Conduction velocity compensation for afferent fiber length in the trunk lateral line of the trout

The trout lateral line contains about 122 trunk scales and is tens of centimeters long. The difference in time of arrival in the hindbrain of simultaneously elicited afferent responses from the neuromasts is unknown. Propagation times of single-fiber afferent responses to water motion revealed that their mean conduction velocity was lowest (13 m s(-1)) for fibers innervating a neuromast close to the operculum and highest (33 m s(-1)) for those close to the tail. Histological examination showed that the nerve close to the operculum comprises about 500 afferents and that this number diminishes from operculum to tail with 4/scale. The mean diameter of the fibers changed from 12.5 mum at the operculum to 7.5 mum at three-quarters of the operculum-to-tail distance. Comparison of the distributions of diameters indicated that the fibers are tapered with the thick end towards the operculum. A model was developed describing the relationship between tapering and local conduction velocity. We conclude that simultaneous stimulation of all trunk neuromasts causes an average time-of-arrival difference in the hindbrain of 2.8 ms, which is 2.1 times less than the difference expected with a distance-independent conduction velocity. This suggests that tapering and velocity compensation are relevant for central processing of lateral line information