Inside Information: The Unique Features of Visceral Sensation

The colon is innervated by five different types of mechanosensory afferent. Five different types of afferent fiber have been reported in the colon: serosal, mesenteric, muscular, mucosal, and muscular/mucosal. Each has characteristic response criteria based on a protocol of probing, stretching, and stroking the colonic wall, and each has a different putative functional role. Most serosal afferents are mechanosensitive and, given that their thresholds for activation are higher than would be expected physiologically, are thought to signal short, sharp events (e.g., muscle contraction). Found only in the splanchnic afferent innervation, mesenteric afferents are predominantly found closely associated with blood vessels and likely signal twisting of the colon wall and some changes in mesenteric blood pressure, with a potential role in inflammation. Muscular afferents, named for their termination in the circular and longitudinal muscle layers of the gut, respond directly to circumferential stretch with a low threshold of activation (though can code into the noxious range). Muscular afferents exhibit slightly different properties depending upon the nerve (splanchnic or pelvic) in which they are found, but generally are considered to contribute to sustained filling, bloating, or distending sensations. Mucosal afferents are stretch-insensitive (at least circular stretch-insensitive) and respond to fine probing and stoking of the mucosal membrane. This suggests a role in providing feedback from physiological stimuli such as the normal passing of fecal material through the gastrointestinal tract. Finally, muscular/mucosal afferents, so named for their ability to detect both circular stretch and fine mucosal stroking, are a class of mechanoreceptor that, in mouse colon at least, are only found in the pelvic innervation. Presumably, these fibers provide a combination of the information that is transmitted from the muscular and mucosal fibers described above. For examples of the responses seen for each of these mechanosensitive fiber types, please see reference (53).