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At least 595 records · Page 33Linked to original sources
PubMed2004

Simulation of the effect of Na+ and Cl- on the velocity of a spreading depression wave using a simplified electrochemical model of synaptic terminals.

In the study of the spreading depression (SD) wave phenomenon and its dynamics, it is necessary to describe the ionic movements along the extracellular space, as well as between this and the intracellular space. In both cases, the ionic movement includes a double coupling involving the concentration and the potential gradients and hence must be described by electrodiffusion mechanisms. Based on this, the effects of the ionic composition on the characteristics of the wave propagation can be predicted. The influence of varying extracellular sodium and chloride concentrations on the velocity of propagation of the SD wave was investigated by simulation. The results achieved are close to the experimental measurement from the literature. These findings suggest the potentiality of the model proposed in supporting the interpretation of experimental data in neuronal tissues, particularly the SD.

Animals↗
PubMed2001

Abnormalities of sensorimotor integration in focal dystonia: a transcranial magnetic stimulation study.

It has been postulated that sensorimotor integration is abnormal in dystonia. We investigated changes in motor cortical excitability induced by peripheral stimulation in patients with focal hand dystonia (12 patients with hand cramps) and with cervical dystonia (nine with spasmodic torticollis) compared with 16 age-matched normal controls. Motor evoked potentials (MEP) to focal (figure-of-eight coil) transcranial magnetic stimulation of the hand area were recorded from the right abductor pollicis brevis (APB), first dorsal interosseus (FDI), flexor carpi radialis and extensor carpi radialis muscles. Changes of test MEP size following conditioning stimulation of the right median nerve (or of the index finger) at conditioning-test (C-T) intervals of 50, 200, 600 and 1000 ms were analysed. Peripheral stimulation significantly reduced test MEP size in the APB and FDI muscles of normal control and spasmodic torticollis patients. The inhibitory effect was larger upon median nerve stimulation and reached a maximum at the C-T interval of 200 ms. On the contrary, hand cramp patients showed a significant facilitation of test MEP size. This study suggests that MEP suppression following peripheral stimulation is defective in patients with focal hand dystonia. Central processing of sensory input is abnormal in dystonia and may contribute to increased motor cortical excitability.

Adult↗
PubMed2005

Transynaptic effects of tetanus neurotoxin in the oculomotor system.

The question whether general tetanus arises from the independent sum of multiple local tetani or results from the actions of the transynaptic tetanus neurotoxin (TeNT) in higher brain centres remains unresolved. Despite the blood-borne dissemination of TeNT from an infected wound, the access to the central nervous system is probably prevented by the blood-brain barrier. However, several long-term sequelae (e.g. autonomic dysfunction, seizures, myoclonus, and sleep disturbances) present after the subsidence of muscle spasms might be indicative of central actions that occur farther away from lower motoneurons. Subsequently, the obvious entry route is the peripheral neurons followed by the transynaptic passage to the brain. We aimed at describing the pathophysiological correlates of TeNT translocation using the oculomotor system as a comprehensive model of cell connectivity and neuronal firing properties. In this study, we report that injection of TeNT into the medial rectus muscle of one eye resulted in bilateral gaze palsy attributed to firing alterations found in the contralaterally projecting abducens internuclear neurons. Functional alterations in the abducens-to-oculomotor internuclear pathway resembled in part the classically described TeNT disinhibition. We confirmed the transynaptic targeted action of TeNT by analysing vesicle-associated membrane protein2 (VAMP2) immunoreactivity (the SNARE protein cleaved by TeNT). VAMP2 immunoreactivity decreased by 94.4% in the oculomotor nucleus (the first synaptic relay) and by 62.1% presynaptic to abducens neurons (the second synaptic relay). These results are the first demonstration of physiological changes in chains of connected neurons that are best explained by the transynaptic action of TeNT on premotor neurons as shown with VAMP2 immunoreactivity which serves as an indicator of TeNT activity.

Abducens Nerve↗
PubMed1982

Neural plasticity: Part 1. Plasticity in the developing nervous system: prenatal maturation.

The origin and development of the nervous system is a gradual process of cell division, migration, and specialization. The establishment of neural circuits requires cell-to-cell recognition. Despite an unceasing search, the mechanisms accounting for this cell-to-cell recognition remain unknown. In contrast, the stages in prenatal development from conception to birth and the sequence of events in the formation of the nervous system are known in considerable detail. The major purpose of Part 1 is to review the ontogeny of the spinal nervous system, with emphasis on the continuous remodeling phenomena that occur as a result of changes in neuronal activity or in the biochemical milieu. The underlying rationale for focusing on the details of prenatal maturation is to identify and analyze cell-to-cell interactions and to define their critical periods. This type of information is expected to provide explanations for previously unexplained developmental phenomena, to improve ability to diagnose and prognosticate in newborns with congenital anomalies of the nervous system, and to provide therapists with insights for improving treatment techniques for neonates with neurological deficits.

