Cutaneous Receptors

Three types of receptors are common to glabrous and hairy skin: pacinian (lamellated) corpuscles, Merkel disks, and free nerve endings. The pacinian corpuscle has been identified as a quickly adapting mechanoreceptor, and its mechanical transduction process has been extensively studied. The primary role of pacinian corpuscles appears to be the sensing of brief touch or vibration.

Merkel disks are slowly adapting mechanoreceptors structured to respond to maintaining deformation of the skin surface. Typically, one afferent fiber of largeto-medium diameter branches to form a cluster of Merkel disks situated at the base of a thickened region of epidermis. Each nerve terminal branch ends in a disk enclosed by a specialized accessory cell (Merkel cell). The distal surface of the Merkel cell is held to nearby epidermal cells by cytoplasmic protrusions and desmosomes, while the base of the cell is embedded in the underlying dermis. Thus movement of the epidermis relative to the dermis will exert a shearing force on the Merkel cell. The Merkel cell also contains numerous granulated vesicles, which suggests that some form of chemical synaptic transmission may occur, although attempts to demonstrate this have failed. Direct mechanical transduction by the nerve ending has not been ruled out as a possibility. However, whatever the transduction mechanism, the Merkel-cell/Merkel-disk ending appears to play a role in the sensing of both touch and pressure.

The so-called free nerve ending is made up of a branching nerve axon, which is entirely or partially surrounded by Schwann cells. The axon/Schwann-cell complex is further surrounded by a basement membrane. Free nerve endings originate from fine myelinated or unmyelinated fibers that branch extensively in the dermis and may penetrate into the epidermis. These endings respond to strong mechanical and thermal stimuli, and they are particularly activated by painful stimuli.

The other receptors found in glabrous skin are Meissner corpuscles (tactile corpuscles), in which the terminal branches of a myelinated axon intertwine in a basketlike array of accessory cells, and Krause end bulbs, in which a fine myelinated fiber forms a club-shaped ending. Meissner corpuscles have been tentatively identified as quickly adapting mechanoreceptors subserving the sense of touch, whereas Krause end bulbs may be thermoreceptors.

The most important receptors in hairy skin are the hair follicle endings, in which axon terminals of sensory nerve fibers wrap themselves around a hair follicle. These endings are quickly adapting mechanoreceptors that provide information about any force applied to the hair and, thus, to the skin. Hairy skin also contains the spraylike Ruffini terminals, which may be involved in the sensing of steady pressure applied to hairy skin.

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