Utaneous Sensations

There are several different types of sensory receptors in the skin, each of which is specialized to be maximally sensitive to one modality of sensation. A receptor will be activated when a given area of the skin is stimulated; this area is the receptive field of that receptor A process known as lateral inhibition helps to sharpen the perceived location of the stimulus on the skin.

The cutaneous sensations of touch, pressure, heat and cold, and pain are mediated by the dendritic nerve endings of different sensory neurons. The receptors for heat, cold, and pain are simply the naked endings of sensory neurons. Sensations of touch are mediated by naked dendritic endings surrounding hair follicles and by expanded dendritic endings, called Ruffini endings and Merkel's discs. The sensations of touch and pressure are also mediated by dendrites that are encapsulated within various structures (table 10.2); these include Meissner's corpuscles and pacinian (lamellated) corpuscles. In pacinian corpuscles, for example, the dendritic endings are encased within thirty to fifty onionlike layers of connective tissue (fig. 10.4). These layers absorb some of the pressure when a stimulus is maintained, which helps to accentuate the phasic response of this receptor. The encapsulated touch receptors thus adapt rapidly, in contrast to the more slowly adapting Ruffini endings and Merkel's discs.

There are far more free dendritic endings that respond to cold than to warm. The receptors for cold are located in the upper region of the dermis, just below the epidermis. These receptors are stimulated by cooling and inhibited by warming. The warm receptors are located somewhat deeper in the dermis and are excited by warming and inhibited by cooling. Nociceptors are also free sensory nerve endings of either myelinated or un-myelinated fibers. The initial sharp sensation of pain, as from a pin-prick, is transmitted by rapidly conducting myelinated axons, whereas a dull, persistent ache is transmitted by slower conducting unmyelinated axons. These afferent neurons synapse in the spinal cord, using substance P (an eleven-amino-acid polypeptide) and glutamate as neurotransmitters.

Hot temperatures produce sensations of pain through the action of a particular membrane protein in sensory dendrites. This protein, called a capsaicin receptor, serves as both an ion channel and a receptor for capsaicin—the molecule in chili peppers that causes sensations of heat and pain. In response to a noxiously high temperature, or to capsaicin in chili peppers, these ion channels open. This allows Ca2+ and Na+ to diffuse into the neuron, producing depolarization and resulting action potentials that are transmitted to the CNS and perceived as heat and pain.

While the capsaicin receptor for pain is activated by intense heat, other nociceptors may be activated by mechanical stimuli that cause cellular damage. There is evidence that ATP released from damaged cells can cause pain, as can a local fall in pH produced during infection and inflammation.

1 Fox: Human Physiology, Eighth Edition

1 10. Sensory Physiology 1 Text

© The McGraw-Hill Companies, 2003

Sensory Physiology

245

Table 10.2 Cutaneous Receptors

Receptor Structure

Sensation

Location

Free nerve endings

Merkel's discs

Ruffini corpuscles (endings)

Meissner's corpuscles Pacinian corpuscles

Unmyelinated dendrites of sensory neurons Expanded dendritic endings Enlarged dendritic endings with open, elongated capsule Dendrites encapsulated in connective tissue Dendrites encapsulated by concentric lamellae of connective tissue structures

Light touch; hot; cold; nociception (pain) Sustained touch and pressure Sustained pressure

Changes in texture; slow vibrations Deep pressure; fast vibrations

Around hair follicles; throughout skin Base of epidermis (stratum basale) Deep in dermis and hypodermis

Upper dermis (papillary layer) Deep in dermis

Merkels's discs

Merkels's discs

Root hair plexus

Meissner's corpuscle

Free nerve ending

Pacinian corpuscle

Ruffini endings

Root hair plexus

Meissner's corpuscle

Free nerve ending

Pacinian corpuscle

Ruffini endings

■ Figure 10.4 The cutaneous sensory receptors. Each of these structures is associated with a sensory (afferent) neuron. Free nerve endings are naked, dendritic branches that serve a variety of cutaneous sensations, including that of heat. Some cutaneous receptors are dendritic branches encapsulated within associated structures. Examples of this type include the pacinian (lamellated) corpuscles, which provide a sense of deep pressure, and the Meissner's corpuscles, which provide cutaneous information related to changes in texture.

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