CUTANEOUS AND SUBCUTANEOUS SENSE ORGANS A Iggo CUTANEOUS AND SUBCUTANEOUS SENSE ORGANS

The skin has a rich afferent nerve supply, but the density of innervation varies greatly from one region to another. The face, like the skin of the extremities, is richly innervated, whereas the posterior aspect of the trunk is much more thinly supplied by nerve fibres. The surfaces of the skin actively concerned with tactile exploration, such as palmar surface of the hands and feet, have a well-organized structure of ridges and a rich sensory supply, in contrast to the more uniform and less highly differentiated skin surface covering the greater part of the body and which contains hairs. The latter, however, act as sensory structures. Most of the sensory nerve-endings in the skin are found in association with, but not necessarily in, the epidermis, although some are in the deeper layers of the skin—the dermis—and in the subcutaneous tissues. The cutaneous receptors and their afferent nerve fibres provide the only direct spatially organized pathway for transmission, to the central nervous system (CNS), of informa-tion about the nature of events occurring in, or on, the skin. In addition, the nerve fibres provide a channel of chemical communication between the CNS and the skin, and vice versa; this is important in the trophic actions that maintain the normal structure and integrity of the skin. 1 Morphology of Sense Organs The existence and structure of cutaneous sense organs have been of interest to morphologists for at least 150 years. With the advent of electron microscopy and improved methods for the histological preparation of tissues many of the earlier uncertainties were resolved. One remaining difficulty is that there are many minor variations in structure, and some investigators conclude that no individual or specific kinds of structure exist. The resolution of this problem has in part depended on the combined use of electrophysiological and morphological experiments to study the sense organs (e.g