2. Somatosensory Systems
Part 4 of 5

Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).

Somatosensory Receptor Types 

Cutaneous Receptors

Some of the somatosensory receptors in skin (i.e., the cutaneous receptors) are classified as encapsulated receptors as the 1° afferent terminal and surrounding cutaneous tissue are encapsulated by a thin sheath (Table II). The encapsulated cutaneous receptors include Meissner corpuscles, Pacinian corpuscles and Ruffini corpuscles (See Figure 2.11). Other cutaneous receptors are unencapsulated and include the hair follicle receptor (the 1° afferent ends on hair follicles) and the Merkel complex (the 1° afferent ends at the base of a specialized receptor cell called the Merkel cell). The sensory receptors of the crude touch, pain and temperature senses are bare or free nerve endings. That is, they are unencapsulated, do not end on or near specialized tissue, and may be mechanoreceptors, nociceptors or thermoreceptors. 

As was noted earlier, the sensitivity (modality specificity) of the somatosensory receptor is determined by its location and by the structure of the non-neural tissue surrounding the 1° afferent terminal. The following describes the most commonly observed cutaneous receptors. 

Meissner Corpuscle. The Meissner corpuscle is found in glabrous (i.e., hairless) skin, within the dermal papillae (Figure 2.11). It consists of an elongated, encapsulated stack of flattened epithelial (laminar) cells with 1° afferent terminal fibers interdigitated between the cells (Figure 2.12).

A force applied to non-hairy skin (Figure 2.13) causes the laminar cells in the Meissner corpuscle to slide past one another, which distorts the membranes of the axon terminals located between these cells. If the force is maintained, the laminar cells remain in a fixed, albeit, displaced position, and the shearing force on the axon terminals' membranes disappears. Consequently, the 1° afferent axons produce a transient, rapidly adapting response to a sustained mechanical stimulus.

The Meissner 1° afferent discharges "follow" low frequency vibrating (30 -50 Hz) stimuli, which produces the sensation of "flutter" (Figure 2.10, left). Because a single 1° afferent axon forms many, dispersed (3-4 mm) Meissner corpuscles, the 1° afferent can detect and signal small movements across the skin. Stimulation of a sequence of Meissner corpuscles have been described to produce the perception of localized movement along the skin.

Consequently, Meissner corpuscles are considered to be the discriminative touch system's flutter and movement detecting receptors in non-hairy skin.

Pacinian Corpuscle. Pacinian corpuscles are found in subcutaneous tissue beneath the dermis (Figure 2.9) and in the connectivetissues of bone, the body wall and body cavity. Therefore, they can be cutaneous, proprioceptive or visceral receptors, depending on their location.

The Pacinian corpuscle is football-shaped, encapsulated, and contains concentrically layered epithelial (laminar) cells (Figure 2.14). In cross section, the Pacinian corpuscle looks like a slice of onion, with a single 1° afferent terminal fiber located in its center. The outer layers of laminar cells contain fluid that is displaced when a force is applied on the corpuscle.

When a force is first applied on the Pacinian corpuscle (Figure 2.15), it initially displaces the laminar cells and distorts the axon terminal membrane. If the external pressure is maintained on the corpuscle, the displacement of fluid in the outer laminar cells dissipates the applied force on the axon terminal. Consequently, a sustained force on the corpuscle is transformed into a transient force on the axon terminal, and the Pacinian corpuscle 1° afferent produces a fast adapting response.

Pacinian corpuscles 1° afferent axons are most sensitive to vibrating stimuli (e.g., a tuning fork vibrating at 100 to 300 Hz, Figure 2.10, left) and unresponsive to steady pressure. The sensation elicited when cutaneous Pacinian corpuscles are stimulated is of vibration or tickle.

Pacinian corpuscles in skin are considered to be the vibration sensitive receptors of the discriminative touch system.

Ruffini Corpuscle. The Ruffini corpuscles are found deep in the skin (Figure 2.11), as well as in joint ligaments and joint capsules and can function as cutaneous or proprioceptive receptors depending on their location. The Ruffini corpuscle (Figure 2.16) is cigar-shaped, encapsulated, and contains longitudinal strands of collagenous fibers that are continuous with the connective tissue of the skin or joint. Within the capsule, the 1° afferent fiber branches repeatedly and its branches are intertwined with the encapsulated collagenous fibers.

