Showing papers by "Instituto Politécnico Nacional published in 1987"

Forward and backward axial synergies in man

Abstract: 1. Upper trunk and head forward and backward movements were analyzed in human subjects standing on a force platform. EMG of several flexor and extensor muscles was recorded together with the kinematics of the movement (EL.I.TE. system). 2. It was found that upper trunk movements are accompanied by movements of hip and knees in the opposite direction, resulting in a slight displacement of the center of gravity projection on the ground. 3. In fast movements, all the body segments were displaced at the same time, which suggests a feedforward control, whereas in slow movements, onset of displacement of the body segments was found to take place sequentially in a cranio-caudal direction. 4. EMG analysis during fast movements revealed two different types of control, utilized in forward and backward movements. With forward bending movements the action of two sets of muscles could be recognized: the prime mover (R. Abd.), the activation of which was not correlated with that of the other muscles and preceded the onset of movement with a fairly constant lead, and a group of postural muscles, the activation (VM, TA) and inhibition (Sol) of which were closely correlated. By contrast, with backward movements, the prime mover (Er.S.) and the postural leg muscles (Hamstrings, Sol) were activated simultaneously. In both cases, a feedforward type of control is evident. 5. Performance of the fast forward movements was accompanied by an initial forward displacement of the knee. The function of this phenomenon is discussed in term of a destabilizing action favouring the forward bending of the body or a prestretching of the knee extensor muscles increasing the strength of their subsequent contraction.

Performance models of multiprocessor systems

Abstract: While there are several studies of computer systems modeling and performance evaluation where models of multiprocessor systems can be found as examples of applications of general modeling techniques, this is the first to focus entirely on the problem of modeling and performance evaluation of multiprocessor systems using analytical methods.Increasingly sophisticated and fast-moving technologies require models that can estimate the performance of a computer system without having actually to build and test it, models that can help designers make the correct architectural choices. The area of distributed computer architectures, or multiprocessor systems, has numerous such choices and can greatly benefit from an extensive use of performance evaluation techniques in the system design stage.The multiprocessor features that are studied here focus on contention for physical system resources, such as shared devices and interconnection networks. A brief overview covers the modeling of other important system characteristics, such as failures of components and synchronizations at the software level.Contents: Stochastic Processes. Queuing Models. Stochastic Petri Nets. Multiprocessor Architectures. Analysis of Crossbar Multiprocessor Architecture. Single Bus Multiprocessors with External Common Memory. Multiple Bus Multiprocessors with External Common Memory. Single Bus Multiprocessors with Distributed Common Memory. Multiple Bus Multiprocessors with Distributed Common Memory.The authors are affiliated with Politecnico and Universita di Torino in Italy. Performance "Models of Multiprocessor Systems" is included in The MIT Press Series in Computer Systems, edited by Herb Schwetman.

Multielement stable resonators containing a variable lens

Abstract: A unified formulation for the analysis of linear stable resonators containing a lens of variable focal length, which represents the rod of a solid-state laser, and other intracavity optical systems is presented. The stability, the mode spot sizes, the dynamical stability, and the misalignment sensitivity are investigated, and general properties that are valid for any resonator are derived. Some important practical consequences for resonator design are discussed.

13C NMR spectroscopy of indole derivatives

Abstract: The chemical shifts of 298 naturally occurring and synthetic compounds containing the indole chromophoric group are listed. Substituent effects on 13C chemical shifts (SCS) induced by substitution on the heteroaromatic five-membered ring are discussed. The data provide a reference set for future 13C NMR investigations and highlight the need for unambiguous experimental evidence to resolve controversial assignments for differently substituted representative indole derivatives. Many original assignments have been changed, and values not considered to be unambiguously assinged are delineated. The 1J(CH) values for the parent indole were measured.

