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Biomedical subjects

J Delbeke

Publications and source records attributed to J Delbeke.

At least 19 recordsLinked to original sources
PubMed2006

Vagus nerve stimulation does not affect spatial memory in fast rats, but has both anti-convulsive and pro-convulsive effects on amygdala-kindled seizures.

Vagus nerve stimulation (VNS) is an adjunctive treatment for refractory epilepsy. Using a seizure-prone Fast-kindling rat strain with known comorbid behavioral features, we investigated the effects of VNS on spatial memory, epileptogenesis, kindled seizures and body weight. Electrodes were implanted in both amygdalae and around the left vagus nerve of 17 rats. Following recovery, rats were tested in the Morris water-maze utilizing a fixed platform paradigm. The VNS group received 2 h of stimulation prior to entering the Morris water-maze. Rats were then tested in the kindling paradigm wherein the VNS group received 2 h of stimulation prior to daily kindling stimulation. Finally, the abortive effects of acute VNS against kindling-induced seizures were determined in fully kindled rats by applying VNS immediately after the kindling pulse. Body weight, water consumption and food intake were measured throughout. Memory performance in the Morris water-maze was not different between control and vagus nerve stimulation rats. Similarly, kindling rate was unaffected by antecedent VNS. However, pro-convulsive effects (P<0.05) were noted, when VNS was administered prior to the kindling pulse in fully kindled rats. Yet, paradoxically, VNS showed anti-convulsant effects (P<0.01) in those rats when applied immediately after the kindling stimulus. Body weight was significantly lower throughout kindling (P<0.01) in VNS-treated rats compared with controls, which was associated with reduced food intake (P<0.05), but without difference in water consumption. VNS appears to be devoid of significant cognitive side effects in the Morris water-maze in Fast rats. Although VNS exhibited no prophylactic effect on epileptogenesis or seizure severity when applied prior to the kindling stimulus, it showed significant anti-convulsant effects in fully kindled rats when applied after seizure initiation. Lastly, VNS prevented the weight gain associated with kindling through reduced food intake.

Amygdala↗
PubMed2006

A model of the mammalian optic nerve fibre based on experimental data.

Several experimental data about membrane dynamics and pharmacological sensitivities of optic nerve axons have been published. The present work summarizes these data and computer simulations have been used to develop a model of the mammalian optic nerve fibre. The ionic currents description were derived from existing membrane models and particularly from a model of the somatic retinal ganglion cell (RGC) impulse generation. However, original equations had to be modified to match experimental data, which suggests that in RGCs, axonal and somatic ion channel expression are different. The new model is consistent with recent experimental results about optic nerve axonal excitability.

4-Aminopyridine↗
PubMed2005

Creating a meaningful visual perception in blind volunteers by optic nerve stimulation.

A blind volunteer, suffering from retinitis pigmentosa, has been chronically implanted with an optic nerve visual prosthesis. Vision rehabilitation with this volunteer has concentrated on the development of a stimulation strategy according to which video camera images are converted into stimulation pulses. The aim is to convey as much information as possible about the visual scene within the limits of the device's capabilities. Pattern recognition tasks were used to assess the effectiveness of the stimulation strategy. The results demonstrate how even a relatively basic algorithm can efficiently convey useful information regarding the visual scene. By increasing the number of phosphenes used in the algorithm, better performance is observed but a longer training period is required. After a learning period, the volunteer achieved a pattern recognition score of 85% at 54 s on average per pattern. After nine evaluation sessions, when using a stimulation strategy exploiting all available phosphenes, no saturation effect has yet been observed.

Artificial Intelligence↗
PubMed2005

Two-way communication for programming and measurement in a miniature implantable stimulator.

