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Biomedical subjects

A R Marchand

Publications and source records attributed to A R Marchand.

At least 19 recordsLinked to original sources
PubMed2006

Entorhinal cortex lesions disrupt fear conditioning to background context but spare fear conditioning to a tone in the rat.

Recent studies have shown that the integrity of the entorhinal cortex (EC) is not required for simple contextual conditioning. In background contextual conditioning, i.e., when a phasic cue is present during training, the involvement of the EC is still a matter of debate. Therefore, the present work further examines whether the EC is required for background contextual conditioning using a tone as the phasic cue. Rats sustaining either excitotoxic lesions of the EC or sham-lesions were trained with one of two procedures differing with respect to the predictive value of the tone: a paired procedure in which the tone perfectly predicts shock occurrence and overshadows context, and an unpaired procedure in which the predictive value of the tone is reduced. Conditioned fear was assessed by freezing responses during conditioning, reexposure to the training context, and reexposure to the tone in a new context. Postshock freezing was reduced in rats with entorhinal lesions. In all rats trained with the paired procedure, freezing to the context was low and freezing to the tone was high, suggesting that the tone has overshadowed the context during the conditioning session. The reverse pattern was observed with the unpaired procedure in sham-operated rats. In rats with entorhinal lesions trained with the unpaired procedure, freezing responses to the context was markedly reduced. In a new context, however, entorhinal-lesioned rats showed higher freezing scores than those of sham-lesioned rats. Freezing to the tone was unaffected by the lesion irrespective of the tone's predictive value. As a whole, these results support the notion that the EC is required for normal background contextual freezing.

Acoustic Stimulation↗
PubMed2004

Trace fear conditioning: a role for context?

Fear conditioning can be rapidly obtained over long trace intervals, but its specificity with respect to both time and stimulus is uncertain. Long-trace fear conditioning often parallels contextual conditioning, and it is sensitive to hippocampal lesions. These properties of trace conditioning are not directly addressed by timing models and multiple-time-scale models of conditioning. It is proposed that during early stages of conditioning, a joint representation of the context and the stimulus trace may underlie conditioned responses, and that discriminative processes allow the emergence of specific responses in a later stage.

Acoustic Stimulation↗
PubMed2000

Time course of cardiac conditioned responses in restrained rats as a function of the trace CS-US interval.

Two experiments aimed at understanding the temporal characteristics of trace-conditioned heart-rate responses to a 0.5-s tone (conditioned stimulus [CS]) in restrained rats. A CS paired with a tail-shock (unconditioned stimulus [US]) elicited lasting bradycardiac responses. The magnitude and extinction rate of conditioned responses (CR) were independent of the CS-US interval (interstimulus interval [ISI], 3 s to 20 s). Unreinforced test trials were analyzed for CR topography. Responding was delayed in groups with longer ISIs, but CR latencies, peak and decay times were not proportional to the ISI. Response peaks tended to cluster either about 6 s after CS onset, or about 10 s with a slow decay, depending on the ISI. The authors postulated 2 components of auditory stimulus traces involved in cardiac conditioning, maximally active 6 s and 10 s respectively after CS onset. The topography of the CR could be constrained to combinations of associative strength and instantaneous activity of these 2 components.

Animals↗
PubMed1997

Perioperative variation in phagocytic activity against Candida albicans measured by a flow-cytometric assay in cardiovascular-surgery patients.

Candidiasis is an opportunistic fungal infection that frequently occurs following modifications of host defenses. Major surgery can be responsible for such alterations, and therefore it increases the risk of fungal infection. The purpose of this study was to evaluate the perioperative impairment of leukocyte function in patients after cardiovascular surgery by measuring the phagocytic activity against Candida albicans by a flow-cytometric method. The average postsurgical decrease in phagocytosis in our patients was 11.4%. By univariate analysis, three factors, all related to antibiotic therapy, were significantly associated with an important decrease in phagocytosis; the use of antimicrobial therapy before surgery, the number of different antibiotics taken, and the length of antibiotic treatment. The results of our study showed that the use of antibiotics in patients undergoing cardiovascular surgery alters the normal phagocytic activity of the host immune system against C. albicans and that flow cytometry is a rapid and simple technique that helps in early identification of patients at high risk for Candida infections. The mechanisms by which surgery and antibiotics decrease phagocytosis remain to be elucidated.

