Les nerfs à fleur de peau.
Skin innervation is extremely dense and reaches the most superficial layers of the epidermis (but not into the stratum corneum). The relationship between skin and the nervous system was neglected for a long time. Nowadays, this represents one of the most widely investigated fields of skin biology. Associations between nerve fibres and skin are so close that these can be observed at the cellular level. They are anatomical and physiological. Neurotransmitters are naturally synthesized by nerve endings, but cells from the skin and the immune system are also able to produce them. They are mediators for transmission of information between skin and the nervous system. All cutaneous and immune cells express specific receptors for these neuromediators, and enzymes for degrading them. Binding of the neurotransmitter to its receptor induces modulation of cell properties and skin functions (immunity, cell differentiation and proliferation, pigmentation, etc.). Hence, keratinocytes, Langerhans cells, melanocytes, endothelial cells, fibroblasts and the other cells of the skin or the immune system are modulated and controlled by the nerves. Reciprocally, skin is able to modulate neuronal activity and growth. All these data suggest that skin, nervous and immune systems are integrated in a unique system that we could name the neuro-immuno-cutaneous system (NICS). Hence, we can understand how the nervous system, and further the psychism, are involved in the maintenance of cutaneous homeostasis. In dermatological diseases or in cosmetological disorders, this equilibrium is broken. New fields of research are open: neuro-dermatology and of course neuro-cosmetics. Neuro-cosmetic products are supposed to modulate NICS functioning at the epidermal level. The French expression 'avoir les nerfs à fleur de peau' means that nervosity is so tense that one feels his nerves closely under or within skin, ready to explode. In fact, nerves are always 'à fleur de peau'. Skin is an organ whose dense innervation involves the outermost skin layers, stratum corneum excepted. Long time neglected, the relation between skin and the nervous system are, today, a growing field of research within cutaneous biology.
Misery
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Service de Dermatologie, CHU Morvan, 5 avenue Foch, 29200 Brest, France.