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.

Slowly-adapting type II afferents contribute to conscious touch sensation in humans: Evidence from single unit intraneural microstimulation.

Slowly-adapting type II (SA-II, Ruffini) mechanoreceptive afferents respond well to pressure and stretch, and are regularly encountered in human microneurography studies. Despite an understanding of SA-II response properties, their role in touch perception remains unclear. Specific roles of different myelinated Aß mechanoreceptive afferents in tactile perception have been revealed using single unit intraneural microstimulation (INMS), via microneurography, recording from and then electrically stimulating individual afferents. This method directly links single afferent artificial activation to perception, where INMS produces specific 'quantal' touch percepts associated with different mechanoreceptive afferent types. However, SA-II afferent stimulation has been ambiguous, producing inconsistent, vague sensations, or no clear percept. We physiologically characterized hundreds of individual Aß mechanoreceptive afferents in the glabrous hand skin and examined the subsequent percepts evoked by trains of low amplitude INMS current pulses (<10 µA). We present 18 SA-II afferents where INMS resulted in a clear, electrically evoked sensation of large (∼36 mm2 ) diffuse pressure, which was projected precisely to their physiologically-defined receptive field in the skin. This sensation was felt as natural, distinctive from other afferents, and showed no indications of multi-afferent stimulation. Stimulus frequency modulated sensation intensity and even brief stimuli (4 pulses, 60 ms) were perceived. These results suggest that SA-II afferents contribute to perceived tactile sensations, can signal this rapidly and precisely, and are relevant and important for computational models of touch sensation and artificial prosthetic feedback. KEY POINTS: Slowly adapting type II mechanoreceptors (SA-IIs) are primary sensory neurons in humans that respond to pressure and stretch applied to the skin. To date, no specific conscious correlate of touch has been linked to SA-II activation. Using microneurography and intraneural microstimulation to stimulate single sensory neurons in human subjects, we find a specific sensation linked to the activation of single SA-II afferents. This sensation of touch was reported as gentle pressure and subjects could detect this with a high degree of accuracy. Methods of artificial tactile sensory feedback and computational models of touch should include SA-IIs as meaningful contributors to the conscious sensation of touch.

Human low-threshold mechanoafferent responses to pure changes in friction controlled using an ultrasonic haptic device.

The forces that are developed when manipulating objects generate sensory cues that inform the central nervous system about the qualities of the object's surface and the status of the hand/object interaction. Afferent responses to frictional transients or slips have been studied in the context of lifting/holding tasks. Here, we used microneurography and an innovative tactile stimulator, the Stimtac, to modulate both the friction level of a surface, without changing the surface or adding a lubricant, and, to generate the frictional transients in a pure and net fashion. In three protocols, we manipulated: the frictional transients, the friction levels, the rise times, the alternation of phases of decrease or increase in friction to emulate grating-like stimuli. Afferent responses were recorded in 2 FAIs, 1 FAII, 2 SAIs and 3 SAIIs from the median nerve of human participants. Independently of the unit type, we observed that: single spikes were generated time-locked to the frictional transients, and that reducing the friction level reduced the number of spikes during the stable phase of the stimulation. Our results suggest that those frictional cues are encoded in all the unit types and emphasize the possibility to use the Stimtac device to control mechanoreceptor firing with high temporal precision.

Evidence for sparse C-tactile afferent innervation of glabrous human hand skin.

C-tactile (CT) afferents were long-believed to be lacking in humans, but these were subsequently shown to densely innervate the face and arm skin, and to a lesser extent the leg. Their firing frequency to stroking touch at different velocities has been correlated with ratings of tactile pleasantness. CT afferents were thought to be absent in human glabrous skin; however, tactile pleasantness can be perceived across the whole body, including glabrous hand skin. We used microneurography to investigate mechanoreceptive afferents in the glabrous skin of the human hand, during median and radial nerve recordings. We describe CTs found in the glabrous skin, with characteristics comparable with those in hairy arm skin, and detail recordings from three such afferents. CTs were infrequently encountered in the glabrous skin and we estimate that the ratio of recorded CTs relative to myelinated mechanoreceptors (1:80) corresponds to an absolute innervation density of around seven times lower than in hairy skin. This sparse innervation sheds light on discrepancies between psychophysical findings of touch perception on glabrous skin and hairy skin, although the role of these CT afferents in the glabrous skin remains subject to future work.NEW & NOTEWORTHY Human touch is encoded by low-threshold mechanoreceptors, including myelinated Aß afferents and unmyelinated C-tactile (CT) afferents. CTs are abundant in hairy skin and are thought to code gentle, stroking touch that signals positive affective interactions. CTs have never been described in human glabrous skin, yet we show evidence of their existence on the hand, albeit at a relatively low density. Glabrous skin CTs may provide modulatory reinforcement of gentle tactile interactions during touch using the hands.

Microneurography as a tool to study the function of individual C-fiber afferents in humans: responses from nociceptors, thermoreceptors, and mechanoreceptors.

The technique of microneurography-recording neural traffic from nerves in awake humans-has provided us with unrivaled insights into afferent and efferent processes in the peripheral nervous system for over 50 years. We review the use of microneurography to study single C-fiber afferents and provide an overview of the knowledge gained, with views to future investigations. C-fibers have slowly conducting, thin-diameter, unmyelinated axons and make up the majority of the fibers in peripheral nerves (~80%). With the use of microneurography in humans, C-fiber afferents have been differentiated into discrete subclasses that encode specific qualities of stimuli on the skin, and their functional roles have been investigated. Afferent somatosensory information provided by C-fibers underpins various positive and negative affective sensations from the periphery, including mechanical, thermal, and chemical pain (C-nociceptors), temperature (C-thermoreceptors), and positive affective aspects of touch (C-tactile afferents). Insights from microneurographic investigations have revealed the complexity of the C-fiber system, methods for delineating fundamental C-fiber populations in a translational manner, how C-fiber firing can be used to identify nerve deficits in pathological states, and how the responses from C-fibers may be modified to change sensory percepts, including decreasing pain. Understanding these processes may lead to future medical interventions to diagnose and treat C-fiber dysfunction. NEW & NOTEWORTHY The technique of microneurography allows us to directly investigate the functional roles of single C-fiber afferents in awake human beings. Here we outline and discuss the current field of C-fiber research on this heterogeneous population of afferents in healthy subjects, in pathological states, and from a translational perspective. We cover C-fibers encoding touch, temperature, and pain and provide perspectives on the future of C-fiber microneurography investigations in humans.