Implantable stimulators are needed for chronic electrical stimulation of nerves and muscles in experimental studies. The device described exploits the versatility of current microcontrollers for stimulation and communication in a miniature implant. Their standard outputs can provide the required selectable constant-current sources. In this device, pre-programmed stimulation paradigms were selected by transcutaneous light pulses. The potential of a programmable integrated circuit (PIC) was thus exploited. Implantable devices must be biocompatible. A novel encapsulation method that require no specialised equipment and that used two classical encapsulants, silicone and Teflon was developed. It was tested for implantation periods of up to four weeks. A novel way to estimate electrode impedance in awake animals is also presented. It was thus possible to follow the evolution of the nerve-electrode interface and, if necessary, to adjust the stimulation parameters. In practice, the electrode voltage at the end of a known constant-current pulse was measured by the PIC. The binary coded value was then indicated to the user as a series of muscle twitches that represented the binary value of the impedance measurement. This neurostimulator has been successfully tested in vitro and in vivo. Thresholds and impedance values were chronically monitored following implantation of a self-sizing spiral cuff electrode. Impedance variations in the first weeks could reflect morphological changes usually observed after the implantation of such electrodes.

Animals↗
PubMed2005

Simulation of intra-orbital optic nerve electrical stimulation.

In blind subjects who still have functional retinal ganglion cells, electrical stimuli applied to the optic nerve can produce localised visual sensations. This has been demonstrated with an intracranially implanted self-sizing spiral cuff electrode, but, to avoid skull opening, intra-orbital cuff implantation is now considered. In its orbital segment, the optic nerve is surrounded by subarachnoidal cerebrospinal fluid (CSF) and dura mater. Dura mater is a tough fibrous tissue that can impede electrical stimulation. In the study, the issue of whether or not to remove the dura mater at the implantation site was addressed using simulation on numerical models. Several volume conductor models were built representing, respectively: the cuff implanted directly around the nerve; the cuff over the nerve after connective tissue encapsulated the implant; and the cuff electrode placed around the dura mater. Stimulation-induced electric potential fields were computed for these configurations using a full 3D finite elements software. Responses of fibres within the nerve were computed. A large range of dural conductivities and several CSF thicknesses were considered. In all simulated conditions, the presence of dura mater around a layer of CSF increased excitation thresholds. Selectivity performance also decreased, but was found to be independent of the CSF thickness. However, simulations showed that, if the diameter of the cuff electrode is adapted to the target nerve, the injected charge associated with activation is limited within a reasonable range. Electrical stimulation of the optic nerve with a cuff electrode implanted around the dura mater should therefore be feasible.

Dura Mater↗
PubMed2004

Biocompatibility of platinum-metallized silicone rubber: in vivo and in vitro evaluation.

Silicone rubber is commonly used for biomedical applications, including implanted cuff electrodes for both recording and stimulation of peripheral nerves. This study was undertaken to evaluate the consequences of a new platinum metallization method on the biocompatibility of silicone rubber cuff electrodes. This method was introduced in order to allow the manufacture of spiral nerve cuff electrodes with a large number of contacts. The metallization process, implying silicone coating with poly(methyl methacrylate) (PMMA), its activation by an excimer laser and subsequent electroless metal deposition, led to a new surface microtexture. The neutral red cytotoxicity assay procedure was first applied in vitro on BALB/c 3T3 fibroblasts in order to analyze the cellular response elicited by the studied material. An in vivo assay was then performed to investigate the tissue reaction after chronic subcutaneous implantation of the metallized material. Results demonstrate that silicone rubber biocompatibility is not altered by the new platinum metallization method.

Animals↗
PubMed2001

Neural morphological effects of long-term implantation of the self-sizing spiral cuff nerve electrode.

The paper reports on the histological effects of chronic implantation of self-sizing spiral cuff nerve electrodes on the cat sciatic nerve. The implantation period is about 4.4 months. Four different experimental conditions are evaluated: control, sham, bare cuff (cuffs without contacts and leads) and full cuff. The total number of axons in the nerves of the control group is compared with the three other groups. The surface occupied by collagen fibres in the nerve section, perineurium thickness, fibre diameter and myelin thickness are also measured. The average number of axons in the control nerves is found to be 16,416 (+/- 1,509) and does not differ significantly from the three other groups (p > 0.1). Collagen measurements show an extrafascicular epineurial fibrosis in the two implanted groups that is found to be significantly different (p < 0.05). No differences are encountered in the perineurium thickness analysis. Fibre diameter distributions show a regular bimodal pattern for all groups. Centrality (mean and Pm) and dispersion statistics (P25 and P75) extracted from fibre diameter distributions do not reveal significant differences. Myelin thickness distributions are also similar for all groups, as well as centrality and dispersion statistics. The present morphometrical results suggest that the effects produced by a chronic spiral cuff implant on this animal model are negligible.

