Electrical mapping of the projections of intrinsic primary afferent neurones to the mucosa of the guinea-pig small intestine

The patterns of innervation of the mucosa by axons of individual primary af ferent neurones with cell bodies in the myenteric plexus were studied by ma pping sites from which electrical stimulation of the mucosa elicited action potentials (APs) in their cell bodies. Segments of guinea-pig ileum were d issected to reveal the myenteric plexus over half of the intestinal circumf erence, leaving the mucosa intact over the other half. Intracellular record ings were taken from myenteric neurones located within 1 mm of the intact m ucosa. Focal electrical stimuli were applied to the mucosa at multiple loca tions separated by about 1 mm. Neurones that responded had round or oval ce ll bodies with several long processes (Dogiel type II) and APs that had an inflection on the falling phase (AH-neurones). Responses consisted of singl e APs or bursts of APs. Maps of the mucosal projections of 30 neurones were generated. The maximum distances from which individual neurones responded were 7 mm circumferential and 2 mm oral or anal to the cell body with a hig her proportion of responses from the oral regions. The areas of intact muco sa calculated to be innervated ranged from 1 mm(2) up to approximate to 15 mm(2) (mean 3.9 mm(2); median 2.5 mm(2)). It is estimated that the areas in nervated would be two to three times larger under conditions where part of the mucosa is not removed. Some neurones also responded to a chemical or a mechanical stimulus applied to the mucosa within the electrically mapped ar ea. It is concluded that intrinsic primary afferent neurones have overlappi ng receptive fields with 230-350 neurones innervating the same region of mu cosa.