The Ruffini corpuscles are oriented with their long axes parallel to the surface of the skin and are most sensitive to skin stretch. Stretching the skin (Figure 2.17) stretches the collagen fibers within the Ruffini corpuscle, which compresses the axon terminals. As the collagen fibers remain stretched and the axon terminals remain compressed during the skin stretch, the Ruffini corpuscle's 1° afferent axon produces a sustained slowly adapting discharge to maintained stimuli.

Figure 2.17

When the skin is stretched (press), the collagen fibers in the Ruffini corpuscles are also stretched and compress their 1° afferent terminals. As the collagen fibers remain stretched and the axon terminals remain compressed during the skin stretch, the Ruffini corpuscle 1° afferent axon produces a sustained generator potential and a slowly adapting discharge to maintained stimuli.

Ruffini corpuscles in skin are considered to be skin stretch sensitive receptors of the discriminative touch system. They also work with the proprioceptors in joints and muscles to indicate the position and movement of body parts.

Hair Follicle.The hair follicle receptor is an unencapsulated cutaneous receptor (Figure 2.10). The 1° afferent terminal axons spiral around the hair follicle base or run parallel to the hair shaft forming a lattice-like pattern (Figure 2.18).

Most hair follicle 1° afferents are the fast-adapting type; displacement of the hair produces a transient discharge of action potentials at the onset of the displacement and a maintained displacement of the hair often fails to produce a sustained discharge (Figure 2.19). The hair follicle afferents respond best to moving objects and signal the direction and velocity of the movement of a stimulus brushing against hairy skin.

Figure 2.19

Bending a hair (press ) produces a transient force on the hair follicle base as the entire follicle is displaced by the bending force. The 1° afferent terminal may produce a rapidly adapting generator potential and the 1° afferent axon a transient discharge of action potentials — even to sustained bending of the hair.

As Meissner corpuscles are absent from hairy skin, the hair follicle endings are considered to be the discriminative touch system's movement sensitive receptors in hairy skin.

Merkel Complex. The Merkel complex is found in both hairy and non-hairy skin and is located in the basal layer of the epidermis (Figure 2.11). The Merkel complex is unencapsulated and consists of a specialized receptor cell, the Merkel cell, and a 1° afferent terminal ending, the Merkel disk³ (Figure 2.20). Thick, short, finger-like protrusions of the Merkel cell couple it tightly to the surrounding tissue. The Merkel cell is a modified epithelial cell, which contains synaptic vesicles that appear to release neuropeptides that modulate the activity of the 1° afferent terminal. Each 1° afferent axon often innervates only a few Merkel cells in a discrete patch of skin (Figure 2.18).

A force applied to the skin overlying the Merkel cell distorts it (Figure 2.21), which stimulates its release of a neuropeptide at its synaptic junctions with the Merkel disk. As the Merkel cell is mechanically coupled to the surrounding skin, it remains distorted for the duration of the force applied on the overlying skin. Consequently, the Merkel complex 1° afferent axon responds to small forces applied to a discrete patch of skin with a slowly adapting, sustained discharge

Merkel cells are considered to be the fine tactile receptors of the discriminative touch system that provide cues used to localize tactile stimuli and to perceive the edges (shape or form) of objects. 

Free Nerve Endings. Free nerve endings are found throughout the body, in skin (Figure 2.11), muscles, tendons, joints, mucous membranes, cornea, body mesentery, the dura, the viscera, etc. The free nerve endings in skin are stimulated by tissue-damaging (nociceptive) stimuli that produce the sensation of pain or by cooling of the skin or the warming of skin or by touch. Notice that although all cutaneous free nerve endings appear very similar morphologically, there are different functional types of free nerve endings, with each responding to specific types of cutaneous stimuli (e.g., nociceptive, cooling, warming or touch). 

Free nerve endings are considered to be the somatosensory receptors for pain, temperature and crude touch.

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