Changes in Tactile Spatial Discrimination and Cutaneous Coding Properties by Skin Hydration in the Elderly

SummaryNeurosensory tactile functions were investigated in human subjects by two different and complementary experimental approaches. First, a conventional psychophysical method (two-point gap discrimination) was used to determine the tactile discrimination threshold by analyzing the subjects' ability to detect a gap of variable width between two contact points when a series of stimuli was applied to the skin. Using this method we confirmed the marked degradation of tactile spatial acuity with age and showed that skin discriminative function was partially restored after hydration of the skin with a moisturizer. The second approach consisted of a microneurographic recording of tactile afferent fibers in response to two types of mechanical stimuli applied reproducibly to the corresponding receptive fields. With this method, we found that the afferent messages were depressed following hydration of the skin surface. Thus, partial restoration of tactile spatial acuity after hydration appears to be due to both a softening of the stratum corneum permitting better localization of the stimulus and a weaker transfer of the stimulus toward the sensory receptors

Contribution a l'analyse du codage des activites motrices chez l'homme : etude du controle de la sensibilite des fuseaux neuro-musculaires et role des informations musculo-tendineuses et cutanees

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Introduction : Amélioration du contrôle postural par une stimulation mécanique imperceptible

National audienceL’application de vibrations mécaniques de fréquences aléatoires et faibles amplitudes (appelées « bruit ») sur les tendons des muscles posturaux de la cheville augmente la sensibilité proprioceptive et améliore la détection de mouvements de l’articulation de la cheville. L’objectif de cette étude a été d’analyser les conséquences de l’application de ce bruit sur la stabilisation posturale chez le sujet adulte jeune et en bonne santé

Muscle proprioceptive feedback can be adapted to the behavioral and emotional context in humans

International audienceTo interact with the external world, incoming sensory information informs us about situational changes that are of relevance to our behavior and we act upon these accordingly, be it in a produced movement or an internal reaction (e.g. an emotion, paying attention). Humans efficiently interact with their world and each other and to do this, a system is required to promote relevant sensory feedback to allow fast, accurate, and appropriate actions. We review findings from single unit recordings of muscle afferents that demonstrate the effects of the descending γ-fusimotor system on their activity. This includes changes in firing from cognitive and emotional influences, which we postulate can prepare the body for responsive and appropriate action to a change in environment

Introduction : Amélioration du contrôle postural par une stimulation mécanique imperceptible

National audienceL’application de vibrations mécaniques de fréquences aléatoires et faibles amplitudes (appelées « bruit ») sur les tendons des muscles posturaux de la cheville augmente la sensibilité proprioceptive et améliore la détection de mouvements de l’articulation de la cheville. L’objectif de cette étude a été d’analyser les conséquences de l’application de ce bruit sur la stabilisation posturale chez le sujet adulte jeune et en bonne santé

Ago-antagonist muscle spindle inputs contribute together to joint movement coding in man

International audienceThe proprioceptive feedback associated with the performance of even quite simple movements is always generated by the whole set of muscles subjected to mechanical deformation (lengthening, shortening, contraction, etc.) during that particular movement. The question was addressed here as to how muscle spindle feedbacks arising from agonist and antagonist muscles may contribute to the coding of movement parameters such as the direction and velocity. For this purpose, the activity of single muscle spindle afferents located in the lateral peroneal nerve was analysed using the microneurographic technique, in human subjects performing repetitive voluntary movements, i.e., plantar/dorsal flexions of the ankle, at three different velocities (3, 4.5 and 6 degrees/s). The data obtained suggest that in humans, the direction of a slow movement may be specified on the basis of the spindle discharge rate, which is greater in the stretched than in the shortened muscle, and that the velocity of this movement might be correlated with the difference between the spindle activity occurring in the agonist and antagonist muscles. These neurophysiological data are in agreement with the results of previous psychophysical studies showing for example that a sensation of illusory movement can be elicited only when there exists an imbalance between the agonist versus antagonist vibration-induced Ia inputs. In addition, the greater the difference between the vibration frequencies applied to the two antagonist muscles, the higher the perceived movement velocity was found to be. All in all, joint movement perception seems to result from the co-processing by the central nervous system of the multiple spindle feedbacks originating from the whole set of muscles involved in the performance of a movement. (C) 1998 Elsevier Science B.V

Improving postural control by applying mechanical noise to ankle muscle tendons

International audienceThe application of subthreshold mechanical vibrations with random frequencies (white mechanical noise) to ankle muscle tendons is known to increase muscle proprioceptive information and to improve the detection of ankle movements. The aim of the present study was to analyze the effect of this mechanical noise on postural control, its possible modulation according to the sensory strategies used for postural control, and the consequences of increasing postural difficulty. The upright stance of 20 healthy young participants tested with their eyes closed was analyzed during the application of four different levels of noise and compared to that in the absence of noise (control) in three conditions: static, static on foam, and dynamic (sinusoidal translation). The quiet standing condition was conducted with the eyes open and closed to determine the subjects' visual dependency to maintain postural stability. Postural performance was assessed using posturographic and motion analysis evaluations. The results in the static condition showed that the spectral power density of body sway significantly decreased with an optimal level of noise and that the higher the spectral power density without noise, the greater the noise effect, irrespective of visual dependency. Finally, noise application was ineffective in the foam and dynamic conditions. We conclude that the application of mechanical noise to ankle muscle tendons is a means to improve quiet standing only. These results suggest that mechanical noise stimulation may be more effective in more impaired populations

Contribution des afférences proprioceptives au codage sensoriel des mouvements chez l'homme (ajustement de la commande fusimotrice au contexte comportemental)

