Conduction velocity and axonal diameter of alimentary C fibres.

Experiments were undertaken to investigate the numerical relationship between the axon diameter and conduction velocity (scaling factor) of non-myelinated axons supplying the duodenum of the sheep and rabbit. Transmission electron microscopy and light microscopy of duodenal nerves showed distinct species differences. In the rabbit, all axons were non-myelinated and three classes of nerves were found each containing axon populations with different diameters. Sheep nerves had a mixed population of non-myelinated axons and contained 5% myelinated axons. There were significantly more Schwann cells containing single axons in sheep nerves than in rabbit nerves. The conduction velocities of functionally identified single afferent units in both species were measured during acute experiments in anaesthetized preparations. For pooled results of five C fibres taken over the temperature range 25-41 degrees C, a linear relationship between conduction velocity and temperature was found: c.v.% = 0. 0114T + 0.317 (c.v.% = ranked conduction velocity; T = temperature (degrees C]. At 39 degrees C a scaling factor between 0.89 and 1.26 is required to convert non-myelinated axon diameter to conduction velocity in duodenal nerves.