The canonical view is that touch is signaled by fast-conducting, thickly myelinated afferents, whereas pain is signaled by slow-conducting, thinly myelinated (“fast” pain) or unmyelinated (“slow” pain) afferents. While other mammals have thickly myelinated afferents signaling pain (ultrafast nociceptors), these have not been demonstrated in humans. Here, we performed single-unit axonal recordings (microneurography) from cutaneous mechanoreceptive afferents in healthy participants. We identified A-fiber high-threshold mechanoreceptors (A-HTMRs) that were insensitive to gentle touch, encoded noxious skin indentations, and displayed conduction velocities similar to A-fiber low-threshold mechanoreceptors. Intraneural electrical stimulation of single ultrafast A-HTMRs evoked painful percepts. Testing in patients with selective deafferentation revealed impaired pain judgments to graded mechanical stimuli only when thickly myelinated fibers were absent. This function was preserved in patients with a loss-of-function mutation in mechanotransduction channel PIEZO2. These findings demonstrate that human mechanical pain does not require PIEZO2 and can be signaled by fast-conducting, thickly myelinated afferents.

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Recent evidence suggests that altered responses to affective touch—a pleasant interoceptive stimulus associated with activation of the C-Tactile (CT) system—may contribute to the aetiology and maintenance of mental conditions characterised by body image disturbances (e.g., Anorexia Nervosa). Here, we investigated whether tactile pleasantness and intensity differ across body sites, and if individual differences in dysmorphic appearance concerns and body and emotional awareness might be associated with touch perceptions across body sites. To this end, we measured perceived pleasantness and intensity of gentle, dynamic stroking touches applied to the palm, forearm, face, abdomen and back of 30 female participants (mean age: 25.87±1.17yrs) using CT-optimal (3 cm/s) and non-CT optimal (0.3 and 30 cm/s) stroking touch. As expected, participants rated CT-targeted touch as more pleasant compared to the two non-CT optimal stroking touch at all body sites. Regardless of stroking velocity, touch applied to the abdomen elicited the lowest pleasantness ratings. Lower levels of emotional awareness, greater levels of interoceptive sensibility and of dysmorphic concerns were associated with lower preference for CT-optimal stroking touch applied to the forearm and the back. These findings begin to elucidate the link between CT sensitivity, dysmorphic appearance concerns and body and emotional awareness, which may have implications for future research looking to inform early interventions. Addressing impaired processing of affective interoceptive stimuli, such as CT-targeted touch, may be the key to current treatment approaches available for those populations at risk of disorders characterised by body image disturbance.

The C-tactile (CLTM) peripheral nervous system is involved in social bonding in primates and humans through its capacity to trigger the brain’s endorphin system. Since the mammalian cochlea has an unusually high density of similar neurons (type-II spiral ganglion neurons, SGNs), we hypothesise that their function may have been exploited for social bonding by co-opting head movements in response to music and other rhythmic movements of the head in social contexts. Music provides one of many cultural behavioural mechanisms for ‘virtual grooming’ in that it is used to trigger the endorphin system with many people simultaneously so as to bond both dyadic relationships and large groups. Changes in pain threshold across an activity are a convenient proxy assay for endorphin uptake in the brain, and we use this, in two experiments, to show that pain thresholds are higher when nodding the head than when sitting still.

First order cutaneous neurons allow object 2 recognition, texture discrimination, and sensorimotor 3 feedback. Their function is well-investigated under passive 4 stimulation while their role during active touch or 5 sensorimotor control is understudied. To understand how 6 human perception and sensorimotor controlling strategy 7 depend on cutaneous neural signals under active tactile 8 exploration, the finite element (FE) hand and Izhikevich 9 neural dynamic model were combined to predict the 10 cutaneous neural dynamics and the resulting perception 11 during a discrimination test. Using in-vivo microneurography 12 generated single afferent recordings, 75% of the data was 13 applied for the model optimization and another 25% was 14 used for validation. By using this integrated numerical model, 15 the predicted tactile neural signals of the single afferent fibers 16 agreed well with the microneurography test results, achieving 17 the out-of-sample values of 0.94 and 0.82 for slowly adapting 18 type I (SAI) and fast adapting type I unit (FAI) respectively. 19 Similar discriminating capability with the human subject was 20 achieved based on this computational model. Comparable 21 performance with the published numerical model on 22 predicting the cutaneous neural response under passive 23 stimuli was also presented, ensuring the potential 24 applicability of this multi-level numerical model in studying 25 the human tactile sensing mechanisms during active touch. 26 The predicted population-level 1st order afferent neural 27 signals under active touch suggest that different coding 28 strategies might be applied to the afferent neural signals 29 elicited from different cutaneous neurons simultaneously. 30 31