La perception des actions de notre corps repose sur l intégration d informations provenant de différentes modalités sensorielles. Dans cette thèse, nous avons analysé le rôle joué d une part, par les informations proprioceptives musculaires en portant une attention plus particulière sur les conditions de réglage de la sensibilité des fuseaux neuromusculaires et d autre part, par les informations d origine cutanée. Pour ce faire, nous avons principalement enregistré l activité unitaire de neurones sensoriels chez l Homme au moyen de la technique microneurographique. Les informations proprioceptives musculaires proviennent des fuseaux neuromusculaires dont la sensibilité peut être contrôlée sélectivement par le système nerveux central grâce à l activité de neurones particuliers : les neurones fusimoteurs gamma (g) statiques et dynamiques. Ainsi, les récepteurs fusoriaux peuvent être rendus plus sensibles aux postures ou aux mouvements. Pour la plupart des auteurs, la commande fusimotrice aurait un rôle très élémentaire. Elle maintiendrait les fuseaux neuromusculaires dans un état tel qu ils soient aptes à remplir leur fonction sensorielle utile à la perception et à la régulation des activités motrices et ce, malgré leur localisation dans un tissu aux propriétés mécaniques changeantes, le muscle. Toutefois, ce rôle est déjà joué par le système squelettofusimoteur. Nous avons fait l hypothèse que le système fusimoteur présente un intérêt fonctionnel qui lui est propre. Nos résultats montrent que le système fusimoteur peut être mis en jeu sélectivement. En effet, les fuseaux neuromusculaires présentent une sensibilité accrue lorsque le sujet est impliqué dans une tâche où il doit reconnaître des mouvements imposés (signes graphiques) sur la seule base des informations proprioceptives (absence de vision et de commande volontaire). Nous montrons également que ce contrôle de sensibilité peut être différentiel. En effet, la sensibilisation fusoriale peut commuter d une nature statique à dynamique selon que l attention du sujet est spécifiquement portée sur la position adoptée dans l espace ou sur la vitesse des mouvements. Enfin, nous montrons que la mise en jeu du système fusimoteur peut améliorer le sens du mouvement et notamment diminuer les seuils de perception. Par ailleurs, l exécution de mouvements déforme tous les tissus autour des articulations, les muscles mais aussi les tissus cutanés qui les enveloppent. Notre dernière étude montre que les récepteurs cutanés partagent des propriétés communes de codage des mouvements avec les récepteurs musculaires des muscles sous-jacents. Ce parallélisme faciliterait l intégration centrale des informations kinesthésiques d origines différentes. En conclusion, pour la première fois chez l Homme, nous soutenons l idée que le système nerveux central peut régler spécifiquement la sensibilité statique ou dynamique des fuseaux neuromusculaires. Ainsi, le feedback proprioceptif musculaire n est pas "automatique" mais peut être ajusté aux conditions environnementales et/ou comportementales. Nous suggérons que cet ajustement présente un intérêt majeur au cours des activités motrices posturales et/ou de locomotion.The perception of our actions relies on feedbacks from various sensory modalities. In this thesis, we analysed the role played on one hand by muscle proprioceptive feedback with particular focus on the conditions of adjustment of the muscle spindle sensitivity and on the other, by cutaneous feedback. We recorded principally the activity of single sensory neurons in humans with the microneurographic technique. Muscle proprioceptive feedback originates from muscle spindles whose sensitivity may be selectively controlled by the central nervous system through activation of static and dynamic gamma (g) fusimotor neurons. For most authors, the role of the fusimotor drive is basic: it would maintain the muscle spindle in a specific state in order to exert its perceptual and sensorimotor functions despite its localisation in a changing mechanical tissue, the muscle. However, this role is already played by the skeletofusimotor system. We hypothesized that the fusimotor system presents a functional interest in itself. Our results show that the fusimotor system may be selectively activated: muscle spindle sensitivity increases when the subject is involved in a movement trajectory recognition task that relies only on proprioceptive cues (in the absence of visual information) and independently of any muscle activity. We also show that the muscle spindle sensitivity may be differentially controlled: focusing attention on movement velocities or on the positions adopted in space can change from dynamic to static fusimotor drive, respectively. Finally, we show that the fusimotor system improves movement sense and particularly decreases the perception thresholds. Moreover, movements deform muscles but also all the skin areas surrounding the joint. Our last study shows that cutaneous and muscle afferents share common movementencoding characteristics. This parallelism may facilitate the central co-processing of the proprioceptive feedbacks subserving kinesthesia. In conclusion, for the first time in humans, our findings support the idea that the central nervous system is able to selectively and differentially control the static or dynamic sensitivity of muscle spindles. The muscle proprioceptive feedback is not "automatic" but may be adjusted to environmental and/or behavioural contexts. We suggest that this adjustment presents a major interest during postural and/or locomotor activities.AIX-MARSEILLE1-BU Sci.St Charles (130552104) / SudocSudocFranceF

Pain management in photoepilation.

International audienceThe hair follicle is a complex, hormonally active structure with permanent and cyclically renewed parts which are highly innervated by myelinated and unmyelinated afferent fibers. Hair removal, a very ancient practice, affects this sensory network and causes both acute and diffuse pain associated with inflammatory reaction. Optic permanent hair removal is becoming a popular alternative to traditional methods such as shaving, waxing, among other methods. These optical removal devices thermally destroy the target chromophore, that is, melanin, without damaging the surrounding skin. The increase in the skin surface temperature causes mild-to-severe pain, and optical hair removal has to be combined with pain relieving devices. Pain management relies on topical anesthetic agents, cooling devices, or non-noxious cutaneous stimulation whose mechanisms of action and efficiency are discussed in this article

Approche microneurographique du sens tactile chez l’homme ; effet des émotions

International audienc