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

Abstract

NEW & NOTEWORTHY Human touch is encoded by low-threshold mechanoreceptors, including myelinated Aβ 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.

A topographical and physiological exploration of C-tactile afferents and their response to menthol and histamine

“…Molecular visualization of the apparent CT afferent mouse homologue have shown that CT afferents are present in all the nerves supplying the hairy skin (Li et al 2011). Combined with ours and previous recordings from nerves innervating the skin of the face, thigh and palm (Edin 2001;Johansson et al 1988;Nordin 1990;Watkins et al 2021), this suggests that CT afferents are ubiquitous across human hairy skin although very sparse in distal parts of the body. Deep sequencing of sensory neurons from mice have revealed that while the most dramatic differences in gene expression patterns are observed between unmyelinated and myelinated neuron subtypes, some genes such as the potassium voltage-gated channel Kv4.3 is uniquely expressed in CLTMR sensory neurons (Zheng et al 2019).…”

“…These estimates are obviously not an exact measure of density, but as our main focus was to record from CT afferents, the higher incidence of CMs in relation to CTs is more likely to be an underestimation, than vice versa. We recently showed that low-threshold mechano C afferents are also found in glabrous skin of the human hand, although their presence here appear to be very sparse (Watkins et al 2021). It is now clear that CT afferents can indeed be found in the distal parts of the limbs such as the hand and lower leg, although the current results suggests that they are considerably sparser in the distal parts of the human body, similar to what has been found in non-human primates (Kumazawa and Perl 1977) CT afferents display several interesting physiological phenomena that are yet to be fully understood.…”

“…CT afferents (lateral antebrachial nerve: n=27/32 experiments, radialis n=8/9 experiments, peroneus n=4/17 experiments) were identified by a low mechanical threshold to monofilament bristles (lateral antebrachial nerve n=27, mean threshold 0.85mN, range 0.04-2.5mN; radial nerve n=8, median threshold 0.68 mN, range 0.27-2.5mN; peroneal nerve n=4, median threshold 1.6 mN, range 0.7 -1.6 mN), slow conduction velocity (lateral antebrachial median 0.9m/s, radialis median 0,98 m/s, peroneus median 1 m/s, range 0.9-1.1m/s m/s), and vigorous response to brush stroking. The majority of units in the sample recorded from the lateral antebrachial nerve have been described previously with respect to their response to brush velocity (Loken et al 2009) and one unit from the radial nerve was included in a previous publication (Watkins et al 2021). The most distal receptive fields were located on the proximal phalanx of the third finger for the superficial branch of the radial nerve, and ~6 cm proximal to the lateral malleolus for the peroneal nerve.…”

The impact of C-Tactile Low threshold mechanoreceptors on affective touch and social interactions in mice

“…In accordance with C-LTMRs known innervation of the hairy skin, we observed a large increase of what we considered skin-to-skin contact (contact side by side and side by side opposite, cumulative duration and number of events Figure 5 d-e) 17 . More surprisingly, we also witnessed an increase in nose-to-nose contacts and even more nose-to-anogenital contacts, involving skin areas that are not supposedly innervated by C-LTMRs, although recent work suggest that C-LTMRs innervates more skin areas than expected in humans, such as glabrous skin 48 . Such an observation may be due to a priming or reinforcing influence of C-LTMRs activation on inter individual interaction to induce more complex social contacts such as social investigation (nose to nose and nose to anogenital contacts).…”

Out of touch: Touch deprivation and affective touch perception during the COVID-19 pandemic

“…Gentle stroking at 1-10 cm/s (optimally at 3 cm/s) activates specific unmyelinated low-threshold mechanosensory C-fibers namely, C-tactile (CT) afferents, therefore it is also referred to as CT-optimal touch. CT-afferents are mostly located in the hairy skin ( (Vallbo, Olausson, & Wessberg, 1999) but see (Watkins et al, 2020)). The CT-system contains a distinct neural pathway projecting to cortical areas mostly involved in affective and emotional processing, i.e.…”