Animals↗
PubMed2001

Electrical stimulation of anterior visual pathways in retinitis pigmentosa.

PURPOSE: To explore electrically induced phosphenes in blind patients with retinitis pigmentosa (RP) in comparison with healthy subjects and to develop a screening test for candidates for an optic nerve visual prosthesis implantation. METHODS: Phosphenes are obtained by charge balanced biphasic pulse stimulations through a surface cathode over the closed eyelids and an anode near the opposite ear. The resulting strength-duration relationship for somatosensory, phosphene, and pain threshold has been recorded in five RP patients as well as in 10 healthy volunteers. RESULTS: In sighted subjects, the average rheobase and chronaxy for phosphene perception are 0.28 mA and 3.07 msec, respectively. For pulse durations longer than 2 msec, phosphenes are usually obtained at current strengths below the level giving rise to any other electrically generated sensation. In RP patients, however, phosphenes are not so easily obtained. One in five had no visual response at all. Another patient reported a flash perception for the longest pulse durations only. Spontaneous phosphenes interfered heavily with the stimulation in a third person. Finally, despite the higher threshold, two patients displayed normally shaped strength-duration curves. CONCLUSIONS: The surface stimulation has proven harmless, adequate, and very helpful to ascertain that the optic nerve can be electrically activated in completely blind individuals. Long-duration stimulation pulses yield very low phosphene thresholds in healthy subjects. Anterior visual pathways activation requires higher currents in RP patients.

Adult↗
PubMed2000

Automatic morphometry of nerve histological sections.

A method for the automatic segmentation, recognition and measurement of neuronal myelinated fibers in nerve histological sections is presented. In this method, the fiber parameters i.e. perimeter, area, position of the fiber and myelin sheath thickness are automatically computed. Obliquity of the sections may be taken into account. First, the image is thresholded to provide a coarse classification between myelin and non-myelin pixels. Next, the resulting binary image is further simplified using connected morphological operators. By applying semantic rules to the zonal graph axon candidates are identified. Those are either isolated or still connected. Then, separation of connected fibers is performed by evaluating myelin sheath thickness around each candidate area with an Euclidean distance transformation. Finally, properties of each detected fiber are computed and false positives are removed. The accuracy of the method is assessed by evaluating missed detection, false positive ratio and comparing the results to the manual procedure with sampling. In the evaluated nerve surface, a 0.9% of false positives was found, along with 6.36% of missed detections. The resulting histograms show strong correlation with those obtained by manual measure. The noise introduced by this method is significantly lower than the intrinsic sampling variability. This automatic method constitutes an original tool for morphometrical analysis.

Animals↗
PubMed2000

Modelling analysis of human optic nerve fibre excitation based on experimental data.

The aim of the study is to determine which of the existing myelinated mammalian nerve fibre models better fits experimental data resulting from electrical stimulation of the human optic nerve and from propagation velocity measured on primates. The macroscopic electric potential is computed in a 3D, inhomogeneous and anisotropic nerve model. The Chiu-Sweeney (CS) and the Schwarz-Wesselink (SW) membrane descriptions are then considered. Variations in parameters that are not well established (encapsulation-tissue thickness, nerve-fascicle conductivity, geometric and electrochemical fibre cable parameters) are taken into account. Results demonstrate that the SW model predictions are in better agreement with the experimental data than those of the CS model, although thresholds are still too high. When channel densities are varied, the SW model turns out to be more robust than the CS model. For a suitable leakage channel density value (about 8% of the original one), the SW model predicts a conduction velocity of 11.4 ms-1 and an excitation threshold of 0.055 mA (for 0.1 ms pulse duration), which is in very good agreement with experimental values (11 ms-1 and 0.055 mA). Potassium current in the SW model is necessary for stability. Introduction of a potassium-like current can restore stability in the CS system.

Electric Stimulation↗
PubMed2000

The conductivity of the human skull: results of in vivo and in vitro measurements.

