Search PubMed⌕ Search

Biomedical subjects

J van Pelt

Publications and source records attributed to J van Pelt.

At least 19 recordsLinked to original sources
PubMed1996

Complex periodic behaviour in a neural network model with activity-dependent neurite outgrowth.

Empirical studies have demonstrated that electrical activity of the neuron can directly affect neurite outgrowth. High levels of activity cause neurites to retract, whereas low levels allow further outgrowth. Previously we studied networks in which all the cells reacted in the same way on electrical activity. Since experiments have shown that neurons may in fact react differentially, we study in this paper networks in which the range of activity where outgrowth takes place varies among cells. We show that this can lead to complex periodic behaviour in electrical activity and connectivity of individual cells. The precise behaviour depends on the spatial distribution of the cells and the distribution of the outgrowth properties over the cells. Any other cellular property that adapts slowly to electrical activity such that neuronal activity is attempted to be maintained at a given level, can lead to similar results.

Animals↗
PubMed1996

[Quality improvement project 'laboratory diagnosis by family physicians' leads to considerable decrease in number of laboratory tests].

OBJECTIVE: Quality improvement of laboratory diagnostics by general practitioners (GPs) through introduction of a simplified, problem-oriented application form, supported by information and feedback. DESIGN: Prospective descriptive study. SETTING: Laboratory of Clinical Chemistry and Haematology of St. Maartens Gasthuis, Venlo, the Netherlands and the GPs in the region. METHODS: The effects of the intervention were measured by counting the analyses requested by all GPs in the region in 1992, 1993 and 1994. Furthermore requests by every GP for fifteen selected analyses in the first 6 months of 1992, 1993 and 1994 were counted and reported together with the anonymous data of all colleagues. RESULTS: After the intervention a 23% reduction of the total number of analyses request by GPs was noticed. Blood sampling increased by 8% and the mean number of laboratory test requests per patient decreased from 5.9 to 4.2. The largest request reductions were noticed for analyses not listed on the application form. Measurements in the first 6 months of 1992, 1993 and 1994 showed continuation of the trend and a fadeaway of 'redundant' analyses. CONCLUSION: The introduction of a simplified problem-oriented application form for GPs supported by feedback caused a marked decrease of the number of (redundant) laboratory requests.

Clinical Laboratory Techniques↗
PubMed1996

The transferability of a candidate reference method for determination of creatinine in serum.

We developed a candidate reference method for the determination of creatinine in serum. For the acceptance of a reference method it is important that it be rigorously validated against a definitive method and that the method can be transferred from one laboratory to another. This study focussed on the transferability and consisted of two parts: introduction and familiarization with the method in four clinical chemistry laboratories in the Netherlands, followed by independent measurements of Standard Reference Material 909a2 and several commercial quality control materials provided with reference method values according to the protocol of the German Quality Assessment Organisation. The criterion for judging transferability was the mean total error (%) of the five sera used in the accuracy experiment. For creatinine we used a total error of < 2.2%. For Standard Reference Material 909a2 all four laboratories were able to comply with this demand, while only two laboratories met this requirement for the other four sera. The results for the Standard Reference Material 909a2 from the collaborating laboratories demonstrate that this candidate reference method can be successfully transferred without loss of precision and accuracy.

Chromatography, High Pressure Liquid↗
PubMed1994

Pitfalls in the differentiation of N-glycosylation variants of prostate-specific antigen using concanavalin A.

We determined the optimal conditions for the separation of N-glycosylation variants of prostate-specific antigen using concanavalin A. Concanavalin A is a lectin that binds to the terminal sugar residues of glycoproteins. We demonstrated that differences in the percentage of prostate-specific antigen bound to concanavalin A-Sepharose in patients with benign prostatic hyperplasia compared with patients with prostatic carcinoma, as described in the literature, arise when insufficient concanavalin A binding sites are added for complete binding of the glycosylation variants of prostate-specific antigen. We observed similar percentages of prostate-specific antigen bound to concanavalin A-Sepharose for benign prostatic hyperplasia (86.3% +/- 7.5, mean +/- SD) and carcinoma patients (81.8% +/- 12.0, mean +/- SD), when sufficient concanavalin A-Sepharose was added to allow optimal binding, and when samples with high prostate-specific antigen concentrations were not pre-diluted before incubation with concanavalin A-Sepharose. We conclude that differentiation of patients with benign prostatic hyperplasia or carcinoma of the prostate on the basis of differences in percentages of prostate-specific antigen bound to concanavalin A-Sepharose, i.e. separation of N-glycosylation variants, is not possible.

Binding Sites↗
PubMed1993

Terminal and intermediate segment lengths in neuronal trees with finite length.

A basic but neglected property of neuronal trees is their finite length. This finite length restricts the length of a segment to a certain maximum. The implications of the finite length of the tree with respect to the segment length distributions of terminal and intermediate segments are shown by means of a stochastic model. In the model it is assumed that branching is governed by a Poisson process. The model shows that terminal segments are expected to be longer than intermediate segments. Terminal and intermediate segments are expected to decrease in length with increasing centrifugal order. The results are compared with data from in vivo pyramidal cells from rat brain and tissue cultured ganglion cells from chicken. A good agreement between data and model was found.

Animals↗
PubMed1992

A simple vector implementation of the Laplace-transformed cable equations in passive dendritic trees.

Transient potentials in dendritic trees can be calculated by approximating the dendrite by a set of connected cylinders. The profiles for the currents and potentials in the whole system can then be obtained by imposing the proper boundary conditions and calculating these profiles along each individual cylinder. An elegant implementation of this method has been described by Holmes (1986), and is based on the Laplace transform of the cable equation. By calculating the currents and potentials only at the ends of the cylinders, the whole system of connected cylinders can be described by a set of n equations, where n denotes the number of internal and external nodes (points of connection and endpoints of the cylinders). The present study shows that the set of equations can be formulated by a simple vector equation which is essentially a generalization of Ohm's law for the whole system. The current and potential n-vectors are coupled by a n x n conductance matrix whose structure immediately reflects the connectivity pattern of the connected cylinders. The vector equation accounts for conductances, associated with driving potentials, which may be local or distributed over the membrane. It is shown that the vector equation can easily be adapted for the calculation of transients over a period in which stepwise changes in system parameters have occurred. In this adaptation it is assumed that the initial conditions for the potential profiles at the start of a new period after a stepwise change can be approximated by steady-state solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

The emergence of long-lasting transients of activity in simple neural networks.

The question was investigated whether long-lasting transients of activity, observed to occur in the intact cerebral cortex (EEG slow (delta) waves and 'K' complexes) as well as in isolated tissues cultured in vitro, can also emerge in a model network of excitatory and inhibitory cells. We show that such transients can indeed occur even if the cells do not have built-in slow kinetics. For certain parameter settings, the network is in a bistable state in which periods of increased activity (long-lasting transients) alternate with minimal activity. Transients are triggered by spontaneously firing cells ('noise'), which, rather than via a build-up of recurrent synaptic inhibition, also initiate their termination. During a transient, the network continually makes transitions from one equilibrium to another as a result of spontaneous firing until it is switched back to the quiescent state, i.e., after a variable period of time of noise-induced transitions the transient is terminated. If the network is small, activity can terminate even without inhibition. In large networks, inhibition keeps the network sensitive to spontaneously firing cells by holding it in the neighbourhood of a critical point between active and quiescent state.

Action Potentials↗