The two main types of lateral line organs of lower vertebrates are the superficial neuromasts (SN), with a cupula that protrudes in the surrounding water, and the canal neuromasts (CN), located in the lateral line canal. The scales of the trunk lateral line canal of fish contain SNs as well as CNs. In this study, we examine whether there exist two functional classes of afferent fibers in the trunk lateral line nerve of the rainbow trout that can be attributed to the SNs and CNs. 2. The response properties of the afferent fibers in the trunk lateral line nerve have been determined during stimulation with sinusoidally varying water motion generated by a small vibrating sphere. Linear frequency response analysis revealed the presence of two distinct populations of afferent fibers in the lateral line nerve. The fibers belonging to the two populations showed significant differences in the frequency at which the sensitivity was maximal, the low-frequency response slope and the low-frequency asymptotic phase angle. 3. One population of fibers has a maximum sensitivity at 36 +/- 13 (SD) Hz (n = 22) and responds up to this frequency to water velocity. The low-frequency slope of the frequency response of these fibers was 20 +/- 3 (SD) dB/decade and the low-frequency phase lead was 121 +/- 11 degrees (mean +/- SD), both with respect to sphere displacement. The fibers of the other population have a maximum sensitivity at 93 +/- 14 (SD) Hz (n = 12) and respond up to this frequency to water acceleration. The low-frequency slope of these fibers was 35 +/- 5 (SD) dB/decade, and the low-frequency phase lead was 188 +/- 13 degrees (mean +/- SD). 4. Analysis of the stochastic properties of the spontaneous activity of both types of fibers revealed that the mean firing rate of the fibers responding to water velocity (26 +/- 12 spikes/s, mean +/- SD; n = 22) was significantly higher than that of the fibers responding to acceleration (36 +/- 11 spikes/s, mean +/- SD; n = 12). The other statistical properties of the spontaneous activity were found to be indistinguishable. 5. From comparison of the results with the available quantitative data on frequency responses of lateral line organs in other species, it has been concluded that the fibers responding (< or = 40 Hz) to water velocity innervate SNs and that the fibers responding (< or = 90 Hz) to water acceleration innervate CNs.(ABSTRACT TRUNCATED AT 400 WORDS)
