Touch, Organs of

special receptors embedded in the skin, in the locomotor system, which includes the muscles, tendons, and joints, and in some mucous membranes, such as those on the lips and tongue.

The organism uses organs of touch to sense the complex action of mechanical, temperature, and pain factors. The tactile organs in the skin are distributed unevenly. In man, for example, they are particularly numerous in the fingers, palms, soles, lips, and genitalia; hence the high sensitivity of these skin areas. The commonest type of tactile organ is the free nerve endings; these are connected to nonmyelinized fibers, which constitute about 80 percent of the skin afferent fibers, and to thin and medium-sized medullated fibers usually less than 6 μm in diameter. The free nerve endings are heavily branched in the tissues so that a single nerve fiber can innervate a large area; in the cornea, for example, about 0.5 cm². The innervated areas, or receptive fields, of the individual nerve fibers usually overlap one another considerably. In hair-covered skin, which constitutes 90 percent of the skin surface, the nerve endings around the root sheaths of the hairs are very numerous. Also richly innervated are the special sensory hairs, the vibrissae, which are usually situated on the muzzle; in climbing animals they are found on the belly as well (man does not have vibrissae). The receptors of the hair follicle are connected to medullated nerve fibers; each hair is innervated by several fibers, and the same fiber can innervate several hairs.

Organs of touch also include various types of encapsulated receptors (dentritic extremities enclosed in special cellular capsules). Among these receptors are Pacini’s corpuscles, Meis-sner’s corpuscles, Golgi-Mazzoni corpuscles, Ruffini’s corpuscles, the bulbs of Krause, and Merkel’s corpuscles. The most finely differentiated encapsulated receptors (Pacini’s and Meis-sner’s corpuscles) are connected to thick (4–13 μm) medullated afferent fibers and thin nonmedullated efferent fibers. The afferent fibers that innervate Meissner’s corpuscles can also terminate simultaneously in Merkel’s corpuscles.

Also participating in the tactile processes are the specialized receptors in the muscles (the muscle spindles) and in the tendons, joints, and fasciae. The afferent fibers of the muscle receptors are the thickest, reaching 20 μm, and therefore the most rapidly conducting sensory fibers in the body. Some organs of touch have a highly specific characters; for example, Pacini’s, Meissner’s, and Merkel’s corpuscles are highly specialized mechanoreceptors. Other organs of touch, such as the free nerve endings, can perceive a great variety of stimuli. The many types of tactile organs and the distinguishing features of their spatial and temporal excitation contribute to the variety of tactile sensations.