General sensory receptors can be classified by the type of stimulus that excites them PDF

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General sensory receptors can be classified by the type of stimulus that excites them...

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General sensory receptors can be classified by the type of stimulus that excites them Using the classification scheme introduced in Chapter 12, we can divide the receptors for the general senses into three types by location of the stimulus. p. 395 Exteroceptors provide information about the external environment. Proprioceptors report the positions and movements of skeletal muscles and joints. Interoceptors monitor visceral organs and functions. However, other classification schemes for the general sensory receptors are also used. One useful scheme is to divide them into four types by the nature of the stimulus that excites them: nociceptors (pain), thermoreceptors (temperature), mechanoreceptors (physical distortion such as touch), and chemoreceptors (chemical concentration). Each class of receptors has distinct structural and functional characteristics, although all general sensory receptors are simple in structure. There are both somatic and visceral versions of these receptors; the difference between them has to do with location, not structure. A pain receptor in the gut looks and acts like a pain receptor in the skin, but the two sensations are delivered to separate locations in the CNS. The visceral organs also have fewer pain, temperature, and touch receptors than we find elsewhere in the body. The sensory information you receive from these receptors is poorly localized because their receptive fields are very large and may be widely separated. The processing of general sensations is widely distributed in the CNS, but most processing takes place in centers along the sensory pathways in the spinal cord or brainstem. Only about 1 percent of the information provided by afferent fibers reaches the cerebral cortex and our awareness. For example, you usually do not feel the clothes you wear. Let’s take a closer look at the structure and function of the receptors as classified by the nature of their stimuli. Nociceptors and Pain Nociceptors (noxa, harm), or pain receptors, are especially common in the superficial portions of the skin, in joint capsules, within the periostea of bones, and around the walls of blood vessels. Other deep tissues and most visceral organs have few nociceptors. Pain receptors are free nerve endings with large receptive fields (see Figure 15–2). As a result, it is often difficult to determine the exact source of a painful sensation. Different nociceptors may be sensitive to (1) temperature extremes, (2) mechanical damage, and (3) dissolved chemicals, such as chemicals released by injured cells. Very strong stimuli, however, will excite all three receptor types. For that reason, people describing very painful sensations—whether caused by acids, heat, or a deep cut—use similar descriptive terms, such as “burning.” Stimulation of the dendrites of a nociceptor causes depolarization. When the initial segment of the axon reaches threshold, an action potential heads toward the CNS....