[Structure and functions of the cutaneous nervous system].

Cutaneous nerves are either somatic sensory afferents to ganglion neurons or terminal efferents from the autonomous nervous system. Myelinated somatic nerves form subcutaneous and intradermal plexus from which arise various types of endings: free, dilated and included in corpuscles. Free terminals are ramified in the dermis, epidermis and the epithelial sheet of hairs. Dilated terminals are lanceolate fibres around hairs and those in close contact with Merkel cells. Well identified corpuscles are those of Meissner, Ruffini and Pacini. Physiologically, nerves are involved as mechano-, thermo- and pain receptors. Five mechanoreceptors are recognized according to their rate (slow/fast) of adaptation and their receptive field (type I, small; type II, large): Merkel cells (SA I), Ruffini corpuscles (SA II), Meissner corpuscles (FA I), Pacini corpuscles (FA II) and palisades of lanceolate nerve endings (FA with various field units). Thermo- and nociceptors are mainly thin unmyelinated free nerve endings with various rates of conduction velocity. The autonomic nervous system supplies sympathetic innervation appearing as unmyelinated small nerve endings in close contact with skin appendages, except sebaceous glands. Only sweat glands are innervated by cholinergic terminals.