Encoding of visceral noxious stimuli in the discharge patterns of visceral afferent fibres from the colon

Abstract

Afferent fibres, in the inferior splanchnic nerves and lumbar white rami, which supply the colon and its mesentery in the cat, were investigated for their responses to distension and contraction of the colon and to local pressure applied to colon and its mesentery. 1) 63% (177 out of 287) of the axons had resting activity (median 0.3 imp/s). These axons were either unmyelinated (conduction velocity below 2 m/s) or thin myelinated (conduction velocity below 18 m/s). Most axons without resting activity (N=95 out of 106 axons) conducted at less than 1.4 m/s, and most were probably sympathetic efferents. 2) 76 out of 80 afferent units with resting activity (95%) and 8 out of 27 units without (30%) were excited by distension of the colon. The thresholds were largely at intraluminal pressures of around 25 mm Hg or higher. 3) Most afferent units (87%) responded with an increased steady state discharge throughout the distension with or without initial dynamic response. The rest of the afferent units responded only with a transient discharged to distension. 4) Most afferent units reacted in a graded manner to variable intraluminal pressures. 5) In only 43% of the distension-sensitive afferent units could mechanoreceptive sites be located on the wall of the colon or in the mesentery. The majority of the afferent units had one mechanoreceptive site only, some had two. 6) Afferent units reacting to colon distension were also excited by contraction of the colon. 7) The excitability spectrum of visceral afferent fibres in the inferior splanchnic nerves, which are activated by colon distension, suggests that these units are involved in visceral nociception from the colon.