Adult↗
PubMed1997

Primary afferent depolarizations of sensory origin within contact-sensitive mechanoreceptive afferents of a crayfish leg.

Recordings from the central branches of single identified dactyl sensory afferent (DSA) neurons in a crayfish in vitro preparation were performed to study modifications of the sensory message occurring before the first central synapse. These afferents comprised hairs and force-sensitive mechanoreceptors with phasic and phasotonic response characteristics in the terminal segment (dactyl) of the crayfish leg. More than one afferent spike size was often observed in intracellular recordings from these afferents, thus indicating the presence of electrical coupling between the central processes of DSA fibers. Additionally, in identified DSA fibers with large spike sizes, primary afferent depolarizations (PADs) of up to 15 mV were observed, which sometimes triggered antidromic spikes in the afferent. Nevertheless, PADs were clearly inhibitory, because they shunted the afferent spikes. They exhibited the following properties. First, each PAD was preceded by an afferent spike from a neighboring hair, indicating that the PADs had a sensory rather than central origin. Second, PADs could follow high frequencies of afferent discharges without failure, a property suggestive of monosynaptic connections, but because PAD latencies varied by +/-0.5 ms it is more likely that they were mediated by a disynaptic pathway. Third, although PADs were evoked in an extremely reliable manner, their amplitude varied in a quantal manner. Most unitary PADs were the result of the release of < 12 quanta, the mean quantal content lying between 4 and 5; quantal size was large, approximately 1 mV. Fourth, PADs showed facilitation in some fibers, whereas in others they became much smaller when occurring at brief intervals. We suggest that PADs may be an efficient and parsimonious way to limit sensory inflow in space and time, allowing the crayfish to identify precisely both weak and strong mechanical stimuli.

Afferent Pathways↗
PubMed1996

Synaptic connections of the cuticular stress detectors in crayfish: mono- and polysynaptic reflexes and the entrainment of fictive locomotion in an in vitro preparation.

The reflex connections made by Cuticular Stress Detector afferents (CSD1 and CSD2) with motorneurones of the four proximal muscle groups in the 5th walking legs of crayfish (Procambarus clarkii, Pacifastacus leniusculus) have been studied in an in vitro preparation. Reflex responses to mechanical stimulation of the CSDs were studied in single neurones by means of intracellular techniques. Within each motorneurone pool, both excitatory and inhibitory reflex responses occurred, although sometimes no reflex connections were found. When present, they could be classified into 'levation' and 'depression' reflexes, corresponding to negative and positive feedback effects respectively. Each motorneurone receives input from a number of different CSD afferents (mean values between 3.0 and 5.8). Using electrophysiological and pharmacological tests, it was demonstrated that at least 32% of all connections were monosynaptic. In preparations showing fictive locomotion, phasic CSD stimulation was shown to be able to entrain anterior levator and depressor motorneurone activity in 95% of cases. The results thus demonstrate the importance of sensory feedback from the CSDs in shaping the final motor output.

Animals↗
PubMed1996

Force dependent response reversal mediated by low- and high-threshold afferents from the same mechanoreceptor in a crayfish leg.

1. The reflex responses initiated by cuticular stress detector one (CSD1) afferents in anterior levator motor neurnons in the fifth walking leg of crayfish were studied in an in vitro preparation by means of intracellular recordings. 2. The response in the anterior levator reversed with strong mechanical stimulation. Inhibitory responses were correlated with the activation of low-threshold CSD1 units, excitatory responses with the activation of high-threshold CSD1 units. Two opposing reflexes thus originate from the same sense organ at different levels of stimulation. 3. Some low-threshold units were inactive at stimulus strengths that activated the high-threshold units. Other low-threshold units remained active. Their reflex effects were reduced by either pre- or postsynaptic mechanisms. 4. Functionally, this force dependent response reversal could be important in switching from postural/locomotor responses to defensive responses such as limb autotomy.

Animals↗
PubMed1995

A simple dual pressure-ejection system and calibration method for brief local applications of drugs and modified salines.

We report a minimal method for dual pressure ejection through a single micropipette, which uses standard theta-type glass tubing and a connection through thinned polythene catheters without any sealing. We also describe a simple calibration method using a standard electrometer. The linearity of the ejected volume with respect to pulse duration and to applied pressure was maintained, as measured by the resistivity of ultrapure water after ejection of a saline solution. The method was used to analyse the inhibitory effect of glutamate on crayfish motoneurones with local and reversible applications of low-chloride saline and picrotoxin.