Isolation of a 220-Kilodalton Protein With Lectin Properties From a Virulent Strain of Entamoeba histolytica

Abstract: A 220-kilodalton (kDa) protein with lectin properties was isolated from Entamoeba histolytica strain HM1:IMSS and was purified by Sepharose 4B chromatography and electroelution from 5% SDS-polyacrylamide gels. The protein contains 9% carbohydrate by weight; is rich in hydrophobic residues; and is very immunogenic in mice, hamsters, and rabbits. The protein binds to fixed monolayers of MDCK cells and inhibits trophozoite attachment to the cultured cells. The 220-kDa protein agglutinates human erythrocytes, and agglutination is inhibited by micromolar concentrations of hyaluronic acid, chitin, chitin-derived products (chitotriose), and antibodies to the purified protein. The 220-kDa protein is recognized by an antibody to the membrane but not by antibodies to other subcellular fractions. We therefore suggest that this 220-kDa protein with lectin properties is a component of the plasma membrane and could be one of the putative "receptor" molecules involved in cell and/or matrix attachment.

Investigation of the phase diagram and critical fluctuations of the system polyvenylmethylether and d‐polystyrene with neutron small angle scattering

Abstract: The system PVME/d‐PS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the Flory–Huggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ΔG in a Flory–Huggins term χ(φ,T) and a configurational term for the entropy ΔS, it turns out that χ is independent of the molecular weight, and of the weight distribution, i.e., it is a local or ‘‘segmental’’ quantity. The critical scattering was carefully investigated at the critical concentration. At temperatures Tc−T>2.4 K(Tc=141.9 °C), a mean field behavior of the susceptibility S(Q=0) was clearly observed, with a critical exponent γ=1. However, in a region of Tc−2.4 K≤T

Multiphoton squeezed states

Abstract: The multiphoton squeezed states defined in this paper are generalizations of the conventional coherent (Glauber) and squeezed (Yuen) states previously discussed by many authors. We define multiphoton generalizations of the latter by a unified class of states that includes the Holstein–Primakoff realizations of SU(2) and SU(1, 1) as well as the standard harmonic oscillator coherent states (Weyl–Heisenberg group) and squeezed states in a general framework that allows also non-Hermitian realizations. We determine the squeezing properties of these states in a unified formalism and study numerically their dependence on the parameter classifying the states.

Effect of plasma waves on the optical properties of metal-insulator superlattices.

Abstract: We develop a simple transfer-matrix formalism in order to obtain the dispersion relation of the electromagnetic normal modes of an infinite insulator-metal (or highly doped semiconductor) superlattice, taking into account the presence of plasma waves, spatial dispersion, and retardation. We also calculate the p-polarized reflectance and give analytical expressions for the surface impedance, reflection amplitude, and the dispersion relation of the surface modes of a semi-infinite superlattice. We find resonant features corresponding to the propagation of guided plasma waves in the metal layers coupled by transverse fields in the insulating layers. This coupling also yields new bulk modes in regions where propagation would not be allowed without spatial dispersion.

Mechanisms involved in the depolarization of cutaneous afferents produced by segmental and descending inputs in the cat spinal cord

Abstract: The relative contribution of specific and unspecific (potassium) components involved in the generation of primary afferent depolarization (PAD) of cutaneous fibres was analyzed in the spinal cord of the anaesthetized cat. To this end we examined the correlation between the intraspinal threshold changes of single afferent fibres in the sural nerve produced by segmental and descending inputs and the negative DC potential shifts produced by these same stimuli at the site of excitability testing, the latter taken as indicators of the changes in extracellular concentration of potassium ions. Stimulation of the ipsilateral brain-stem reticular formation and of the contralateral red nucleus with 100–200 Hz trains reduced very effectively the intraspinal threshold of sural nerve fibres ending in the dorsal horn practically without producing any negative DC potential shifts at the site of excitability testing. However, negative DC potential shifts were produced more ventrally, in the intermediate nucleus and/or motor nucleus. Stimulation of the sural and superficial peroneus nerves with pulses at 2 Hz and strengths below 2×T, also reduced the intraspinal threshold of single SU fibres without producing significant DC potential changes at the site of excitability testing. On the other hand, 100 Hz trains with strengths above 2×T produced negative DC potential shifts and a proportional reduction of the intraspinal threshold of the SU fibres. The PAD of sural fibres produced by stimulation of rubro-spinal and reticulospinal fibres as well as by stimulation of sensory nerves with low frequency trains was unaffected or slightly increased, by i.v. injection of strychnine (0.2 mg/kg), but was readily abolished 5–10 min after the i.v. injection of picrotoxin (2 mg/kg). The results suggest that activation of reticulo-spinal and rubrospinal fibres, as well as stimulation of cutaneous nerves with low frequencies and low strengths, produce PAD of cutaneous fibres involving activation of specific interneuronal pathways with interposed last-order GABAergic interneurons. The potassium component of the PAD produced by cutaneous fibres becames dominant with high stimulus frequencies and strengths.

