An unsupervised neural network model for the development of reflex co-ordination

In this paper, we present a model for the development of connections between muscle afferents and motoneurones in the human spinal cord. The model consists of a limb with six muscles, one motoneurone pool, one pooled (Ia-like) afferent for each muscle and a central programme generator. The weights of the connections between the afferents and the motoneurone pools are adapted during centrally induced movements of the limb. The connections between the afferents and the motoneurone pools adapt in a hebbian way, using only local information present at the synapses. This neural network is tested in two examples of a limb with two degrees of freedom and six muscles. Despite the simplifications, the model predicts the pattern of autogenic and heterogenic monosynaptic reflexes quite realistically.     

Reactivity of consecutive basic amino acid residues in peptides

Different tetrapeptides of general formula L-Ala-X-X-Gly, possessing a basic doublet in the second and third position (X = Arg or Lys), have been synthesized as free or N-acetylated molecules. The chemical reactivity of the arginine guanidino group and of the lysine epsilon-amino group were studied using respectively the Sakaguchi and the ortho-diacetylbenzene reactions, in the tetrapeptides as well as in related molecules. In both cases, the colour yield is markedly influenced by the length of the polypeptide chain and by the relative positions of the arginine and lysine residues, suggesting the occurrence of intramolecular bonds within the tetrapeptide molecule. Tryptic hydrolysis of the tetrapeptides was followed by evaluating the amino acids or peptides which appear to be specific for the different possible cleavages at the arginyl or at the lysyl bonds. The susceptibility to trypsin of the carboxylic group of the second basic amino acid decreases progressively in the order Lys-Arg greater than Arg-Arg much greater than Lys-Lys greater than Arg-Lys, which shows a fair correlation with the intra-cellular cleavage of the bonds observed during the processing of preproteins of of the precursors of several physiologically active peptides.     

A split-pot experiment with sorghum to test a root water uptake partitioning model

Correct modeling of root water uptake partitioning over depth is an important issue in hydrological and crop growth models. Recently a physically based model to describe root water uptake was developed at single root scale and upscaled to the root system scale considering a homogeneous distribution of roots per soil layer. Root water uptake partitioning is calculated over soil layers or compartments as a function of respective soil hydraulic conditions, specifically the soil matric flux potential, root characteristics and a root system efficiency factor to compensate for within-layer root system heterogeneities. The performance of this model was tested in an experiment performed in two-compartment split-pot lysimeters with sorghum plants. The compartments were submitted to different irrigation cycles resulting in contrasting water contents over time. The root system efficiency factor was determined to be about 0.05. Release of water from roots to soil was predicted and observed on several occasions during the experiment; however, model predictions suggested root water release to occur more often and at a higher rate than observed. This may be due to not considering internal root system resistances, thus overestimating the ease with which roots can act as conductors of water. Excluding these erroneous predictions from the dataset, statistical indices show model performance to be of good quality.     

Positive colloidal thorium dioxide as an electron microscopical contrasting agent for glycosaminoglycans,compared with ruthenium red and positive colloidal iron

Summary In electron microscopy Thorotrast has been used as a specific contrasting agent for acid glycosaminoglycans. Because of its high atomic number, thorium (Z=90) gives good contrast in the electron microscope, but at present it is less frequently used for this purpose. We prepared a positive colloidal solution of ThO₂ without stabilizers to compare its properties with those of ruthenium red and positive colloidal iron for contrasting fetal mouse epiphyseal cartilage. The results indicate that colloidal ThO₂, which is easy to prepare in any laboratory, gives better results than ruthenium red and colloidal iron do in this kind of cartilage. Furthermore, as judged from data in the literature and obtained in our laboratory, it penetrates this tissue better than Thorotrast does, probably because of the absence of stabilizers.     

