Figure 1411

Electron micrograph of a Merkel s cell. The cell has small neurosecretory granules in the cytoplasm and makes contact with a peripheral terminal (NT) of a neuron. The dermis (D) is in the lower part of the micrograph. x 14,450. (Courtesy of Dr. Bryce L. Munger.)

epidermal cell, called a Merkel's corpuscle, is a sensitive mechano recep to r.

v structures of skin Nerve Supply

The skin is endowed with sensory receptors of various types that are peripheral terminals of sensory nerves (Fig. 14.12). It is also well supplied with motor nerve endings to the blood vessels, arrector pili muscles, and sweat glands.

Free nerve endings are the most numerous neuronal receptors in the epidermis

Free nerve endings in the epidermis terminate in the stratum granulosum. The endings are "free" in that they lack a connective tissue or Schwann cell investment. Such neuronal endings subserve multiple sensory modalities including fine touch, heat, and cold, without apparent morphologic distinction. Networks of free dermal endings surround most hair follicles and attach to their outer root sheath (Fig. 14.13). In this position they are particularly sensitive to hair movement and serve as mechanorecep-tors. This relationship imparts a sophisticated degree of specialization in the receptors that surround tactile hairs (vibrissae), such as the whiskers of a cat or rodent, in which each vibrissa has a specific representation in the cerebral cortex.

Other nerve endings in the skin are enclosed in a connective tissue capsule. Encapsulated neive endings include

• Pacinian corpuscles • Ruffini's corpuscles

• Meissner's corpuscles

Pacinian corpuscles are deep pressure receptors for mechanical and vibratory pressure

Pacinian corpuscles are large ovoid structures found in the deeper dermis and hypodermis (especially in the fingertips), in connective tissue in general, and in association with joints, periosteum, and internal organs. Pacinian corpuscles usually have macroscopic dimensions, measuring more than 1 mm along their long axis. They are composed of a myelinated nerve ending surrounded by a capsule structure (see Figs. 14.12 and 14.13a). The nerve enters the capsule at one pole with its myelin sheath intact. The myelin is retained for one or two nodes and is then lost. The unmyelinated portion of the axon extends toward the opposite pole from which it entered, and its length is covered by a series of tightly packed, flattened Schwann cell lamellae that form the inner core of the corpuscle. The remainder or bulk of the capsule, the outer core, is formed by a series of concentric lamellae; each lamella is separated from its neighbor by a narrow space containing lymph-like fluid. The appearance of the concentric lamellae as observed in the light microscope is reminiscent of the cut surface of a hemisected onion. Each lamella is composed of flattened cells that correspond to the cells of the endoneurium outside the capsule. In addition to fluid between the lamellae, collagen fibrils are present, although sparse, as well as occasional capillaries.

Pacinian corpuscles respond to pressure and vibration through the displacement of the capsule lamellae. This displacement effectively causes depolarization of the axon.

Meissner's corpuscles are localized within dermal papillae and serve as touch receptors

Meissner's corpuscles (see Figs. 14.12 and 14.13b) are touch receptors that are particularly responsive to low-frequency stimuli in the papillary layer of hairless skin, terminal disk of afferent axon Merkel's cell

free endings of afferent axon

capsule terminal branches of afferent axon terminal of afferent axon multilayered capsule dermal papilla

terminal branches of afferent axon capsule

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