The human touch: A connected neuro‐cellular skin‐brain network

Abstract

In addition to its highly important function as a protective barrier, the skin is a key player in the sense of touch. The latter provides a role that is too often neglected despite its major importance in many aspects, being intimate or social but crucial in the application of cosmetic products or medical procedures. The present review aims at describing the human skin as a connected bio-electronic tissue, acting as an emitter and receiver in the image of electronic devices such as sophisticated smartphones. In late 2021, the Nobel Prize for Physiology or Medicine was awarded to D. Julius and A. Patapoutian for their joint successful research on some skin receptor channels that sense heat, pressure, and body position shedding new light on the human sense of touch.1-3 Indeed, with regard to its very large surface (1.5–2 m2 in adults), skin comprises a vast and dense network of cells and transmitter channels. The human skin can therefore be viewed as a bio-electronic sensor that covers each µm2 of the skin with such high precision that the image of a tactile screen comes spontaneously to mind. If the sensorial feeling is the primary role of this bio-electronic skin, conveying centripetal signals (up to the brain), some of this information can be centrifuged, that is, emitted to the environment, being self-perceived or sent to the “others”. Like the two-faced Latin God Janus, skin and its neuro-sensorial organization would then present two faces, acting as both a receiver and a transmitter “antenna”. The present review, therefore, aims at describing the various skin sensorial facets, appraised at the light of this densely connected neural organization, its dual ability to perform as both a receiver and an emitter, the physics bases of touch, and the challenge of cosmetic products in preserving or maintaining at best its sensorial performance. Skin is logically first viewed as a body-protecting tissue, organized in different strata that all act as barriers against external foes or invaders of a different nature (repeated frictions, ultraviolet [UV], microbes, aggressive chemicals, superficial burns, etc.) using different molecular mechanisms of defense. On the second hand, skin can also be seen as a sophisticated electronic/neuronal structure as its neural organization (nerve fibers, corpuscles, and cells) acts, in final, as electron transducers via messenger molecules such as nitric oxide (NO) or peptides of neuronal origin such as Substance P.4-6 In addition, viewing skin as a biologically designed “electronic” network fits with the embryological origin of some skin cells that derive from the neural crest. Hence, the complex “electronic” organization of the skin grounds on a dense network of fibers differently myelinated (A, C), corpuscles (Pacini, Ruffini, and Meissner), and Merkel cells that all contribute to acting as efficient sensors of different stimuli subjected to epidermal and dermal cells (mechanoreceptors/pressure, heat/cold, roughness/softness, etc.).7, 8 These electron-based signals, of very low voltages and intensities (mV, nA) are transmitted to the brain at speeds ranging from 33 to 75 m/s.8-10 For example, the pain inflicted by a splinter suddenly inserted in the foot of an adult of 1.8 m reaches the brain within some 30–40 ms. This fast response explains how the painful contact of skin with nettle or the skin immersion in too-hot water (>45°C) is immediately felt. Hence, when transmitted to the brain cortex, the sense of touch carries two vectors of information, that is, objective (cold/warm and smooth/rough) and hedonistic (pain/pleasure and contentment).11 The sense of touch is operational at birth, much before the four other senses.11 Touch, together with gustatory, requires close contact, inversely to the three others, that is, hearing, olfactory/smell, and vision. From birth to infancy, touch progressively loses its importance with the acquisition of vision and speech. However, during these early times of