RESPONSE PROPERTIES OF LATERAL-LINE AND AUDITORY UNITS IN THE MEDULLA-OBLONGATA OF THE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The objective of this study was to investigate the response properties of second-order lateral line and auditory neurones in the medulla obl ongata of the rainbow trout. The frequency response of 20 medullary un its was measured by recording sin-le-unit spike activity in response t o a mechanical stimulus provided by an oscillating membrane or by a sm all vibrating sphere. These 20 units were categorized, according to th eir frequency response properties. into two classes. Ten units respond ed to relatively low frequencies (<50 Hz) and showed a maximum in the frequency response between 70 and 120 Hz. The other units responded to higher frequencies, showing a maximum in the frequency response above 150 Hz. Significant differences between these two classes were also o bserved with respect to recording site, latency and sensitivity. It is concluded that these two classes of medullary units are lateral line units and auditory units, respectively. In the medulla, the input from the two mechanosensory systems appears to be mainly processed separat ely. The majority of mechanically sensitive units (95 %) showed a sust ained, phase-locked response; 25 % displayed a transient response comp onent, mostly in addition to a sustained response component. All units were spontaneously active, with a mean firing rate of 27 spikes s-1. Two units responded to a visual stimulus. No topographical representat ion of lateral line receptive fields was found in the caudal part of t he medulla. The response characteristics of primary afferents reported in the literature differ from those of the medullary units of this st udy, so we conclude that the latter are higher-order units. Medullary lateral line units stimulated by the vibrating sphere appeared to be l ess sensitive than units stimulated by the vibrating membrane. The sen sitivity of the units and the size of their receptive fields indicate that lateral line input converges in the medial nucleus.