Porth's Essentials of Pathophysiology, 4e

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Somatosensory Function, Pain, and Headache

C h a p t e r 3 5

Hairy skin

Nonhairy skin

Merkel disk

Free nerve ending

Epidermis

Meissner corpuscle

FIGURE 35-6. Somatic sensory receptors in the skin. Hairy skin and nonhairy skin have a variety of sensory receptors. (Adapted from Bear MF, Connors BW, Paradiso MA. Neuroscience: Exploring the Brain. Baltimore, MD: Lippincott-Raven; 1996:311.)

Hair follicle receptor

Dermis

Pacinian corpuscle

Ruffini ending

by way of small myelinated fibers (i.e., type A δ ) with conduction velocities of 10 to 30 m/second. The sensory information for tactile sensation enters the spinal cord through the dorsal roots of the spinal nerves. All tactile sensation that requires rapid trans- mission is conducted through the discriminative path- way to the thalamus by way of the medial lemniscus. Tactile sensation also uses the more primitive and crude anterolateral pathway. Because of these multiple routes, total destruction of the anterolateral pathway seldom occurs. The only time this crude alternative system becomes essential is when the discriminative pathway is damaged. Then, despite projection of the anterolateral system information to the somatosensory cortex, only a poorly localized, high-threshold sense of touch remains. Such persons lose all sense of joint and muscle movement, body position, and two-point discrimination. Thermal Sensation Thermal sensation is discriminated by three types of receptors: cold, warmth, and pain. The cold and warmth receptors are located immediately under the skin at discrete but separate points, each serving an area of approximately 1 mm 2 . Different gradations of heat and cold reception result from the relative degrees of stimulation of the different types of nerve endings. Warmth receptors respond proportionately to increases in skin temperature above resting values of 34°C and cold receptors to temperatures below 34°C. 2 The ther- mal pain receptors are stimulated only by extremes of temperature such as “freezing cold” (temperatures below 5°C) and “burning hot” (temperatures above 45°C) sensations. 2 Thermal receptors respond rapidly to sudden changes in temperature and then adapt over the next few minutes. They do not adapt completely, how- ever, but continue to respond to steady states of tem- perature. For example, the sensation of heat one feels on entering a tub of hot water is the initial response to a change in temperature, followed by an adaptation in which one gets accustomed to the temperature change but still feels the heat because the receptors have not adapted completely.

Thermal afferents, with receptive thermal endings in the skin, send their central axons into the segmental dorsal horn of the spinal cord. On entering the dorsal horn, thermal signals are processed by second-order input association neurons. These association neurons activate projection neurons whose axons then cross to the opposite side of the cord and ascend in the mul- tisynaptic, slow-conducting anterolateral system to the opposite side of the brain. Thalamic and cortical somatosensory regions for temperature are mixed with those for tactile sensibility. Conduction of thermal information through periph- eral nerves is quite slow compared with the rapid con- duction of tactile sensation that travels through the discriminative system. If a person places a foot in a tub of hot water, the tactile sensation occurs well in advance of the burning sensation. The foot has been removed from the hot water by the local withdrawal reflex well before the excessive heat is perceived by the forebrain. Local anesthetic agents block the small- diameter afferents that carry thermal sensory informa- tion before they block the large-diameter axons that carry discriminative touch information. Position Sense Proprioception refers the sense or perception of limb and body movement and position without the use of vision. It is mediated by input from proprioceptive receptors (muscle spindle receptors and Golgi tendon organs) found primarily in muscles, tendons, and joint capsules (see Chapter 36). There are two submodali- ties of proprioception: the stationary or static com- ponent (limb position sense) and the dynamic aspects of position sense (kinesthesia). Both of these depend on constant transmission of information to the CNS regarding the degree of angulation of all joints and the rate of change in angulation. In addition, stretch- sensitive receptors in the skin (Ruffini end-organs, pacinian corpuscles, and Merkel cells) also signal pos- tural information. Signals from these receptors are processed through the dorsal column–medial lemnis- cus pathway. In addition, stretch-sensitive receptors in the skin (Ruffini end-organs, pacinian corpuscles, and