The parietal peritoneum (PP) is innervated by somatic and visceral afferent nerves. PP receives sensitive branches from the lower intercostal nerves and from the upper lumbar nerves. Microscopically, a dense network of unmyelinated and myelinated nerve fibers can be found all over the PP. The unmyelinated fibers are thin and are ending just underneath the PP. The myelinated fibers can penetrate the PP to reach the peritoneal cavity, where they lose their myelin sheath and are exposed to somatic and nociceptive stimuli. PP is sensitive to pain, pressure, touch, friction, cutting and temperature. Noxious stimuli are perceived as a localized, sharp pain. The visceral peritoneum (VP) itself is not innervated, but the sub-mesothelial tissue is innervated by the autonomous nerve system. In contrast to the PP, the visceral submesothelium also receives fibers from the vagal nerve, in addition to the spinal nerves. VP responds primarily to traction and pressure; not to cutting, burning or electrostimulation. Painful stimuli of the VP are poorly localized and dull. Pain in a foregut structure (stomach, duodenum or biliary tract) is referred to the epigastric region, pain in a midgut structure (appendix, jejunum, or ileum) to the periumbilical area and pain from a hindgut source (distal colon or rectum) is referred to the lower abdomen or suprapubic region. Peritoneal adhesions can contain nerve endings. Neurotransmitters are acetylcholine, VIP, serotonin, NO, encephalins, CGRP and substance P. Chronic peritoneal pain can be exacerbated by neurogenic inflammation, e.g. by endometriosis.
Keywords: enteric nervous system; innervation; myelinated fibers; peritoneum; unmyelinated fibers; visceral pain.