Occurrence, distribution and neurochemical features of small intestinal neurons projecting to the cranial mesenteric ganglion in the pig.

The small intestine of the pig has been investigated for its topographical distribution of enteric neurons projecting to the cranial mesenteric ganglion, by using Fast Blue or Fluorogold as a retrogradely transported neuronal tracer. Contrary to the situation in small laboratory animals such as rat and guinea-pig, the intestinofugally projecting neurons in the porcine small intestine were not restricted to the myenteric plexus, but were observed in greater numbers in ganglia of the outer submucous plexus. The inner submucous plexus was devoid of labelled neurons. Retrogradely labelled neurons were mostly found, either singly or in small aggregates, in ganglia located within a narrow border on either side of the mesenteric attachment. For both nerve networks, their number increased from duodenum to ileum. All the retrogradely labelled neurons exhibited a multidendritic uniaxonal appearance. Some of them displayed type-III morphology and stained for serotonin. This study indicates that, in the pig, not only the myenteric plexus but also one submucous nerve network is involved in the afferent component of intestino-sympathico-intestinal reflex pathways. The finding that some of the morphologically defined type-III neurons participate in these reflexes is in accord with the earlier proposal that type-III neurons are supposed to fulfill an interneuronal role, whether intra- or extramurally.