Inside Information: The Unique Features of Visceral Sensation

The viscera are innervated by low- and high-threshold mecha-noreceptors. Hollow viscera are exquisitely sensitive to distension, a phenomenon that can be observed experimentally by recording the electrical activity induced in pelvic nerve afferent fibers (mechanoreceptors) during distension of the visceral organ under study. A) Low-threshold mechanore-ceptors (blue) detect both non-noxious and noxious distension pressures, whereas high-threshold mechanoreceptors (green) only respond to noxious distention pressures. Data are recorded from distension of the rat bladder. B) Mechanoreceptors (of either the high- or low-threshold type) in the pelvic nerve can be sensitized by the addition of irritants (e.g., xylenes) into the bladder. C) Mice lacking the TRPV1 receptor show impaired visceral nocicep-tion. The visceromotor response (electrical activity recorded in the abdominal musculature) produced during distension of the colon to a noxious pressure (e.g., 60 mmHg) is lower in mice that do not have functional TRPV1 receptors compared to wild-type controls. Even at the highest tested colonic distension pressure, TRPV1 knockout mice only show a level of response equivalent to that of the wild-type mice at 30 mmHg, the pressure at which this stimulus is likely to be noxious (dashed red line). Panels A and B are adapted with permission from (64), and panel C is adapted with permission from (48).