Heterogenous distribution of peptide-containing nerve fibres within the circular muscle layer of the human pylorus.

The distribution of nerve fibres immunoreactive for vasoactive intestinal polypeptide (VIP), substance P (SP), methionine-enkephalin (ENK), calcitonin gene-related peptide (CGRP) and neuropeptide Y (NPY) within the circular muscle layer was examined histochemically in the human pylorus, adjacent antrum and duodenum. Longitudinal cryostat sections of the pyloric and surrounding regions were stained by an indirect immunofluorescence method, and the total length of each type of peptide-containing fibre per unit sectional area (micron/mm2) was measured using an image-analysing system. The narrow region of the circular muscle layer bordering the submucosa in the pylorus contained a rich supply of VIP, SP, ENK and CGRP immunoreactive fibres; VIP fibres were most prominent with less SP and ENK fibres and moderate amounts of CGRP. These peptide-containing nerve fibres were more dense than in the pyloric circular muscle, the longitudinal muscle layer and also the adjacent muscle layer. NPY-immunoreactive fibres were sparsely distributed throughout the pyloric region. These results suggest that the inner edge of the circular muscle, lying adjacent to the submucosa and densely innervated with peptide-containing fibres, may be a characteristic feature of the human pyloric sphincter.