Physiological Mechanisms of Visceral Pain

Pain is a complex sensory experience significantly modulated by the central nervous system (CNS). Despite the high prevalence of chronic visceral pain and its profound negative impact on patients’ quality of life, the understanding of the mechanisms underlying the physiology of visceral pain has prominently lagged behind the understanding of non-visceral pain (e.g., neuropathic and inflammatory). This was partly due to the complexity of the interactions between the visceral organs, the autonomic nervous system, and the CNS, and partly because early clinicians disagreed about whether the viscera received any sensory innervation and later, whether this innervation included nociceptors. The mechanisms of visceral pain share many features with pain occurred in other parts of the body. However, the anatomical basis with dual innervation and a central projection to areas controlling emotion, as well as the associated clinical data showing significant effects on mood, distinguish visceral pain from skin or other somatic pain. The fact that the viscera are more inaccessible than the skin and other tissues also hinders the understanding of visceral pain mechanisms. Therefore, in this review, an introduction to the basis for discussing the anatomical complexity of visceral sensory innervation was provided first. Then, considering recent and emerging knowledge about the location and functional characteristics of sensory receptive terminals in visceral organs, the impact of nociception/pain and nociceptor sensitization on chronic visceral pain conditions was explored. This chapter concludes with a brief description of central processing of visceral pain.