Mucosal Afferents

Mucosal afferents in the vagal pathway have been extensively documented throughout the upper gastrointestinal tract. These afferents are silent at rather than respond to fine stroking of the luminal surface with rapidly adapting responses, and are unresponsive to distension. Moreover, they are polymodal as they are also chemosensitive to a range of chemical and osmotic stimuli including serotonin, bradykinin, purines, prostaglandins and cholecystokinin (28). Recent in vitro studies have highlighted the relative importance of these afferents in terms of their exact location, proportions, and modality (71,72). The major role of vagal mucosal receptors is thought to be in the generation of sensations such as satiety, nausea, and vomiting, with a minor role in direct generation of reflex responses (28).

Mucosal afferents in the lower gastrointestinal tract have not been studied as extensively as those in the upper gut; however, spinal mucosal afferents have been characterized functionally in the colon, anal canal, and perianal mucosa. In the distal colon, mucosal afferents were first identified using an in vitro preparation and recording from the LSN. These colonic muco-sal afferents have similar properties to vagal mucosal afferents, responding to fine tactile stimulation of the mucosa with a 10 mg von Frey hair and did not respond to circumferential stretch of the colon. Mucosal afferents account for 24% of the colonic afferents recorded from the rat LSN and are also polymodal as they are responsive to a variety of chemical stimuli, including 5-hydroxytryptamine (5-HT), NaCl, HCl, bile, and capsaicin (74,76).

In mouse colon, both pelvic nerve and LSN pathways have been shown to contain muco-sal afferents (80). These afferents are consistent with previous reports of mucosal afferents throughout the gastrointestinal tract, suggesting that these afferents may respond to particu-late material within the colonic lumen. In the mouse, afferents sensitive to mucosal stroking account for nearly 50% of the pelvic nerve afferent population (including both mucosal and muscular/mucosal afferents—see below) but only 4% of the LSN innervation (mucosal afferents) (80). These afferents exhibited different distributions, with pelvic nerve mucosal and muscular/mucosal afferents localized in the rectum and most aboral regions of the distal colon while the few LSN mucosal afferents were found more orally in the distal colon. Although the responsiveness of individual afferents to mucosal stroking is similar between the two pathways, these results suggest that fine mechanical stimulation of the colonic mucosa is signaled predominantly via the pelvic nerve pathway to the lumbosacral spinal cord. In particular, this signal occurs when mechanical stimulation of the colonic mucosa occurs in the distal colon/ rectum. The high proportion of pelvic nerve mucosal afferents recorded in this study may in fact correspond to the large proportion of distension-insensitive pelvic nerve afferents reported previously in vivo (36,64), which could not be ascribed any function.

Afferents with similar properties to mucosal afferents have been identified from the pelvic nerves in vivo, with receptive fields in the anal canal of the cat (36,78) and perianal mucosa of the rat (69). In the anal canal of the cat, these afferents responded to proximodistal shearing stimuli within the lumen, had discrete receptive fields, and were usually not activated by distension. The afferents also had significantly faster conduction velocities than colonic afferents (36). Similarly, afferents documented in the perianal mucosa of the rat responded with a burst of firing to stoking of the mucosa or by rotation or movement of the experimental balloon within the colon. These afferents also had discrete receptive fields with the majority unresponsive to colorectal distension (69). Unlike the distension sensitive afferents (which were C-fibers), the majority of these perianal mucosal afferents were classified as AS fibers (64). As mucosal afferents are sensitive to mechanical deformation of the mucosa, they may respond to particulate material within the lumen, which can refine the quality of perceived stimuli and alter reflexes controlling motility (81,82).

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