Figure 1. Representative traces of the cardiovascular, intestinal motor and mesenteric afferent responses to 5-HT (10 μg), distension and intraluminal acid (150 mM HCl).

The afferent response to 5-HT consisted of a brief but pronounced initial increase in firing (highlighted by the filled area) followed by a more prolonged secondary component (highlighted by the shaded area), the latter occurring coincident with a small increase in intraluminal pressure. Rapid distension caused a slowly adapting increase in afferent discharge which returned rapidly to baseline on draining of the intestinal loop. Perfusion of the intestinal segment with acid caused a slowly developing increase in firing which was well maintained. The response to acid occurred in the absence of any increase in intestinal pressure.