Chapter 13 Summary
1. The sense of touch produces a number of distinct sensory experiences. Each type of experience is mediated by
sensory receptor system(s). Touch receptors are responsive not only to pressure, but also to vibration, changes in temperature, and noxious stimulation. The
system, which also contributes to our sense of touch, is further involved in sensing limb position and the movement of our limbs in space. Pleasant or emotional touch is another form of sensory specialization.
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2. Four classes of pressure-sensitive (
-) receptors have been found within hairless skin, and another five classes within hairy skin. The organs used to sense limb position and movement (namely, our muscles, tendons, and joints) are more deeply situated within the body. Thermoreceptors respond to changes in skin temperature that occur, for example, when we contact objects that are warmer or cooler than our bodies.
signal tissue damage (or its potential) and give rise to sensations of pain.
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3. The pathways from touch receptors to the brain are complex. Two major pathways have been identified: a fast one (the dorsal column–medial lemniscal pathway) that carries information from mechanoreceptors, and a slower one (the spinothalamic pathway) that carries thermal and nociceptive information. Both enter the dorsal horn of the spinal cord, which itself has dense neural connectivity. The pathways project to the thalamus and from there to the primary somatosensory area, located in the
lobe just behind the central sulcus. This area contains several somatotopically organized subregions, in which adjacent areas of the body project to adjacent areas of the brain. The neural organization of the brain for touch has been shown to be remarkably plastic, even in adults.
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4. Downward pathways from the brain play an important role in the perception of pain. According to the
control theory, signals along these pathways interact at the spinal cord with those from the periphery of the body. Such interactions can block the pain signals that would otherwise be sent forward to the brain. The sensation of pain is further moderated by areas in the cortex.
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5. Investigators have measured sensitivity to mechanical force by applying nylon hairs of different diameters to the skin. They determine spatial acuity of the skin by measuring the
-point touch threshold, and more precisely by discriminating the orientation of gratings applied to the skin. Tactile pressure sensitivity and spatial acuity
body site because of varying concentrations of different types of mechanoreceptors. The minimum depression of the skin needed to feel a stimulus vibrating at a particular rate (frequency) provides a measure of vibration sensitivity.
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6. The sense of touch is intimately related to our ability to perform actions. Signals from the mechanoreceptors are necessary for simple actions such as grasping and lifting an object. Conversely, our own movements determine how touch receptors respond and, hence, which properties of the concrete world we can feel. Touch is better adapted to feeling the
of objects than it is to feeling their
, particularly when an object is large enough to extend beyond the fingertip.
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7. Like other sensory modalities, touch gives rise to internal representations of the world, which convey the positions of objects using the
as a spatial reference system. Touch-derived representations are inputs to higher-level functions like allocation of attention and integration with information from other modalities.
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8. The psychological study of touch is useful for a number of applications. Virtual touch environments that transmit forces to the touch receptors can provide a basis for training people to perform remote operations like surgery and perhaps, in the future, will convey the illusion of touched objects over the Internet.