Functional impact of vibratory proprioceptive assistance in patients with facioscapulohumeral muscular dystrophy.

INTRODUCTION: In this study we analyzed the effects of a rehabilitation method based on the use of vibratory proprioceptive assistance (VPA) in subjects with facioscapulohumeral muscular dystrophy. METHODS: Eight subjects were given 1 month of mechanical vibratory treatment that consisted of 8 sessions of 40-min stimulation on the more affected side. During each session, illusory movements were induced as follows: sensations of extension or flexion of the forearm or elevation of the arm via vibration applied to the distal tendon of the biceps brachialis (BB), triceps brachialis (TB), or pectoralis major muscles (PM), respectively, and of elevation of the arm with extension or flexion of the forearm via vibration of PM+BB or PM+TB, respectively. RESULTS: Treatment led to a significant increase in the amplitude of voluntary shoulder flexion, constant score, and self-rated health. CONCLUSION: VPA may serve as a rehabilitation method for reducing the deleterious effects of decline in motor activities.

Relations between tactile sensitivity of the finger, arm, and cheek skin over the lifespan showing decline only on the finger.

Touch sensitivity generally declines with age, contributing to loss of manual dexterity and tactile function. We investigated how touch changes over the lifespan, using different tests and on three body sites. We used a classical test of force detection sensitivity, where calibrated monofilaments were applied passively to the right index finger pad, forearm, and cheek. In addition, at the index, we used an active touch spatial discrimination task, developed by our group. Spatial discrimination was estimated through participants' ability to evaluate the distance between parallel bands printed on acrylic plates. Data were collected from 96 healthy women, aged 20-75 years. Force detection and tactile spatial discrimination on the index deteriorated significantly with age; however, no change was found for tactile detection on the forearm or cheek. Tactile detection on the cheek remained remarkably highly sensitive throughout life. There was a significant positive relationship between force detection and spatial discrimination on the index. Further, force detection on the forearm was significantly associated with detection on the index and cheek. Our results suggest a decrease in touch perception with age on the index finger pad, yet a preservation of tactile sensitivity in hairy skin. This opens discussion about the impact of daily activities upon the glabrous hand skin and on the function of hairs in tactile sensitivity. We highlight the need for new methods in evaluating tactile sensitivity on hairy skin.

Noise-enhanced kinaesthesia: a psychophysical and microneurographic study.

We first explored whether the ability of subjects to detect the direction of slow ramp imposed movements may be improved by the application of mechanical noise to muscle tendons. Movements were plantar/dorsal flexion of the ankle at 0.04°/s, and the amplitude was just sub-threshold for each subject. A white noise signal (random vibration), low-pass filtered to 100 Hz and distributed uniformly in amplitude, was applied to both the extensor and the flexor ankle muscle tendons with four different mean amplitudes (20, 30, 100, 280 µm). The population of subjects was observed to exhibit clear stochastic-type behaviour: their ability to determine the direction of sub-threshold movements significantly increased when the two lower levels of noise were added and subsequently decreased when the noise magnitude was enhanced. Second, using microneurography, we explored the response of 9 primary muscle spindle afferents and 8 cutaneous afferents to the same imposed movements with and without noise application. While these conditions of ankle mobilisation were too small to induce a response in most of the recorded afferents, two muscle afferents exhibited responses that were characteristic of aperiodic stochastic resonance behaviour: the unit movement response was either triggered or improved by the application of an optimal level of noise. All cutaneous afferents were unresponsive to the imposed movements with or without noise application. We conclude that ankle movement sense can be significantly improved by adding an optimal level of mechanical noise to ankle muscle tendons and discuss the optimisation of the response of movement-encoding receptors that may account for this improvement. The application of a mechanical noise on ankle muscle tendons may constitute a means of improving postural stability in subjects with sensory deficits.

Effects of skin moisturization on various aspects of touch showing differences with age and skin site.

The human body is encompassed by a thin layer of tissue, the skin, which is heterogenous and highly specialized to protect the body and encode interactions with the external world. There is a fundamental scientific drive to understand its function, coupled with the need to preserve skin as we age, which impacts on our physiological and psychological well-being. In the present study, we aimed to define differences in touch perception between age groups and with skin cream application. We investigated touch on the finger, the forearm and cheek in younger (20-28 years, n = 22) and older (65-75 years, n = 22) females. We measured skin hydration, touch detection, finger spatial discrimination, forearm tactile pleasantness together with electrodermal activity, and perceptual ratings about cream use, skin dryness, and cosmetic habits. Glabrous finger skin became drier and touch performance was impaired with age, but these aspects were preserved in hairy skin. Skin moisturization immediately increased hydration levels, but did not significantly change touch perception. We also found that touch appreciation increased with age. We conclude that reduced finger capacity may impact self-evaluation of the skin and that long-term skin care strategies should focus on hydrating the hand to preserve touch capacities.