Female↗
PubMed1982

Neural plasticity: Part 2. Postnatal maturation and function-induced plasticity.

At birth, the central nervous system has completed most of its early stages of cell division, migration, and specialization. Much of the neural circuitry has been laid down. Neuroblasts are continuing to divide only in limited brain regions. Hence, at birth, most mammals have a nearly full complement of neurons. Nonetheless, the functional capabilities of the central nervous system of the newborn have little resemblance to those of the adult. Postnatal maturation must proceed in the proper sequence and at the proper rate if central nervous system deficits in the adult are to be avoided. Many of the prenatal maturational phenomena described in the previous paper continue well into the postnatal period. The purpose of this paper is to describe some postnatal maturational mechanisms and to show, by selected examples, the important role that function and experience play in central nervous system development.

Animals↗
PubMed1998

The mechanism of alpha2-adrenergic inhibition of sympathetic ganglionic transmission.

UNLABELLED: Alpha2-adrenergic agonists produce analgesia and reduce hemodynamic stress through central and peripheral mechanisms, but the effect of adrenergic agonists on pre- and postganglionic sites has not yet been clarified. In this study, we examined the effects of dexmedetomidine (DMT), an alpha2-agonist, on neural conduction and neurotransmitter release in sympathetic ganglia. The stellate ganglia from 48 mongrel dogs were isolated, desheathed, and superfused with Krebs' solution. Compound action potentials were evoked, and chromatography was used to detect acetylcholine released by preganglionic stimulation in the presence or absence of DMT. To further elucidate the mechanism of alpha2 effects, DMT was applied in combination with the alpha2-antagonist atipamezole (AT) or the imidazoline antagonist idazoxan (ID). In other experiments, DMT was applied in the presence of exogenous nicotinic stimulation with 1,1-dimethyl-4-phenylpiperazinium iodide or muscarinic stimulation with (+)cis-dioxolane. DMT dose-dependently inhibited synaptic transmission with a 50% effective dose of 71.6 (26.0-174.3) microM. Neurotransmitter release was reduced 25% by 70 microM DMT during low-frequency (0.4 Hz) stimulation, but this effect was abolished at higher frequency (5 Hz) stimulation. AT but not ID blocked the inhibitory action of DMT. DMT inhibited the excitatory postsynaptic response to exogenous muscarinic stimulation but not nicotinic stimulation. These results indicate that alpha2-receptor activation depresses ganglionic transmission through postsynaptic inhibition of muscarinic stimulation, although reduction of neurotransmitter release through a presynaptic autofeedback mechanism is also involved. IMPLICATIONS: This article provides novel insights into the mechanism of drug action of alpha2-receptor agonists in the sympathetic ganglia of dogs by directly measuring the relative contribution of pre- and postganglionic receptors. Our study indicates that the central sympatholytic effects of alpha2-adrenoceptor stimulation are augmented by peripheral inhibition of ganglionic transmission.

Acetylcholine↗
PubMed2002

Vecuronium directly inhibits hypoxic neurotransmission of the rat carotid body.

UNLABELLED: Previous studies have suggested that partial neuromuscular blockade by vecuronium may inhibit the chemoreceptor neural response to hypoxia. Because acetylcholine and its receptors are critically involved in the hypoxic neurotransmission of the carotid body, we examined whether vecuronium interferes with nicotinic processes in the carotid body and inhibits the chemoreceptor neural response to hypoxia. The carotid body was harvested from anesthetized adult Wister rats. Carotid sinus nerve activity (CSNA) was recorded in vitro, whereas the carotid body was perfused with Krebs solutions equilibrated with 5% CO(2)/air or 5% CO(2)/N(2). Vecuronium (0.1, 0.5, and 5 microM) was administered via perfusion. Hypoxic perfusion increased CSNA and the response remained stable for two hours. With vecuronium 0.5 and 5 microM, the increase in CSNA (DeltaCSNA) in response to hypoxia was significantly attenuated. The inhibitory effect of vecuronium was dose-related. Acetylcholine and nicotine increased CSNA, and the values of DeltaCSNA were significantly attenuated by vecuronium. These results indicate that vecuronium directly inhibits the carotid body neural response to hypoxia, possibly because of the inhibition of neuronal nicotinic receptors in the carotid body. IMPLICATIONS: We investigated the effect of vecuronium on the chemoreceptor response to hypoxia with perfused rat carotid bodies. The results indicate that vecuronium significantly reduces carotid body neural responses to hypoxia, acetylcholine, and nicotine by inhibiting neuronal nicotinic receptors in the carotid body.