Animals↗
PubMed1994

Functional aspects of central electrical coupling in mechanoreceptor afferents of crayfish.

Cuticular stress detector afferents (CSD1 and CSD2) in the walking legs of crayfish have been studied in an in vitro preparation allowing intracellular recordings of single neurones during mechanical stimulation of the organs. Lucifer yellow staining of single fibres showed them to be dye-coupled through junctions presumably located near their point of entry into the ganglion. In intracellular recordings from identified sensory fibres with 30-80-mV impulses, coupled afferent spikes were seen as depolarizing potentials of short duration but smaller amplitude. Coupling sometimes involved fibres with quite different response characteristics to mechanical stimuli. Coupled spikes summated during strong mechanical stimuli. In silent CSD fibres, antidromic spikes could occur which were triggered by the depolarization due to coupling. Antidromic spikes were more frequent during strong stimuli and were in some cases shown to be triggered in a cooperative way when two afferent spikes synchronously reached the ganglion. Our results suggest that central coupling of primary afferents may be a widespread phenomenon in Crustacea and that it provides a threshold mechanism for selective amplification of strong stimuli.

Animals↗
PubMed1994

Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons.

In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

Animals↗
PubMed1994

Cross-correlation analysis of the lateral hip strategy in unperturbed stance.

Subjects standing heel-to-toe on either hard ground or soft support were instructed to stand upright keeping optimal balance. Lateral accelerometric measurements at head, hip and ankle levels were subjected to conjugate cross-correlations analysis in order to determine the co-ordinated movements or strategies. The results strongly suggest that there exists a hip lateral strategy which is very similar to the hip strategy previously described in fore-aft body oscillations. This lateral hip strategy was only observed when the greatest body oscillations were observed, namely on the soft supporting surface, and its descending sequence of co-ordinated movements is consistent with the idea of a top-down organization of postural control during movement or difficult stance conditions.

Acceleration↗
PubMed1994

A statistical approach to sensorimotor strategies: conjugate cross-correlations.

A simple method, based on cross-correlation functions (CCFs) between two time series of kinematic or physiological measurements, is proposed for the analysis of multisegmental movements. Special emphasis is placed on measuring accelerations. When the movements of two body segments are coordinated but consistently time lagged, their CCF displays a peak at the corresponding time abscissa. The reproducible positions of the peaks reflect biomechanical or physiological constraints. Several significantly large peaks can be observed in a CCF. It is possible to identify coordinated movements involving more than two segments by applying simple rules of compatibility between the time lags and between the signs of the correlation peaks. With the method proposed, it is possible to determine the signs of relative variation and the time lags of the successive statistically correlated segmental movements. This is particularly useful in the case of both continuous and periodic sensorimotor control, where classical poststimulus methods cannot be applied. Unlike the classical poststimulus methods, this method does not require a time origin, and it is not necessary to monitor the muscles or even to specify exactly which ones are involved. The method is also applicable to experiments involving a time origin (e.g., and applied perturbation), although in this case it is less accurate than the averaging technique. Individual postural strategies can be identified, which suggests some interesting potential applications of the method to clinical studies.

Journal Article↗
PubMed1993

Temporal response properties of lumbar-projecting vestibulospinal neurons to roll tilt in decerebrate cats.