Applying cosmetic oil with added aromatic compounds improves tactile sensitivity and skin properties.

Tactile sensitivity generally decreases with aging and is associated with impairments in skin properties. Products that hydrate the skin can combat touch deficits and aromatic compounds have been shown to improve skin mechanical properties. Thus, we tested a base cosmetic oil against a perfumed oil, applied to the skin of females aged 40-60 years, on tactile sensitivity and skin properties after repeated application. Tactile detection thresholds were assessed using calibrated monofilaments applied at the index finger, palm, forearm, and cheek. Spatial discrimination on the finger was assessed using pairs of plates with different inter-band spaces. These tests were performed before and after 1 month of base or perfumed oil use. We found that tactile detection thresholds and spatial discrimination improved only in perfumed oil group. A complementary immunohistological study using human skin was conducted to estimate the expression of olfactory receptor OR2A4 and elastic fiber length. Further, the expression of OR2A4 intensity and the length of elastic fibers increased significantly with oil application, where larger effects were seen with the perfumed oil. We conclude that the application of a perfumed oil may be of additional benefit and could repair, and even prevent, tactile decline with aging by ameliorating skin condition.

Ankle joint movements are encoded by both cutaneous and muscle afferents in humans.

We analyzed the cutaneous encoding of two-dimensional movements by investigating the coding of movement velocity for differently oriented straight-line movements and the coding of complex trajectories describing cursive letters. The cutaneous feedback was then compared with that of the underlying muscle afferents previously recorded during the same "writing-like" movements. The unitary activity of 43 type II cutaneous afferents was recorded in the common peroneal nerve in healthy subjects during imposed ankle movements. These movements consisted first of ramp-and-hold movements imposed at two different and close velocities in seven directions and secondly of "writing-like" movements. In both cases, the responses were analyzed using the neuronal population vector model. The results show that movement velocity encoding depended on the direction of the ongoing movement. Discriminating between two velocities therefore involved processing the activity of afferent populations located in the various skin areas surrounding the moving joint, as shown by the statistically significant difference observed in the amplitude of the sum vectors. Secondly, "writing-like" movements induced cutaneous neuronal patterns of activity, which were reproducible and specific to each trajectory. Lastly, the "cutaneous neuronal trajectories," built by adding the sum vectors tip-to-tail, nearly matched both the movement trajectories and the "muscle neuronal trajectories," built from previously recorded muscle afferents. It was concluded that type II cutaneous and the underlying muscle afferents show similar encoding properties of two-dimensional movement parameters. This similarity is discussed in relation to a central gating process that would for instance increase the gain of cutaneous inputs when muscle information is altered by the fusimotor drive.

Large Postural Sways Prevent Foot Tactile Information From Fading: Neurophysiological Evidence.

Cutaneous foot receptors are important for balance control, and their activation during quiet standing depends on the speed and the amplitude of postural oscillations. We hypothesized that the transmission of cutaneous input to the cortex is reduced during prolonged small postural sways due to receptor adaptation during continued skin compression. Central mechanisms would trigger large sways to reactivate the receptors. We compared the amplitude of positive and negative post-stimulation peaks (P50N90) somatosensory cortical potentials evoked by the electrical stimulation of the foot sole during small and large sways in 16 young adults standing still with their eyes closed. We observed greater P50N90 amplitudes during large sways compared with small sways consistent with increased cutaneous transmission during large sways. Postural oscillations computed 200 ms before large sways had smaller amplitudes than those before small sways, providing sustained compression within a small foot sole area. Cortical source analyses revealed that during this interval, the activity of the somatosensory areas decreased, whereas the activity of cortical areas engaged in motor planning (supplementary motor area, dorsolateral prefrontal cortex) increased. We concluded that large sways during quiet standing represent self-generated functional behavior aiming at releasing skin compression to reactivate mechanoreceptors. Such balance motor commands create sensory reafference that help control postural sway.

Seeing Your Foot Move Changes Muscle Proprioceptive Feedback.

