Embryologic, Histologic, and Anatomic Aspects
Both somatic sensory and autonomic motor nerves are present in skin (Fig. 1.82). The somatic sensory system mediates sensations of pain, itch, temperature, light touch, discriminative touch, pressure, vibration, and proprioception. Nerves of the autonomic motor system control cutaneous vascular tone, pilomotor responses, and sweating. The somatic sensory and autonomic motor parts of the peripheral nervous system are derived mostly from the neural crest, which gives rise to dorsal root and sympathetic ganglia, and to their nerve processes, Schwann cells, endoneural sheath cells, and laminar cells of specialized sensory end organs. Nerves in the reticular dermis travel along the course of the superficial and deep vascular plexuses, whence derives the term neurovascular plexuses.
Unmyelinated cutaneous nerve endings transmit sensations of touch, pressure, temperature, pain, and itch to the central nervous system via dorsal root ganglia. All motor fibers in skin are supplied by the autonomic nervous system. Adrenergic fibers activate arterioles, glomus bodies, muscles of hair erection, and apocrine glands; cholinergic fibers stimulate eccrine glands.
Cutaneous sensory receptors can be divided into two groups: One is of “specialized” receptors (“end organs”) that, viewed by conventional microscopy, are observed to possess terminal nerve endings surrounded by lamellar condensations of connective tissue and Schwann cells, e.g., Pacini’s corpuscles, Meissner’s corpuscles, and mucocutaneous end organs, each of these being distinctive morphologically; the other is of “unspecialized” receptors devoid of distinctive structural features. In the latter group are most of the sensory nerves that supply the skin, including those linked to Merkel cells.
Pacini’s corpuscles are located chiefly on weight-bearing surfaces in the deep portion of the dermis and in the subcutaneous fat (Fig. 1.83). They also populate the oral lips, penis, clitoris, and nipples. Pacini’s corpuscles are mechanoreceptors designed to detect pressure. Each corpuscle is supplied by a myelinated axon that makes several turns before entering a capsule composed of concentrically layered connective tissue that, in turn, envelops a sensory terminal. The concentric lamellae consist of flattened cells (“laminar cells”) that probably are modified Schwann cells. In the center of each corpuscle, an unmyelinated, bulbous, terminal nerve ending is in direct contact with the innermost laminar cells. The lamellar organization of a Pacini’s corpuscle enables it to act as a primary receptor that, when its architecture is distorted, gives rise to action potentials, which may be amplified or modulated by the concentric arrangement, in parallel, of laminar cells.
A&B. Pacini’s corpuscles that mediate sensation of pressure are pictured in the subcutaneous fat at a site near volar skin. (From AB Ackerman, H Jacubovic. In: Moschella SL, Hurley HJ, eds. Dermatology, 3rd ed. Philadelphia: W.B. Saunders, 1992.)
Meissner’s corpuscles (Fig. 1.84), lodged at the tip of dermal papillae on volar skin and most plentiful on fingertips, are thought to function as mechanoreceptors that adapt rapidly to the sensation of light touch. Each corpuscle consists of layers of flattened laminar cells, those presumably being modified Schwann cells, among which ramify axon terminals derived from myelinated nerves. Typically, the axons weave back and forth between stacks of lamellae. When viewed by electron microscopy, laminar cells are noted to possess a basal lamina and, at the periphery of a corpuscle, to be separated from one another by collagen fibers, elastic fibers, and fibrillary intercellular material.
A Meissner’s corpuscle that mediates the sensation of touch is found at the tip of a dermal papilla on volar skin. (From AB Ackerman, H Jacubovic. In: Moschella SL, Hurley HJ, eds. Dermatology, 3rd ed. Philadelphia: W.B. Saunders, 1992.)
Mucocutaneous end organs are positioned in the subpapillary connective tissue of the glans penis, prepuce, clitoris, labia minora, perianal region, eyelids, and lips. They are comparable to Meissner’s corpuscles and act as touch receptors. In contrast to Pacini’s corpuscles and Meissner’s corpuscles, however, mucocutaneous end organs cannot be visualized in sections prepared conventionally and stained by hematoxylin and eosin. The structures, on exposure to silver stains, are seen to consist of loops of loosely wound, branching axons that form oval masses.
The millions of “unspecialized” sensory receptors that supply the skin may be divided conveniently into those that are associated with follicles and those that are not. To visualize those fine nerve endings by conventional microscopy, it is necessary to employ specialized stains that utilize silver salts, methylene blue, or cholinesterase. Hair follicles are supplied by myelinated sensory nerves that branch extensively (Fig. 1.85). One myelinated axon may serve numerous follicles, and each follicle may be supplied by several different axons. These sensory endings are most plentiful in the region of the isthmus, where axons and processes of Schwann cells, arranged in a palisade, encircle a follicle.
An end organ of a vellus hair follicle is demonstrated well by silver impregnation after treatment with cholinesterase. (Courtesy of William Montagna, Ph.D.)
Unmyelinated axons surrounded by Schwann cells are arrayed in a branching and overlapping network. Many of the unmyelinated axons terminate near the dermoepidermal junction, where basal lamina of their Schwann cells make contact with the basal lamina of the epidermis. Others extend into the epidermis. Free nerve endings are abundant particularly in the glans penis, where they reside in almost every dermal papilla, as well as being scattered throughout the lower part of the dermis.
The most vexing of all cutaneous symptoms is pruritus, defined as a sensation that evokes a desire to scratch. The mechanisms responsible for pruritus are not well understood. Itching is thought to be either a subthreshold sensation of pain or a particular sensory quality transmitted from a network of nerve endings situated near the dermoepidermal junction. Although sensations of pain and itch travel along the same nerve pathways and have stimuli in common, they seem to be different from one another. Many inflammatory diseases of the skin are associated with itching; among the most notorious of these are dermatitis herpetiformis, Grover’s disease, and scabies. Scratching is thought to relieve itching by disturbing the rhythm of afferent impulses that travel toward the central nervous system. The frequency of nerve impulses associated with itching is lower than that with painful stimuli; by scratching or rubbing vigorously, a patient with pruritus may convert slower, maddening nerve impulses to faster more tolerable ones. Scratching and rubbing leave specific signs in the skin that are detectable easily by conventional microscopy, the former being seen as ragged necrosis of keratocytes in the upper part of the epidermis, small discrete erosions or ulcerations, and deposits of fibrin or a locus of fibrosis immediately beneath the epidermis, and the latter being lichen simplex chronicus, prurigo nodularis, and picker’s nodule, which share findings of compact orthokeratosis, hypergranulosis in conjunction with a thickened surface epidermis, pseudocarcinomatous (infundibular and eccrine ductal) proliferation, and coarse bundles of collagen in vertical streaks in a thickened papillary dermis.