of the stratum oriens in area CA3. Staining was more prominent on the side contralateral to the stimulation. Systemic administration of the opioid receptor antagonist naloxone, sufficient to block mossy fiber LTP induction, did not block the development of Timm's staining in the infrapyramidal region ipsilateral to stimulation, but it did block stimulation-induced increases in Timm's staining observed contralaterally. Systemic administration of (+/-) CPP, a competitive NMDA receptor-antagonist, by contrast, did not block the induction of LTP and did not alter the increase in Timm's staining observed either ipsilaterally or contralaterally. The increase in Timm's staining in the infrapyramidal region suggests that mossy fiber synaptogenesis occurs in response to non-seizure inducing stimulation. Synaptogenesis does not appear to be directly related to opioid receptor-dependent mossy fiber LTP induction, because it occurs in the presence of naloxone which blocks LTP. The mossy fiber synaptogenesis occurring contralaterally appears to be regulated by endogenous opioid peptides, because it is blocked by naloxone.

Animals↗

PubMed1997

Filipino personality structure and the big five model: a lexical approach.

In lexically based studies, we derived Filipino personality dimensions and related them to the Big Five model. In Study 1, Filipino high-school and college students (N = 629) rated themselves on a near-comprehensive list of 861 Filipino (Tagalog) trait adjectives. In Study 2, Filipino high-school and college students (N = 1,531) rated 280 markers of dimensions identified in Study 1. Some students (n = 473) also completed the NEO Five-Factor Inventory. Seven comparable Filipino dimensions were identified in factor analyses in the two studies. We concluded that the dimensions we labeled Concern for Others (vs. Egotism), Conscientiousness, Gregariousness, and Intellect were quite similar to Big Five Agreeableness, Conscientiousness, Extraversion, and Intellect, respectively. The Filipino Self-Assurance dimension was most similar to Big Five Neuroticism. The Filipino Temperamentalness dimension was more complex in Big Five terms, overlapping Agreeableness, Conscientiousness, and Neuroticism. A final Filipino factor resembled a Negative Valence or Infrequency dimension. More than five factors had to be extracted to obtain Philippine dimensions resembling all of the Big Five.

Adaptation, Psychological↗

PubMed1991

Reproductive toxicity of commercial PCB mixtures: LOAELs and NOAELs from animal studies.

This paper reviews the developmental/reproductive toxicity of commercial polychlorinated biphenyl (PCB) mixtures in animals and reports on the "no-observable-adverse-effect levels" (NOAELs) and "lowest-observable-adverse-effect levels" (LOAELs) from these studies. Identification of the lowest effective doses for reproductive toxicity of PCB mixtures is difficult because a variety of reproductive and developmental effects have been reported in several species using different commercial mixtures. Factors to be considered include sensitivity of the end point, sensitivity of species, study quality, biological plausibility, and relevance to humans. End points affected at the lowest doses (sensitive end points) included postnatal growth, development, and function. Among species for whom sensitive end points have been evaluated, a LOAEL of 0.25 mg/kg/day was identified for rodents on the basis of developmental delays in growth and behavioral function, and a LOAEL of 0.008 mg/kg/day was identified for nonhuman primates based on postnatal skin hyperpigmentation. NOAELs were not identifiable for these sensitive end points because effects were reported at the lowest doses tested.

Animals↗

PubMed1990

Rapid assessment of the visual field in glaucoma using an analysis based on multiple correlations.

We wished to determine which areas of the visual field contribute most strongly and most frequently to the deterioration of the visual fields in patients with high ocular pressure. The computerized analysis, using linear and multiple regressions of the findings in 382 visual fields, allowed us to select four test points sensitive enough to estimate the actual global sensitivity of the central visual field (correlation coefficient, 0.98). This system also allowed us to estimate the number of pathological points and even to classify accurately over 75% of the cases in terms of their normal or pathological condition, as well as to determine the depth of the defect. The remaining cases show that the deterioration, if it exists, is not significant. Requiring less than 1 min a threshold examination at these four test points enables us to acquire greater insight into the glaucomatous defect.

Adolescent↗