Multisensory effects are found when the input from single senses combines, and this has been well researched in the brain. Presently, we examined in humans the potential impact of visuo-proprioceptive interactions at the peripheral level, using microneurography, and compared it with a similar behavioral task. We used a paradigm where participants had either proprioceptive information only (no vision) or combined visual and proprioceptive signals (vision). We moved the foot to measure changes in the sensitivity of single muscle afferents, which can be altered by the descending fusimotor drive. Visual information interacted with proprioceptive information, where we found that for the same passive movement, the response of muscle afferents increased when the proprioceptive channel was the only source of information, as compared with when visual cues were added, regardless of the attentional level. Behaviorally, when participants looked at their foot moving, they more accurately judged differences between movement amplitudes, than in the absence of visual cues. These results impact our understanding of multisensory interactions throughout the nervous system, where the information from different senses can modify the sensitivity of peripheral receptors. This has clinical implications, where future strategies may modulate such visual signals during sensorimotor rehabilitation.

Emotions can alter kinesthetic acuity.

Kinesthesia, the perception of our own body movements, relies on the integration of proprioceptive information arising mostly from muscle spindles, which are sensory receptors in skeletal muscles. We recently showed that emotions alter the proprioceptive messages from such muscle afferents, making them more sensitive to muscle lengthening when participants were listening sad music. Presently, we investigated whether these changes in proprioceptive feedback relating to emotional state may affect the perception of limb movements. Kinesthetic acuity was tested in 20 healthy, young adults by imposing ramp-and-hold movements that consisted of either plantar flexion or dorsiflexion movements of the ankle at 0.04°/s, or no movement. These were imposed during four emotional conditions (listening to neutral, sad, or happy music, or no music). The participants were asked to relax and focus on music (or nothing), and then they shifted their focus to the direction of an incoming movement. Once this had finished, they were asked its direction. Muscle activity, heart rate, and electrodermal activity were recorded during each trial, and after each music condition the participants rated the emotion felt on a visual analog scale. The rating of the emotional content of the music corroborated with changes in physiological measures. Kinesthetic acuity was also affected by the emotional state and found to be larger during the sad condition, as compared to the no music or neutral conditions. We conclude that emotion can shape our perception of movements, which we show here where feeling sadness significantly increase our kinesthetic acuity, this may be functionally relevant for the preparation of appropriate behavioral responses.

Long Term Cosmetic Application Improves Tactile Discrimination in the Elderly; a New Psychophysical Approach.

Introduction: Tactile sensitivity is impaired in older adults, which contributes to the loss of manual dexterity and mobility function. The reliability of classical psychophysical tests, such as two-point gap discrimination, has been questioned. Here we tested a new method to determine tactile acuity during dynamic touch, which is more functional than static touch. The aim was to validate a method providing a high level of discrimination of tactile acuity in the elderly. Methods: We tested the ability of subjects to evaluate the distance between bands printed on poly-methyl-methacrylate (PMMA) sheets. Pairs of sheets were compared in two groups of participants aged from 60 to 74 years; the test group was required to apply a cosmetic foam with an active ingredient on both their hands twice a day for 1 month, the control group had an identical task but used the same cosmetic foam without any active ingredient. The tests were run in a double-blind, placebo-controlled study. Results: The tactile discrimination threshold decreased by 83 µm after 1 month of cosmetic application in the group using the active ingredient, while it was unchanged in the control group. Discussion: The test presented here provided highly accurate results and should be useful to determine tactile performance. It allows the monitoring of tactile rehabilitation and/or skin treatments used to restore tactile acuity in the elderly.

Changes in human muscle spindle sensitivity during a proprioceptive attention task.

The aim of the present study was to test whether fusimotor control of human muscle spindle sensitivity changed when attention was selectively directed to the recognition of an imposed two-dimensional movement in the form of a written symbol. The unitary activities of 32 muscle spindle afferents (26 Ia, 6 II) were recorded by microneurography at the level of the common peroneal nerve. The patterns of firing rate in response to passive movements of the ankle, forming different letters or numbers, were compared in two conditions: control and recognition. No visual cues were given in either condition, but subjects had to recognize and name the character in one condition compared with not paying attention in the control condition. The results showed that 58% of the tested Ia afferents presented modified responses to movements when these had to be recognized. Changes in Ia afferent responses included decreased depth of modulation, increased variability of discharge, and changes in spontaneous activity. Not all changes were evident in the same afferent. Furthermore, the percentage of correctly recognized movements amounted to 63% when changes were observed, but it was only 48% when the primary ending sensitivity was unaltered. The responses of group II afferents were only weakly changed or unchanged. It is suggested that the altered muscle spindle sensitivity is because of selective changes in fusimotor control, the consequence of which might be to feed the brain movement trajectory information that is more accurate.

