Equine myenteric plexus with special reference to the pelvic flexure pacemaker.

Sellers et al. (1979, Am. J. Phys., 237: E457-E464) proposed a pelvic flexure pacemaker mechanism to account for the bidirectional contraction waves needed to both retain ingesta within the right ventral colon for cellulose digestion and terminal fermentation and to transport the digesta distad once the process has been completed. To corroborate the presence of a pelvic flexure pacemaker, we prepared whole mount samples of the tunica muscularis from 23 horses at ten sites along the large colon, cecum and jejunum. Following smooth muscle enzymatic digestion, somata of the myenteric plexus were stained with an RNA-specific agent, Cuprolinic blue. These somata were quantified at each site to establish any regional variations in neuronal density. Results indicated an increased neuronal density at the level of the pelvic flexure, especially in the region of the left dorsal colon. The increased neuronal density at the left dorsal colon compared to the other sampling sites was statistically significant (Wilcoxon signed rank test, P less than .01 at each sampling site). There was remarkable size variation (from 10-60 microns) among neurons at the individual sampling sites. However, no statistically significant size discrepancy existed between sampling sites (Friedman's rank test, P = .10). The 23 horses ranged from 6 months to 15 years of age. No age-related differences in neuronal density was discovered (Kruskal-Wallis ANOVA test, P greater than .05). Neuronal densities did not vary on the basis of sex (Wilcoxon signed pairs test, P greater than .05).(ABSTRACT TRUNCATED AT 250 WORDS)