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Input from the somatosensory systems informs the organism about events impinging on it. Sensation can be divided into four types: superficial, deep, visceral, and special. Superficial sensation is concerned with touch, pain, temperature, and two-point discrimination. Deep sensation includes muscle and joint position sense (proprioception), deep muscle pain, and vibration sense. Visceral sensations are relayed by autonomic afferent fibers and include hunger, nausea, and visceral pain (see Chapter 20). The special senses—smell, vision, hearing, taste, and equilibrium—are conveyed by cranial nerves (see Chapters 8, 15, 16, and 17). In addition, nociceptive sensation or pain-signaling serves to warn the organism when there is contact with noxious or potentially damaging elements in the environment, or when tissue is damaged.
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Receptors are specialized cells for detecting particular changes in the environment. Exteroceptors include receptors affected mainly by the external environment: Meissner's corpuscles, Merkel's corpuscles, and hair cells for touch; Krause's end-bulbs for cold; Ruffini's corpuscles for warmth; and free nerve endings for pain (Fig 14–1). Receptors are not absolutely specific for a given sensation; strong stimuli can cause various sensations, even pain, even though the inciting stimuli are not necessarily painful. Proprioceptors receive impulses mainly from pacinian corpuscles, joint receptors, muscle spindles, and Golgi tendon organs. Painful stimuli are detected at the free endings of nerve fibers.
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Each efferent fiber from a receptor relays stimuli that originate in a receptive field and gives rise to a component of an afferent sensory system. Each individual receptor fires either completely or not at all when stimulated. The greater the intensity of a stimulus, the more end-organs that are stimulated, the higher the rate of discharge is, and the longer the duration of effect is. Adaptation denotes the diminution in rate of discharge of some receptors on repeated or continuous stimulation of constant intensity; the sensation of sitting in a chair or walking on even ground is suppressed.
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A chain of three long neurons and a number of interneurons conducts stimuli from the receptor or free ending to the somatosensory cortex (Figs 14–1, 14–2, 14–3).
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The cell body of a first-order neuron lies in a dorsal root ganglion or a somatic afferent ganglion (eg, trigeminal ganglion) of cranial nerves.