Merkel Cells Sense Cooling with TRPM8 Channels.

In the skin, Merkel cells connect with keratinocytes and Aß nerve fibers to form a touch receptor that functions as a slow adapting mechanoreceptor (slow adapting type 1). In human and mouse Merkel cells, we observed an increased concentration of intracellular Ca2+ ions in response to cold temperature and transient receptor potential melastatine 8 (TRPM8) ion channel agonists. A reduction in the response to cooling and TRPM8 agonists occurred after the addition of TRPM8 antagonists, as well as in TRPM8 knockout mice. Cold temperature and TRPM8 agonists also induced a current that was inhibited by a TRPM8 antagonist. Our results indicate that Merkel cells sense cooling through TRPM8 channels. We hypothesized that cooling modulates the slow adapting type 1 receptor response. Cooling mouse skin to 22°C reduced the slow adapting type 1 receptor discharge frequency. Interestingly, we observed no such reduction in TRPM8 knockout mice. Similarly, in human skin, a temperature of 22°C applied to the slow adapting type 1 receptive field reduced the spiking discharge. Altogether, our results indicate that Merkel cells are polymodal sensory cells that respond to mild cold stimuli through the activation of TRPM8 channels. Thermal activation of Merkel cells, and possibly other TRPM8-expressing non-neuronal cells, such as keratinocytes, potentially adapts the discharge of slow adapting type 1 receptors during cooling.

Sensory training with vibration-induced kinesthetic illusions improves proprioceptive integration in patients with Parkinson's disease.

The present study investigates whether proprioceptive training, based on kinesthetic illusions, can help in re-educating the processing of muscle proprioceptive input, which is impaired in patients with Parkinson's disease (PD). The processing of proprioceptive input before and after training was evaluated by determining the error in the amplitude of voluntary dorsiflexion ankle movement (20°), induced by applying a vibration on the tendon of the gastrocnemius-soleus muscle (a vibration-induced movement error). The training consisted of the subjects focusing their attention upon a series of illusory movements of the ankle. Eleven PD patients and eleven age-matched control subjects were tested. Before training, vibration reduced dorsiflexion amplitude in controls by 4.3° (P<0.001); conversely, vibration was inefficient in PD's movement amplitude (reduction of 2.1°, P=0.20). After training, vibration significantly reduced the estimated movement amplitude in PD patients by 5.3° (P=0.01). This re-emergence of a vibration-induced error leads us to conclude that proprioceptive training, based on kinesthetic illusions, is a simple means for re-educating the processing of muscle proprioceptive input in PD patients. Such complementary training should be included in rehabilitation programs that presently focus on improving balance and motor performance.

Emotions alter muscle proprioceptive coding of movements in humans.

Emotions can evoke strong reactions that have profound influences, from gross changes in our internal environment to small fluctuations in facial muscles, and reveal our feelings overtly. Muscles contain proprioceptive afferents, informing us about our movements and regulating motor activities. Their firing reflects changes in muscle length, yet their sensitivity can be modified by the fusimotor system, as found in animals. In humans, the sensitivity of muscle afferents is modulated by cognitive processes, such as attention; however, it is unknown if emotional processes can modulate muscle feedback. Presently, we explored whether muscle afferent sensitivity adapts to the emotional situation. We recorded from single muscle afferents in the leg, using microneurography, and moved the ankle joint of participants, while they listened to evocative classical music to induce sad, neutral, or happy emotions, or sat passively (no music). We further monitored their physiological responses using skin conductance, heart rate, and electromyography measures. We found that muscle afferent firing was modified by the emotional context, especially for sad emotions, where the muscle spindle dynamic response increased. We suggest that this allows us to prime movements, where the emotional state prepares the body for consequent behaviour-appropriate reactions.

Pain management in photoepilation.

The 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.

Kinesthetic illusions attenuate experimental muscle pain, as do muscle and cutaneous stimulation.