Acetylcholine↗
PubMed2001

Passive and active lower-limb movements delay upper-limb balance reactions.

This study investigated the influence of rhythmic lower-limb activity on the timing of upper-limb balance reactions. Compensatory grasping reactions were evoked in healthy subjects by rapid sagittal tilts of a chair under three conditions: (1) active leg pedaling, (2) passive (motor-driven) leg pedaling, and (3) no lower-limb movement (control task). Compared with control trials, both active and passive pedaling resulted in similar delays in the initiation (43-47 ms) and execution (12-17 ms) of grasping reactions. The similarity between effects due to active and passive movement suggests that the conditioning arose predominantly from sensory discharge associated with lower-limb movement. These results may have important implications for understanding the influence of locomotion or other ongoing movement on the control of stability.

Adult↗
PubMed2001

Is frontal lobe involved in the generation of auditory evoked P50?

This study examined the functional substrate of P50 suppression. Auditory evoked potentials (AEPs) and magnetic fields (AEFs) were recorded from healthy subjects simultaneously and analyzed using spatio-temporal source analysis. The resulting equivalent dipole model for the AEP consisted of one source in the auditory cortex (AC) of each hemisphere and an radially oriented medial frontal source, both with maximum AEP activity around 50 ms. The frontal source was functionally separated from the AC sources since it peaked significantly later and showed significantly larger P50 amplitude suppression. P30m showed neither suppression nor substantial frontal activity. In sum, this study relates P50 suppression to reduction of AC source activity and is the first to yield direct evidence for frontal involvement in P50 suppression.

Acoustic Stimulation↗
PubMed2001

Effects of cerebellar dentate nucleus GABAergic cells on rat inferior olivary neurons.

The present study was undertaken to analyze the effects on unitary activity of inferior olive (IO) neurons elicited by activation of cerebellar lateral nucleus (LN), in rats submitted to the chronic destruction of MDJ structures, i.e. in animals in which the LN-evoked effects in IO should be depended only on activation of GABAergic cells of LN. It has been observed that about two-thirds of the olivocerebellar neurons are significantly affected by LN stimulation, and > 68% of those cells were inhibited. Two-thirds of the inhibitory responses were compatible with a monosynaptic linkage, whereas the remaining inhibitions were probably due to polisynaptic linkages. The majority of LN-induced inhibitions was abolished or greatly reduced following application of GABA antagonists.

Action Potentials↗
PubMed2002

Rapid adaptation to internal states as a coding strategy in visual cortex?

Adaptation is a prominent feature of biological neuronal systems. A common interpretation of adaptation in terms of function is that it provides flexibility for a neuronal system to perform well under varying external conditions, for example by adjusting the input/output relation of a sensory system with reference to the ensemble of stimuli the organism currently perceives. This interpretation, however, only applies if the time-scale of adaptation is slower than the time-scale at which the environment changes. Experimentally it is observed, however, that adaptation can be very rapid. Spike-frequency adaptation of cortical neurons, for example, occurs on a time-scale of approximately 100 ms. Here we show that those rapid adaptation processes can also be understood within the framework of information theory. We start with the hypothesis that neuronal codes are designed to optimize the information a neuronal representation conveys about an input stimulus for any increasing time window beginning with stimulus onset, and we show that this implies a rapid adaptation of the neuronal code on the time-scale of stimulus presentation. Adaptation, however, does not occur because the state of the environment changes. Rather it is a reaction to changes of the organisms own internal state, e.g. the level of noise in the neuronal representation. We apply this approach to a model of an orientation hypercolumn in the primary visual cortex, and predict that inter-columnar interactions should adapt on the time-scale of a typical fixation period ( approximately 300 ms).

Adaptation, Psychological↗
PubMed2002

Spatial coincidence modulates interaction between visual and somatosensory evoked potentials.

The time course of interaction between concurrently applied visual and somatosensory stimulation with respect to evoked potentials (EPs) was studied. Visual stimuli, either in the left or right hemifield, and electric stimuli to the left wrist were delivered either alone or simultaneously. Visual and somatosensory EPs were summed and compared to bimodal EPs (BiEP, response to actual combination of both modalities). Temporal coincidence of stimuli lead to sub-additive or over-additive amplitudes in BiEPs in several time windows between 75 and 275 ms. Additional effects of spatial coincidence (left wrist with left hemifield) were found between 75 and 300 ms and beyond 450 ms. These interaction effects hint at a temporo-spatial pattern of multiple brain areas participating in the process of multimodal integration.