Predicting Effects of Water Regime Changes on Waterbirds: Insights from Staging Swans

Predicting the environmental impact of a proposed development is notoriously difficult, especially when future conditions fall outside the current range of conditions. Individual-based approaches have been developed and applied to predict the impact of environmental changes on wintering and staging coastal bird populations. How many birds make use of staging sites is mostly determined by food availability and accessibility, which in the case of many waterbirds in turn is affected by water level. Many water systems are regulated and water levels are maintained at target levels, set by management authorities. We used an individual-based modelling framework (MORPH) to analyse how different target water levels affect the number of migratory Bewick’s swans Cygnus columbianus bewickii staging at a shallow freshwater lake (Lauwersmeer, the Netherlands) in autumn. As an emerging property of the model, we found strong non-linear responses of swan usage to changes in water level, with a sudden drop in peak numbers as well as bird-days with a 0.20 m rise above the current target water level. Such strong non-linear responses are probably common and should be taken into account in environmental impact assessments.     

Education and timing of parenthood among Canadian women: a cohort analysis.

This research examines factors associated with the timing of first birth in Canada, focusing primarily on the role of women's educational attainment. Proportional hazards modelling techniques are applied to data from the 1984 Canadian Fertility Survey (CFS) in order to determine how educational attainment, estimated as close as possible to the date of first birth, influences the timing of first birth and whether the importance of this variable varies according to age cohorts. The results suggest that among a number of variables useful for distinguishing different levels of risk, educational attainment proves to be the most important predictor in the model. Education exerts a substantial positive influence on birth timing for women of all age groups. As expected, moreover, significant cohort differences are also evident, with the greatest to the smallest impact on the risk from the youngest to the oldest cohorts. These clear-cut cohort differences indicate a fundamental change in the effects of education over time, a trend most likely resulting from substantial changes in both the content and social significance of formal schooling during the past few decades.     

On the mechanism of enzyme action. XLV. The role of certain dicarboxylic acids in the formation of oxalic acid by wood-destroying molds

Alkalinity of the medium was shown to be the chief factor involved in the accumulation of oxalate by T. cinnabarina. Glutamate and aspartate are shown to lead to oxalate with this organism and with L. lepideus by dehydrogenation to α-ketoglutarate and oxaloacetate, respectively. Malate was also shown to be dehydrogenated. It is proposed that oxaloacetate may either undergo β-decarboxylation to yield CO₂ and pyruvate, or splitting by coenzyme A to yield oxalate and acetylated coenzyme A. The reversal of this latter reaction is suggested as the explanation of the disappearance of oxalate from culture media. The reduction of resazurin by the dehydrogenase systems of the molds is inhibited by cyanide, indicating the participation of metal systems, such as the cytochromes.     

A Mechanism of Energy Dissipation in Cyanobacteria

When grown under a variety of stress conditions, cyanobacteria express the isiA gene, which encodes the IsiA pigment-protein complex. Overexpression of the isiA gene under iron-depletion stress conditions leads to the formation of large IsiA aggregates, which display remarkably short fluorescence lifetimes and thus a strong capacity to dissipate energy. In this work we investigate the underlying molecular mechanism responsible for chlorophyll fluorescence quenching. Femtosecond transient absorption spectroscopy allowed us to follow the process of energy dissipation in real time. The light energy harvested by chlorophyll pigments migrated within the system and eventually reaches a quenching site where the energy is transferred to a carotenoid-excited state, which dissipates it by decaying to the ground state. We compare these findings with those obtained for the main light-harvesting complex in green plants (light-harvesting complex II) and artificial light-harvesting antennas, and conclude that all of these systems show the same mechanism of energy dissipation, i.e., one or more carotenoids act as energy dissipators by accepting energy via low-lying singlet-excited S₁ states and dissipating it as heat.     

Incidence of cystic fibrosis in Saguenay-Lac-St.-Jean (Quebec, Canada).

The incidence of cystic fibrosis (CF) in Saguenay-Lac-St.-Jean, a geographically isolated region of Quebec, was estimated to be 1 in 902 during the period 1975-1988. The carrier rate was calculated to be 1 in 15 inhabitants. The high incidence of CF in Saguenay-Lac-St.-Jean is probably the result of a founder effect and genetic drift for one or more mutations. Historical, demographic, and social factors also may have contributed to the high incidence.