Nociceptors of the deep somatic tissue encode noxious stimuli applied to the normal tissue. This is important for the protection of the tissue against damage. Noxious stimuli, such as twisting of a joint, cause immediate motor responses and a conscious pain experience both of which are parts of a strategy to avoid further damage. It is thought that loss of sensory mechanisms causes damage of the joint such as in Charcot's joint. Essentially, treatment of joint pain should not impair the normal nociceptive function.
Importantly, significant changes of the nociceptive processing are induced by inflammation and tissue injury which are called peripheral sensitization (sensitization of primary afferents) and central sensitization (development of hyperexcitability of nociceptive neurons in the central nervous system). Figure 2.1 summarizes the structures of the nociceptive system and the sequence of inflammation-evoked events in the nociceptive system. Inflammation leads to peripheral sensitization which in turn causes the development of hyperexcitability in the spinal cord.7,12 Ascending axons in the spinothalamic tract activate the lateral and medial thalamocortical system which evoke the conscious pain sensation with its sensory discriminative and the affective components.17 In parallel, ascending projections to the brainstem are activated. The activation of the brainstem contributes to the activation of the brain by noxious stimuli, but it also acts back on the spinal cord through descending systems.18,19,20
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