Touch Receptors

The five major classes of touch receptors illustrated in Fig. 1 include three (Pacinian corpuscles, Merkel's disks, and Ruffini's corpuscles) that are common to both hairy and glabrous (nonhairy) skin. Hair follicle receptors are unique to hairy skin, and Meissner's corpuscles are found primarily in glabrous skin. All of these types of somato-sensory mechanoreceptors have fibers designated as type II, indicating that they have a similar diameter, a similar degree of myelination, and a similar conduction rate.

Pacinian corpuscles and Ruffini's corpuscles are found deep within the dermal layers. Both have relatively large, specialized end organs that are widely spaced. These attributes permit each sensory fiber to respond to stimuli over a wide expanse of skin, thus producing a relatively large receptive field for each cell. A difference is noted, however, in the ability of these two receptor types to sustain their responses to long-lasting stimuli. Pacinian corpuscles respond rapidly to a change in pressure on the skin, then quickly adapt and stop firing even though the stimulus continues. Ruffini's endings are relatively slow to adapt to a sustained stimulus and continue to respond during long stimulus periods.

These different adaptation characteristics reflect the cellular organization of the end organ rather than electrophysiological properties of the individual nerves. An experimental demonstration of this fact is illustrated in Fig. 3. The end organ of the Pacinian corpuscle is encapsulated by 20 to 70 concentric layers of connective tissue arranged like layers of an onion. In response to

A. Hairy skin

A. Hairy skin

stratum corneum epidermis dermis

Hair follicle receptor

Pacinian corpuscle

Hair follicle receptor

49. Sensory Receptors of the Somatosensory System B. Glabrous skin stratum corneum epidermis dermis

Pacinian corpuscle

stratum corneum epidermis C fibers dermis

Pacinian corpuscle NAD

stratum corneum epidermis C fibers dermis

Pacinian corpuscle NAD

II (AP) neurons 6 - 12 |xm diameter 35 - 75 m/sec conduction rate

Pacinian corpuscle Hair follicle Merkel's disk Ruffini's corpuscle

RA pressure, vibration (300 Hz)

RA touch, vibration ( 40 Hz)

SA no feeling

SA touch & pressure

III (A8) neurons 1 - 5 |im diameter 5 - 30 m/sec conduction rate

II (AP) neurons 6 - 12 |im diameter 35 - 75 m/sec conduction rate

Pacinian corpuscle RA pressure, vibration (300 Hz)

Meissner's corpuscle RA touch, vibration ( 40 Hz)

Merkel's disc SA touch

Ruffini's corpuscle SA touch & pressure

III (A8) neurons 1 - 5 |im diameter 5 - 30 m/sec conduction rate

Thermo receptors Nociceptors

SA cooling SA pricking pain & tickle

IV (C) neurons 0.2 - 1.5 |im diameter 0.5 - 2 m/sec conduction rate

Thermo receptors Nociceptors

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