The unmyelinated fibre spectrum of the main trunk and side branches of the intestinal nerve in the chicken (Gallus gallus var. domesticus).

An electron microscopy study was undertaken to determine the numbers and diameters of unmyelinated axons in the main trunk and side branches of the intestinal nerve at the level of the small intestine. Three techniques were used to determine axon diameter. These were: length of the least chord (D), diameter determined from cross sectional area (DA) and diameter determined from perimeter (DP). A measure of the degree of circularity of the axons was also made. On average, the nerve trunk contained 4729 unmyelinated axons and these outnumbered myelinated axons by 78:1. The mean number of unmyelinated axons in the side branches was 463. Myelinated axons were not seen in the side branches. When a test for circularity was applied to the data it was found that larger axons tended to be less circular than smaller ones and, because of these differences in the degree of circularity, DA and more particularly D were subject to a variable inaccuracy. The mean values for DA were 0.70 micron for the nerve trunk and 0.57 micron for the side branch; the mean values for DP were 0.78 and 0.66 microns respectively. The peak conduction velocity at 40 degrees C of unmyelinated axons in the nerve trunk was determined from the single (C-fibre) deflection in the compound action potential. The following relationship was found, peak conduction velocity CV = DA0.59 = DP0.85.