Velocity- and acceleration-sensitive units in the trunk lateral line of the trout.

1. 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

Neuronal encoding of sound direction in the auditory midbrain of the rainbow trout

Acoustical stimulation causes displacement of the sensory hair cells relative to the otoliths of the fish inner ear. The swimbladder, transforming the acoustical pressure component into displacement, also contributes to the displacement of the hair cells. Together, this (generally) yields elliptical displacement orbits. Alternative mechanisms of fish directional hearing are proposed by the phase model, which requires a temporal neuronal code, and by the orbit model, which requires a spike density code. We investigated whether the directional selective response of auditory neurons in the midbrain torus semicircularis (TS; homologous to the inferior colliculus) is based on spike density and/or temporal encoding. Rainbow trout were mounted on top of a vibrating table that was driven in the horizontal plane to simulate sound source direction. Rectilinear and elliptical (or circular) motion was applied at 172 Hz. Generally, responses to rectilinear and elliptical/circular stimuli (irrespective of direction of revolution) were the same. The response of auditory neurons was either directionally selective (DS units, n = 85) or not (non-DS units, n = 106). The average spontaneous discharge rate of DS units was less than that of non-DS units. Most DS units (70%) had spontaneous activities < 1 spike per second. Response latencies (mode at 18 ms) were similar for both types of units. The response of DS units is transient (19%), sustained (34%), or mixed (47%). The response of 75% of the DS units synchronized to stimulus frequency, whereas just 23% of the non-DS responses did. Synchronized responses were measured at stimulus amplitudes as low as 0.5 nm (at 172 Hz), which is much lower than for auditory neurons in the medulla of the trout, suggesting strong convergence of VIIIth nerve input. The instant of firing of 42% of the units was independent of stimulus direction (shift <15 degrees), but for the other units, a direction dependent phase shift was observed. In the medial TS spatial tuning of DS units is in the rostrocaudal direction, whereas in the lateral TS all preferred directions are present. On average, medial DS units have a broader directional selectivity range, are less often synchronized, and show a smaller shift of the instant of firing as a function of stimulus direction than lateral DS units. DS response characteristics are discussed in relation to different hypotheses. We conclude that the results are more in favor of the phase mode

The pattern of trunk lateral line afferents and efferents in the rainbow trout (Salmo gairdneri).

The primary projections of the mechanosensory posterior lateral line nerve of the rainbow trout Salmo gairdneri, a teleost without lateral line specializations, were studied by applying horseradish peroxidase (HRP). The afferents project nearly exclusively to the nucleus medialis and a small nucleus caudalis in the hindbrain, and to the eminentia granularis. In addition, there is a sparse projection to the cerebellum, but a projection to the magnocellular nucleus is lacking. The afferent projection to the lateral part of the eminentia granularis is more dense than the projection to the medial nucleus, as shown by image processing of the HRP labeling. The efferent fibers originate from a bilateral pair of large octavolateral nuclei in the hindbrain. Characteristic for these nuclei are their large, fusiform and bipolar neurons. Another specific feature is the oblique orientation of the somata, with the main dendritic shaft coursing ventrolaterally and the axonal shaft coursing dorsomedially. The axons of the efferent neurons bifurcate. The shape, orientation and size of the efferent somata are the same in the entire efferent nucleus. In contrast to other teleosts, the occurrence of efferent somata found contralaterally (21%) is substantial. The part of the octavolateral efferent nucleus innervating the neuromasts of the trunk is estimated to contain at most 150 cell bodies unilaterall

Relationships between plasma lipids, proteins, surface tension and post-dive bubbles

Decompression sickness (DCS) in divers is caused by bubbles of inert gas. When DCS occurs, most bubbles can be found in the venous circulation: venous gas emboli (VGE). Bubbles are thought to be stabilized by low molecular weight surfactant reducing the plasma-air surface tension (?). Proteins may play a role as well. We studied the interrelations between these substances, ? and VGE, measured before and after a dry dive simulation. VGE of 63 dive simulations (21-msw/40-minute profile) of 52 divers was examined 40, 80, 120 and 160 minutes after surfacing (precordial Doppler method) and albumin, total protein, triglycerides, total cholesterol and free fatty acids were determined pre- and post-exposure. To manipulate blood plasma composition, half of the subjects obtained a fat-rich breakfast, while the other half got a fat-poor breakfast pre-dive. Eleven subjects obtained both. VGE scores measured with the precordial Doppler method were transformed to the logarithm of Kisman Integrated Severity Scores. With statistical analysis, including (partial) correlations, it could not be established whether ? as well as VGE scores are related to albumin, total protein or total cholesterol. With triglycerides and fatty acids correlations were also lacking, despite the fact that these compounds varied substantially. The same holds true for the paired differences between the two exposures of the 11 subjects. Moreover, no correlation between surface tension and VGE could be shown. From these findings and some theoretical considerations it seems likely that proteins lower surface tension rather than lipids. Since the findings are not in concordance with the classical surfactant hypothesis, reconsideration seems necessary