Porth's Essentials of Pathophysiology, 4e

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Nervous System

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Central Processing of Somatosensory Information

between different intensities of sensory stimulation. Although sensory receptors can respond to many forms of sensory information at high levels, they are highly sensitive to low levels of a particular type of sensation. For example, a receptor may be particularly sensitive to a small increase in local skin temperature, yet stimu- lation with strong pressure also can result in receptor stimulation. Cool versus warm, sharp versus dull pain, and delicate touch versus deep pressure are all based on different populations of afferent neurons or on central integration of simultaneous input from several differ- ently tuned afferents. Tactile Sensation The tactile system, which relays sensory information regarding touch, pressure, and vibration, is considered the basic somatosensory system. Loss of temperature or pain sensitivity leaves the person with no awareness of deficiency. If the tactile system is lost, however, total anesthesia (i.e., numbness) of the involved body part results. Touch sensation results from stimulation of tactile receptors in the skin and in tissues immediately beneath the skin, pressure from deformation of deeper tissues, and vibration from rapidly repetitive sensory signals. There are at least six types of specialized tactile receptors in the skin and deeper structures: free nerve endings, Meissner corpuscles, Merkel disks, pacinian corpuscles, hair fol- licle end-organs, and Ruffini end-organs 1,2 (Fig. 35-6). Free nerve endings are found in skin and many other tissues, including the cornea. They detect touch and pressure. Meissner corpuscles are elongated, encapsu- lated nerve endings present in nonhairy parts of the skin. They are particularly abundant in the fingertips, lips, and other areas where the sense of touch is highly devel- oped. Merkel disks are dome-shaped receptors found in nonhairy and hairy parts of the skin. In contrast to Meissner corpuscles, which adapt within a fraction of a second, Merkel disks transmit an initial strong signal that diminishes in strength but is slow in adapting. They are responsible for giving steady-state signals that allow for continuous sense of touch against the skin. Pacinian corpuscles are located immediately beneath the skin and deep in the fascial tissues. They are stimu- lated by rapidmovements of the tissues and are important in detecting tissue vibration. The hair follicle end-organs consist of afferent unmyelinated fibers entwined around most of the length of the hair follicle. These receptors, which are rapidly adapting, detect movement on the surface of the body. Ruffini end-organs are found in the skin and deeper structures, including the joint capsules. These receptors, which have multibranched encapsu- lated endings, have very little adaptive capacity and are important for signaling continuous states of deforma- tion, such as heavy and continuous touch and pressure. Almost all the specialized touch receptors, with the exception of free nerve endings, transmit their signals through large myelinated nerve fibers (i.e., types A α , A β ) that have transmission velocities ranging from 25 to 70 m/second. Most free nerve endings transmit signals

Perception, or the final processing of somatosensory information, involves awareness of the stimuli, local- ization and discrimination of their characteristics, and interpretation of their meaning. 1,2 As sensory informa- tion reaches the thalamus, it begins to enter the level of consciousness, is roughly localized, and is perceived as a crude sense. The full localization, discrimination of the intensity, and interpretation of the meaning of the stimuli require processing by the somatosensory cortex. The somatosensory cortex is located in the parietal lobe, which lies behind the central sulcus and above the lateral sulcus (Fig. 35-5). The strip of parietal cortex that borders the central sulcus is called the primary somato- sensory cortex because it receives primary sensory infor- mation by direct projections from the thalamus. Parallel to and just behind the primary somatosensory cortex (i.e., toward the occipital cortex) lies the somatosensory association area, which is required to transform the raw sensory information into a meaningful learned percep- tion. Most of the perceptive aspects of body sensation, or somesthesia, require the function of this association area. The perceptive aspect, or meaningfulness, of a stimulus pattern—such as the perception of sitting on a soft chair rather than on a hard bicycle seat—involves the integration of present sensation with past learning. Somatosensory Modalities Earlier, we noted that somatosensory experience can be divided into modalities, qualitative distinctions between the sensations of touch, temperature, position, and pain. 1,2 Somatosensory experience also involves quanti- tative discrimination; that is, the ability to distinguish

Central sulcus

Primary sensory cortex

Somatosensory association area

Lateral sulcus FIGURE 35-5. Primary somatosensory cortex and somatosensory association area.