Porth's Essentials of Pathophysiology, 4e

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Nervous System

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Merkel cells) also signal postural information. Signals from these receptors travel through the discrimina- tive pathway and are processed in the thalamus before reaching the cerebral cortex. Lesions affecting the pos- terior column of the spinal cord impair position sense. The vestibular system (see Chapter 38) also plays an essential role in position sense.

experience. Despite its unpleasantness, pain can serve a useful purpose in that it warns of impending tissue injury, motivating the person to seek relief. For example, an inflamed appendix could progress in severity, rup- ture, and even cause death were it not for the warning afforded by pain. Pain is a common symptom that varies widely in intensity and spares no age group. It can be equally devastating for infants and children, young and middle- aged adults, and the elderly. Both acute pain and chronic pain can be major health problems. It is the most com- mon reason for visits to health care facilities. Acute pain often results from injury, surgery, or invasive medical procedures. It also can be a presenting symptom of infections, such as otitis media. Chronic pain can be symptomatic of a wide range of health problems includ- ing arthritis, back injury, and cancer. PainTheories Many theories, including the specific and pattern theo- ries, have been offered to explain the physiologic basis for the pain experience. The specificity theory regards pain as a separate sensory modality evoked by the activ- ity of specific receptors that transmit information by special nerve endings to pain centers or regions in the forebrain where pain is experienced. 5 Pattern theory proposes that pain receptors share endings or pathways with other sensory modalities, but that different patterns of activity (i.e., spatial or temporal) of the same neurons can be used to signal painful and nonpainful stimuli. 5 For example, light touch applied to the skin would pro- duce the sensation of touch through low-frequency fir- ing of the receptor; intense pressure would produce pain through high-frequency firing of the same receptor. Both of these theories focus on the neurophysiologic basis of pain, and aspects of both probably apply. Specific noci- ceptive afferents have been identified; however, almost all afferent stimuli, if driven at a very high frequency, can be experienced as painful. Gate control theory, a modification of specificity the- ory proposed by Melzack and Wall in 1965, postulated that the presence of a neural gating mechanism at the segmental spinal cord level could block projection of pain information to the brain. 6 According to this theory, internuncial neurons involved in the gating mechanism are activated by large-diameter, faster-propagating fibers that carry tactile information capable of blocking the transmission of impulses from small-diameter myelinated and unmyelinated pain fibers. Pain therapists have long known that pain intensity can be temporarily modified by the stimulation of other sensory fibers. For example, repeated sweeping of a soft-bristled brush on the skin (i.e., brushing) over or near a painful area may result in pain reduction for several minutes to several hours. Pain modulation is now known to be a much more complex phenomenon than that proposed by these the- ories. Tactile information is transmitted by small- and large-diameter fibers. Major interactions between sensory modalities, including the so-called gating phenomenon,

SUMMARY CONCEPTS

Pain Sensation Pain is an unpleasant sensory and emotional sensation associated with actual and potential tissue damage. 1–4 Unlike other somatic modalities, pain has an urgent and primitive quality, a quality responsible for the psycho- logical, social, cultural, and cognitive aspects of the pain three neurons to rapidly transmit information such as position sense and discriminative touch from sensory receptors to somatosensory cortex. The anterolateral pathway, which crosses within the first few segments of entering the spinal cord, consists of bilateral, multisynaptic, slow-conducting tracts that transmit information such pain, thermal sensation, crude touch, and pressure. ■■ The somatosensory component of the nervous system relays information about four major sensory modalities of touch, temperature, pain, and body position. ■■ Somatosensory information is sequentially transmitted over three types of neurons: first- order neurons, which transmit information from receptors in the sensory units of the system to dorsal horn neurons; second-order association neurons in the spinal cord, which communicate with various reflex circuits and transmit information to the thalamus, where it is roughly localized and perceived as a crude sense; and third-order neurons, which forward the information from the thalamus to the somatosensory cortex, where full localization, intensity discrimination, and interpretation occurs. ■■ The somatosensory system is organized segmentally into dermatomes, with each segment supplied by a single dorsal root ganglion that contains the neuronal cell bodies for the sensory units of the segment. ■■ There are two ascending pathways for transmission of somatosensory information: the discriminative and anteriolateral pathways.The discriminative pathway, which crosses at the medulla, uses only