Adult↗
PubMed2002

Limb flexion reflex-related areas in human cerebellum.

Numerous studies have shown that the cerebellum is involved in classical conditioning of specific aversive reactions. Only few studies, however, have examined the exact localization of cerebellar areas involved in the control of unconditioned reflex responses. The present study investigated cerebellar areas, which are activated during eliciting of the limb flexion reflex in healthy controls using [15O]H2O PET. Limb flexion reflex related areas were found in vermal lobules III-VI with the local maximum in vermal lobule V. Areas of activation were more widespread compared to a voluntary foot movement and a sensory foot stimulation condition. Limb flexion reflex areas probably reflect efferent reflex modulation as well as sensory inputs from the stimulus itself and/or the reflex response.

Adult↗
PubMed2004

Contribution of somatosensory cortex to evoked cerebellar blood flow responses.

Projections from the trigeminocerebellar pathway and the somatosensory cortex coincide spatially in the granule cell layer of Crus I/II of the cerebellar hemisphere. A biphasic field potential was seen: one peak at 10 ms (trigeminal input) and another at 20 ms (somatosensory input). Linear correlation analysis revealed only a weak coupling between somatosensory input and cerebellar blood flow responses to infraorbital nerve stimulation. In separate experiments, cortical spreading depression attenuated the field potential peak at 20 ms while blood flow responses remained unaltered. Thus, trigeminocerebellar activity explained the evoked blood flow responses. Our data provide further evidence that activity-dependent blood flow responses are context-sensitive and that interaction between excitatory neuronal circuits targeting the same cells may occlude vascular responses.

Action Potentials↗
PubMed1990

Psychophysical measures of central auditory dysfunction in multiple sclerosis: neurophysiological and neuroanatomical correlates.

Central auditory function was assessed in 15 patients with multiple sclerosis (MS) to determine whether the demyelinating lesions resulted in disruption of temporal processing. Auditory evoked potential (AEP) recordings included all three latency regions: Auditory brain stem responses (ABRs), midlatency responses (MLRs), and long-latency responses (LLRs). Two psychophysical tasks thought to involve temporal processing were used: a monaural-processing task (gap-detection) and a binaural-processing task (masking level difference; MLD). Further, AEP abnormalities and psychophysical performance deficits were related to lesion location, based on magnetic resonance imaging (MRI) scans. Reduced MLDs were seen in six MS subjects. Abnormal MLDs were always accompanied by abnormal ABRs and MLRs, and compared to subjects with normal MLDs, the subjects with abnormal MLDs were more likely to have bilateral abnormalities in the AEPs. Further, MLR indices of abnormal binaural interaction appeared to be specifically related to the psychophysical measure of binaural processing. The MRI data of these patients indicated widespread involvement of the auditory pathway. MS subjects with abnormal MRI signals restricted to levels caudal to the lateral lemniscus did not have abnormal MLDs. Gap-detection thresholds were more resistant to the effects of the demyelinating lesions; only two subjects had abnormal gap-detection thresholds. These subjects had extensive AEP abnormalities (bilaterally, in all three latency regions). The gap-detection thresholds were most specifically related to abnormalities of the LLRs. In addition, the subjects with elevated gap-detection thresholds were the only ones with a prolonged interval between the ABRs and MLRs. Thus, efficient neural conduction between the upper brain stem and auditory cortex appears to be crucial for normal monaural temporal processing. The results indicate that demyelinating lesions can cause deficits in temporal processing in the central auditory pathway. However, auditory temporal processing is not a unitary phenomenon since abnormalities at different levels of the auditory system disrupt different types of temporal processing. Finally, abnormal psychophysical performance was not seen in all subjects with AEP and MRI evidence of involvement of the auditory pathway; rather, these psychophysical measures appeared to be sensitive to disruption only in specific portions of the auditory system.

Adolescent↗
PubMed1998

Transcranial magnetic stimulation in study of the visual pathway.

The authors critically reviewed experiments in which transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS) of the higher visual pathway were used. Topics include basic mechanisms of neural excitation by TMS and their relevance to the visual pathway (excitatory and inhibitory effects), TMS and rTMS of calcarine cortex (suppression, unmasking, and phosphenes), TMS of V5 (suppression), TMS and rTMS of higher level temporoparietooccipital areas (perceptual errors, unmasking, and inattention), the role of frontal lobe output in visual perception, and vocalization of perceived visual stimuli (role of consciousness of linguistic symbols).

Attention↗