Use of antibodies to characterize a 220-kilodalton surface protein from Entamoeba histolytica.

Abstract: Antibodies were prepared against a 220-kilodalton (kDa) protein partially purified by Sepharose 4B chromatography of Entamoeba histolytica strain HM38:IMSS homogenates, and the protein was found to have lectin properties. The antibodies specifically recognized this protein in trophozoite homogenates. Immunologically related molecules with the same molecular weight were identified by polyclonal antibodies in strains HM1:IMSS, Entamoeba invadens, and Entamoeba histolytica Laredo. Six monoclonal antibodies recognized only the 220-kDa protein present in strains HM38 and HM1, a result indicating the presence of similar epitopes in the proteins from virulent strains isolated from humans. All the antibodies against the 220-kDa protein have the following properties: (1) they bind to the plasma membrane of live or fixed trophozoites, (2) they partially inhibit the adhesion of trophozoites to erythrocytes and cultured cells, and (3) they inhibit erythrophagocytosis.

NMR studies of indoles and their N-carboalkoxy derivatives

Abstract: The 13C NMR assignment of indoles and 5-methoxyindoles substituted at position 3, and some of their N-carboalkoxy derivatives, was achieved from one- and two-dimensional NMR experiments. Substituent chemical shifts and 1J(CH) values were evaluated. The dynamic processes of N-carboalkoxyindoles and N-carboalkoxyindoline observed by high-field 1H NMR show the existence of two preferred rotamers around the carbamate NC(O)OR bond, which was interpreted in terms of frequency/field strength-dependent phenomena.

Squeezing versus photon-number fluctuations.

Abstract: After deriving a general formula for the quantum probability distribution function higher moments, we apply it to the multiphoton squeezed states [the usual Gaussian and the new non-Gaussian Weyl-Heisenberg, SU(2), and SU(1,1)]. The resulting moments are discussed as functions of the photon-number fluctuations. General criteria are considered to determine optimal squeezing properties with respect to photon-number noise. There result interesting generalized uncertainty relations in the form of scaling laws.

Boundary element elastic analysis of layered soils by a successive stiffness method

Abstract: The elastostatic analysis of layered systems (such as a soil consisting of a set of L individually homogeneous strata) is tackled here on the basis of discretized boundary integral equations (boundary element method). By exploiting the peculiar chain-like pattern of the system, a recursive formula is obtained which generates economically a ‘stiffness matrix’ of the first n layers (from bottom) at the upper interface with the subsequent layer (n = 1…L). The ‘successive stiffness’ method proposed is shown to imply noteworthy advantages with respect to both the standard boundary element method by zones (or subregions) and another ad hoc, earlier method resting on a boundary element approach combined with the transfer matrix concept. This conclusion is corroborated by two-dimensional examples.

A numerical scheme for integrating the rate plasticity equations with an “a priori” error control

Abstract: An alternative scheme for the integration of incremental plasticity constitutive equations is presented and discussed. The method is based on a linear inequality theory approach and exploits the idea of the “a posteriori” local linearization of the yield surface. Results of both the flow theory of plasticity and the deformation theory can be obtained with a predetermined maximum violation of both the yield condition and the flow rule. Numerical results are shown, and compared to analytical solutions as well as to the results of other methods for the simple case of the von Mises yield function.

Pump power stability range of single-mode solid-state lasers with rod thermal lensing

Abstract: The pump power stability range of solid-state laser resonators operating in the TEM 00 mode has been thoroughly investigated. It has been shown that, for a very general resonator containing intracavity optical systems, rod thermal lensing engenders a pump power stability range which is a characteristic parameter of laser material and pump cavity, but is independent of resonator configuration. Stability ranges have been calculated and critically discussed for Nd:YAG, Nd:glasses, Nd: Cr: GSGG, and alexandrite. The independence of the pump power stability range from the resonator configuration has been experimentally demonstrated for a CW Nd:YAG laser.