Further interpretation of referred pain with hyperalgesia is more complex than can be explained by the convergence-projection theory. Two predominant theories exist for the explanation of hyperalgesia one based upon central mechanisms and one by peripheral. The convergence-facilitation theory states that an abnormal visceral input would produce an "irritable focus" in the relative spinal cord segment, thus facilitating messages from somatic structures. Because of the central convergence of visceral and somatic fibers, this results in an increase in the visceral signaling as well as an increase in the normal somatic input centrally. The outcomes of these increased inputs are a more intense painful sensation than would typically be sensed. According to this theory, referred pain with hyperalgesia is mainly caused by central mechanisms.
The second theory postulates that pain-causing conditions develop in the periphery (i.e., the referral area), with subsequent excitation of pain receptors, because of several viscerocutaneous and visceromuscular reflexes that are triggered by the afferent visceral barrage. Referred pain with hyperalgesia can be produced by stimulation of the sensory afferents from the viscera and the efferent fibers would be triggered via a reflex arc. According to some authors, hyperalgesia and related phenomena at skin level are induced by sympathetic efferents, because an experimental, local anesthetic block of the sympathetic ganglia led to disappearance or marked decrease of referred pain, hyperalgesia, and dermographic skin alterations. In contrast, hyperalgesia in muscle is caused by somatic efferents in this view. These are responsible for a sustained contraction that in turn sensitizes muscular nociceptors and becomes a new source of pain.
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