The conductivity of the human skull was measured both in vitro and in vivo. The in vitro measurement was performed on a sample of fresh skull placed within a saline environment. For the in vivo measurement a small current was passed through the head by means of two electrodes placed on the scalp. The potential distribution thus generated on the scalp was measured in two subjects for two locations of the current injecting electrodes. Both methods revealed a skull conductivity of about 0.015 [symbol: see text]/m. For the conductivities of the brain, the skull and the scalp a ratio of 1:1/15:1 was found. This is consistent with some of the reports on conductivities found in the literature, but differs considerably from the ratio 1:1/80:1 commonly used in neural source localization. An explanation is provided for this discrepancy, indicating that the correct ratio is 1:1/15:1.

Biomedical Engineering↗
PubMed1999

Hybrid finite elements and spectral method for computation of the electric potential generated by a nerve cuff electrode.

An original numerical method is developed to compute the 3D electric potential generated by a dot-contact cuff electrode implanted around an axisymmetrical, inhomogeneous, anisotropic nerve. The technique is based on a 2D finite-element approach coupled with a semi-analytical Fourier spectral decomposition to approximate the solution behaviour in the azymuthal direction. The method only requires a 2D FEM mesh and allows an accurate electrode description, with any number of contacts at different angular positions. Results show that the convergence of the Fourier series is very fast: typically, the relative error due to series truncation (estimated by the norm of the difference between the solution computed with M modes and the one computed with M-1 modes, normalised by the norm of the solution computed with M modes) reaches the order of 10(-3) with six spectral modes (M = 6). As a consequence, the whole algorithm has the complexity of a 2D approach.

Computer Simulation↗
PubMed1998

Visual sensations produced by optic nerve stimulation using an implanted self-sizing spiral cuff electrode.

A blind volunteer with retinitis pigmentosa was chronically implanted with a self-sizing spiral cuff electrode around an optic nerve. Electrical stimuli applied to the nerve produced localized visual sensations that were broadly distributed throughout the visual field and could be varied by changing the stimulating conditions. These results demonstrate the potential for constructing a visual prosthesis, based on electrical stimulation of the optic nerve, for blind subjects who have intact retinal ganglion cells.

Electric Stimulation↗
PubMed1982

Electrophysiological evaluation of cross-face nerve graft in treatment of facial palsy.

A cross-face autogenous single graft was performed in eight patients with facial palsy. Several electrophysiological techniques have been used in a follow-up study of one to three years' duration. Clinical results are extremely disappointing. The only recovery observed could not be ascribed to the graft. The investigations and the motor unit count in the facial muscles led to the conclusion that the surgical technique employed is a failure. The orbicularis oculi muscle in particular has no chance whatsoever of being satisfactorily reinnervated. Various reasons for this poor recovery are analysed: too small a number of properly grafted axons, syncinesis, muscle degeneration and poor myelination. These observations nevertheless suggest some experimental fields which may lead to improvements in the technique to a point where it may become clinically useful.

Adolescent↗
PubMed1978

Refractory period studies in a human neuromuscular preparation.

A subtraction technique has been employed to study refractoriness in the median nerve and thenar muscles in man. The absolutely refractory periods of the distal motor nerve branches and of the muscle fibres were found to be similar, the majority being within the 2.5--3.0 ms range; the values were distributed in a unimodal manner. The relatively refractory periods of the motor nerve terminals were about 5 ms. In some experiments it was possible to record non-propagated potentials at the endplate zones of the muscle fibres.

Adult↗
PubMed1978

Effects of age, temperature, and disease on the refractoriness of human nerve and muscle.

Absolutely and relatively refractory periods have been investigated in terminal median nerve axons and in thenar muscle fibres in a variety of pathophysiological conditions. Marked increased in refractoriness were produced by cooling to 20 degrees C. Nerve fibre refractoriness was more pronounced in older subjects and diminished in myotonic dystrophy. In contrast, no significant changes in muscle fibre excitability could be demonstrated in either condition or in patients with renal failure or motor neuropathies.

Adult↗
PubMed1978

Analysis of evoked lumbosacral potentials in man.

Surface electrodes have been used to record potentials evoked in the lumbosacral region of 15 healthy volunteers after tibial nerve stimulation. By monitoring the M waves and H reflexes in the triceps surae muslces and by comparing the responses recorded over the roots with those over the lower cord, it was possible to identify the neural substrates responsible for several of the components in the responses. The findings are compared with those of previous studies in man and in other mammalian preparations.

Adolescent↗