In decerebrate cats, rotation about the longitudinal axis of the animal, leading to sinusoidal stimulation of labyrinth receptors, produces a tonic contraction of limb extensors during side-down tilt (alpha responses) and of dorsal neck extensors during side-up tilt (beta responses). These changes in posture are mediated, at least in part, by lateral vestibular nucleus (LVN) neurons, with response characteristics to stimulation of macular and/or canal receptors that have so far been evaluated at the level of either unidentified vestibulospinal (VS) neurons or vestibulo-collic neurons projecting to the upper cervical cord. In the present study we investigated the dynamics of the responses of VS neurons projecting to the lumbosacral segments of the spinal cord to increasing frequencies of tilt (from 0.026 to 0.32 Hz, +/- 10 degrees). All the recorded units showed an average phase lead with respect to position of +25.6 +/- 5.5 degrees (SE) at the tilt frequency of 0.026 Hz. Most of these neurons (n = 32) were particularly activated during side-down tilt (alpha responses) and showed either a stable phase or an increase in phase lead of the responses with increasing frequency of tilt. At the tilt frequency of 0.026 Hz, the smaller the phase lead of the responses, the larger was the response gain. Moreover, the smaller the phase lead of the responses at that frequency of tilt, the smaller the increase in gain but the larger was the increase in lead of the responses obtained by increasing the stimulation frequency up to 0.32 Hz. Through this set of finely organized changes in unit response characteristics, the overall output of this population of neurons increased, while the phase angle of the responses reached the mean value of +64.9 +/- 2.6 degrees (SE), thus becoming more related to the velocity than to the positional signal. The remaining units (n = 7), which were mainly activated during side-up tilt (beta responses), displayed an increase in phase lag of the responses to increasing frequency of stimulation, which reached the mean value of -118.9 +/- 14.5 degrees (SE) at 0.32 Hz. The differences in the dynamic properties of these VS neurons projecting to the lumbosacral cord, with respect to those of previously recorded populations of VS neurons, are discussed.

Animals↗
PubMed1990

Early sensory determinants of locomotor speed in adult cats: I. Visual compensation after bilabyrinthectomy in cats and kittens.

After a study of locomotion in cats deprived of their vestibular receptors early in life, the average speed of locomotion was measured in 3 adult cats, first before and then several months after bilateral labyrinthectomy, in a task involving free locomotion across platforms, rails or ladders and under normal illumination versus stroboscopic illumination or darkness. In the absence of visual cues, cats which received lesions as adults showed, on both wide and narrow supports, the same speed deficits as those of subjects which underwent lesions soon after birth. Visual cues were, however, used by some cats with early lesions to recover near normal performances on wide platforms. Evidence was found that the vestibular deficit comprised at least 3 components: a severe loss of dynamic balance occurred in all subjects with lesions and was mostly not compensated for by vision; a loss of the ability to keep to a straight course in darkness was observed on wide platforms, but vision considerably improved this function, even under stroboscopic illumination; a slight paw adjustment deficit was also observed on irregular supports, due to the fact that vision played a decreased role after the vestibular lesion. These effects point to the specificity of the vestibular control of balance, the availability of several sensorimotor strategies for orientation, and the complementarity of vestibular and visual inputs in guiding the subjects' step. We confirm here that vestibular inputs do not play a critical role during locomotor development, and stress the fact that peripheral visual cues about position or orientation (rather than motion), play a leading role in compensatory strategies.

Afferent Pathways↗
PubMed1990

Early sensory determinants of locomotor speed in adult cats: II. Effects of strobe rearing on vestibular functions.

Cats raised under stroboscopic illumination are known to exhibit oculomotor and visuomotor deficits, but little is known about their locomotor abilities. Four strobe-reared cats with intact labyrinths were tested in a locomotor test involving various walking surfaces and various illumination conditions. Apart from their general slowness under all the experimental conditions, these strobe cats showed no special deficit on narrow rails, which indicates that their dynamic balancing abilities were normal. In these subjects, the decrease in the use of kinetic visual cues was roughly compensated for by an increase in the use of position cues. When tested after chronic bilateral labyrinthectomy, the strobe-reared cats' locomotor speeds were identical to those of control labyrinthectomized cats, except on wide platforms involving orientation towards a visual goal. These results show that in the absence of motion-vision, vestibular control of dynamic balance can mature normally, but they suggest that other aspects of locomotion involving the processing of vestibular and kinesthetic inputs may be impaired.

Aging↗
PubMed1989

Discrete visual samples may control locomotor equilibrium and foot positioning in man.

The static or dynamic visual cues required for equilibrium as well as for foot guidance in visually guided locomotion in man were studied using a variety of locomotion supports and illumination and visual conditions. Stroboscopic illumination (brief flashes) and intermittent lighting (longer flashes) were used to control and to vary the visual sampling frequency of static (positional/orientational) visual cues. There were three main findings: First, visual control of foot positioning during locomotion over a narrow support depends mainly upon the availability of high frequency static visual cues (up to about 12 Hz); and third, static visual cues required for equilibrium control are extracted from both the peripheral and the central visual field. Assuming that discrete demands for feedback occur, a simple probabilistic model was proposed, according to which the mean time that elapses following presentation of static visual cues about positions or changes of position accounts for the differences in the difficulty of the various illumination conditions.

Journal Article↗