In the present study, muscle pain was induced experimentally in healthy subjects by administrating hypertonic saline injections into the tibialis anterior (TA) muscle. We first aimed at comparing the analgesic effects of mechanical vibration applied to either cutaneous or muscle receptors of the TA or to both types simultaneously. Secondly, pain alleviation was compared in subjects in whom muscle tendon vibration evoked kinesthetic illusions of the ankle joint. Muscle tendon vibration, which primarily activated muscle receptors, reduced pain intensity by 30% (p<0.01). In addition, tangential skin vibration reduced pain intensity by 33% (p<0.01), primarily by activating cutaneous receptors. Concurrently stimulating both sensory channels induced stronger analgesic effects (-51%, p<0.01), as shown by the lower levels of electrodermal activity. The strongest analgesic effects of the vibration-induced muscle inputs occurred when illusory movements were perceived (-38%, p=0.01). The results suggest that both cutaneous and muscle sensory feedback reduce muscle pain, most likely via segmental and supraspinal processes. Further clinical trials are needed to investigate these new methods of muscle pain relief.

Presynaptic and disynaptic inhibition induced by group I muscle afferents.

The task related changes in the Gp I inputs were investigated in type-identified motor units in the wrist extensor muscles. During wrist extension, the monosynaptic inputs generated by applying radial nerve stimulation were distributed among the motoneurone pool in line with the size principle. Their effectiveness was enhanced in the same way during hand clenching and during wrist extension combined with stimulation of the palm and finger cutaneous receptors. The orderly distribution of the monosynaptic Gp I inputs was reversed by the presynaptic inhibition induced by stimulating the Gp I flexor afferents. The effects of the presynaptic inhibition were partially released by applying cutaneous stimulation. During wrist extension, the Gp I flexor afferents generated disynaptic excitatory inputs acting specifically on high-threshold motor units together with disynaptic inhibitory inputs distributed in line with the size principle among the wrist extensor motor nucleus. During hand lenching, their effectiveness was differentially modulated depending on the motor unit type.

Fusimotor drive may adjust muscle spindle feedback to task requirements in humans.

The aim of the present study was to investigate whether the fusimotor control of muscle spindle sensitivity may depend on the movement parameter the task is focused on, either the velocity or the final position reached. The unitary activities of 18 muscle spindle afferents were recorded by microneurography at the common peroneal nerve. We compared in two situations the responses of muscle spindle afferents to ankle movements imposed while the subject was instructed not to pay attention to or to pay attention to the movement, both in the absence of visual cues. In the two situations, three ramp-and-hold movements were imposed in random order. In one situation, the three movements differed by their velocity and in the other by the final position reached. The task consisted in ranking the three movements according to the parameter under consideration (for example, slow, fast, and medium). The results showed that paying attention to movement velocity gave rise to a significant increase in the dynamic and static responses of muscle afferents. In contrast, focusing attention on the final position reached made the muscle spindle feedback better discriminate the different positions and depressed its capacity to discriminate movement velocities. Changes are interpreted as reflecting dynamic and static gamma activation, respectively. The present results support the view that the fusimotor drive depends on the parameter the task is focused on, so that the muscle afferent feedback is adjusted to the task requirements.

Cutaneous afferents provide a neuronal population vector that encodes the orientation of human ankle movements.

The aim of this study was to analyse the directional coding of two-dimensional limb movements by cutaneous afferents from skin areas covering a multidirectional joint, the ankle. The activity of 89 cutaneous afferents was recorded in the common peroneal nerve, and the mean discharge frequency of each unit was measured during the outward phase of ramp and hold movements imposed in 16 different directions. Forty-two afferents responded to the movements in the following decreasing order (SA2, n = 24/27; FA2, n = 13/17; FA1, n = 3/24; SA1, n = 2/21). All the units activated responded to a specific range of directions, defining their 'preferred sector', within which their response peaked in a given direction, their 'preferred direction'. Based on the distribution of the preferred directions, two populations of afferents, and hence two skin areas were defined: the anterior and the external lateral parts of the leg. As the directional tuning of each population was cosine shaped, the neuronal population vector model was applied and found to efficiently describe the movement direction encoded by cutaneous afferents, as it has been previously reported for muscle afferents. The responses of cutaneous afferents were then considered with respect to those of the afferents from the underlying muscles, which were previously investigated, and an almost perfect matching of directional sensitivity was observed. It is suggested that the common movement-encoding characteristics exhibited by cutaneous and muscle afferents, as early as the peripheral level, may facilitate the central co-processing of their feedbacks subserving kinaesthesia.