Cutaneous nervous system

The nervous system is organised in central nervous system and peripheral nervous system. The central nervous system includes the brain, spinal cord and retina. The peripheral nervous system contains nerves through which information is circulating between the central nervous system and the rest of the body. The peripheral nervous system is functionally subdivided in, the somatic nervous system responsible for coordinating the body movements and also for receiving external stimuli and the autonomic nervous system that controls visceral functions (heart rate, respiration rate, perspiration, salivation, digestion, micturition, diameter of the pupils, and sexual arousal).The autonomic nervous system is classically divided into two subsystems: the parasympathetic nervous system and sympathetic nervous system. The nervous system of the skin is part of the peripheral nervous system.

The skin being one of the principal sites of host interaction with the environment, the cutaneous peripheral nervous system is constantly receiving and responding to various types of stimuli which can be either physical (thermal, mechanical, electrical, ultraviolet light), chemical, or indirect such as those produced by allergens, haptens, microbiological agents, trauma, or inflammation  . Cutaneous nerves can also respond to stimuli from the blood circulation and to emotions. Moreover, the central nervous system can modulate either directly via efferent autonomic nerves or CNS-derived mediators or indirectly, via the immune cells or the adrenal glands, a large number of functions within the skin including vasomotricity, thermoregulation  , pilo-erection, barrier function, gland and cell secretion, tissue growth and differentiation, and also, wound healing, immune response, and inflammation.

Cutaneous nerves have been initially classified as being either “efferent” controlling eccrine functions, blood flow and hair erection, or “afferent” sending informations about the external surrounding to the central nervous system . However, rapidly, it appeared that the cutaneous sensory nervous system not only relays sensory information but also plays an effector role in the skin functions by acting in an efferent neurosecretory fashion through their terminals.

 The cutaneous autonomic nervous system

In the skin, autonomic nerve fibers almost completely derive from sympathetic (cholinergic) and, in the face, rarely parasympathetic (also cholinergic) neurons. Autonomic nerve fibers constitute only a minority of cutaneous nerve fibers and are restricted to the dermis, innervating blood vessels, arteriovenous anastomoses, lymphatic vessels, erector pili muscles, eccrine glands, apocrine glands, and hair follicles.
The cutaneous autonomic nervous system plays a crucial part in regulating sweat gland function, vasomotricity, skin blood flow and thereby body temperature homeostasis. Acetylcholine is an important regulator of sweating but adult human sweat gland innervation coexpresses also all of the proteins required for full noradrenergic function (tyrosine hydroxylase, aromatic amino acid decarboxylase, dopamine b-hydroxylase, and the vesicular monoamine transporter VMAT2).

 The cutaneous sensory nervous system

The sensory function is mediated by a network of sensory myelinated or non-myelinated fibres, free terminal nerve endings and tactile corpuscles (Meissner, Pacini, and Ruffini corpuscules). Sensory nerves represent the majority of cutaneous nerves and innervate the three layers of the skin, epidermis  , dermis and hypodermis  . The sensory fibers can be grouped by conduction   velocity into three broad categories: myelinated Aβ and Ad, and unmyelinated C subtypes. Nociceptors and temperature receptors are primarily of the Ad and C subtypes, whereas Aβ fibers are mechanoreceptors which feels pressure, stretch, or hair movement.The Aβ fiber subclass can be further subdivided by the adaptation characteristics of the fibers: Slowly adapting type I (SAI) fibers innervate Merkel cell-neurite complexes, Slowly adapting type II (SAII) fibers are thought to innervate Ruffini corpuscles  , and rapidly adapting (RA) fibers innervate Meissner and Pacinian corpuscles  . Each of these subclasses is important for detecting a specific form of touch.

The distribution of receptors are regionally variable and specific with a high density in glabrous skin such as labia, areollae, and glans penis.
Sensory fibers transmit sensory signals (temperature and pH changes, chemicals, inflammatory mediators, pressure) via dorsal root ganglia and the spinal cord to specific areas of the CNS, resulting in the perception of burning, burning pain, pain, itching, or touch.

In addition, when stimulated, peripheral nerve endings induce neighboring afferent nerve fibers in the dermis and epidermis to release neuropeptides, a process known as “axon reflex”. It has been shown that epidermal nerves contain the tachykinins substance P (SP)   and neurokinin A   (NKA), calcitonin gene-related protein (CGRP  ), and dermal nerves contain SP, NKA, vasoactive intestinal peptide (VIP  ), and CGRP. Other skin cells (keratinocytes, Langerhans cells  , melanocytes, fibroblasts and endothelial cells), or infiltrating inflammatory cells are also able to produce neuropeptides.

 Bibliography

Boulais N, Misery L. The epidermis: a sensory tissue. Eur J Dermatol. 2008;18(2):119-27.