The sensory innervation of the colon and its modulation

The visceral afferents innervating the colon play a significant role in regulatory reflexes and consciously evoked sensations. These afferents travel centripetally in the pelvic and lumbar splanchnic nerves and enter the spinal cord via white rami. Afferents innervating the muscles of the hollow viscera encode graded intensities of intraluminal distension. A large proportion of them have low thresholds for response and a small proportion have high thresholds for response. The complexity of visceral pain cannot be explained fully on the basis of response characteristics of afferents. It can be proposed, however, that in the colon, which gives rise to sensations from fullness to intense pain, activation of low-threshold afferents contribute to regulatory reflexes and nonpainful sensations and activation of low-and high-threshold afferents contribute to acute pain. A prolonged activation and sensitization of both low-and high-threshold and sensitization of previously mechanically insensitive afferents may contribute to the visceral hyperalgesia, Recent studies indicate that kappa-opioid receptor agonists attenuate visceral hypersensitivity by acting at a peripheral site.