THE SENSORY INNERVATION OF THE COLON AND ITS MODULATION

The visceral afferents innervating the colon play a significant role i n regulatory reflexes and consciously evoked sensations. These afferen ts travel centripetally in the pelvic and lumbar splanchnic nerves and enter the spinal cord via white rami. Afferents innervating the muscl es of the hollow viscera encode graded intensities of intraluminal dis tension. A large proportion of them have low thresholds for response a nd a small proportion have high thresholds for response. The complexit y 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 reflexe s and nonpainful sensations and activation of low-and high-threshold a fferents contribute to acute pain. A prolonged activation and sensitiz ation of both low-and high-threshold and sensitization of previously m echanically insensitive afferents may contribute to the visceral hyper algesia, Recent studies indicate that kappa-opioid receptor agonists a ttenuate visceral hypersensitivity by acting at a peripheral site.