Response Properties of Lateral Line and Auditory Units in the Medulla Oblongata of the Rainbow Trout (Oncorhynchus Mykiss)

Abstract

ABSTRACT The objective of this study was to investigate the response properties of second-order lateral line and auditory neurones in the medulla oblongata of the rainbow trout. The frequency response of 20 medullary units was measured by recording single-unit spike activity in response to a mechanical stimulus provided by an oscillating membrane or by a small vibrating sphere. These 20 units were categorized, according to their frequency response properties, into two classes. Ten units responded to relatively low frequencies (<50Hz) and showed a maximum in the frequency response between 70 and 120Hz. The other units responded to higher frequencies, showing a maximum in the frequency response above 150Hz. Significant differences between these two classes were also observed 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 separately. The majority of mechanically sensitive units (95%) showed a sustained, phase-locked response; 25% displayed a transient response component, mostly in addition to a sustained response component. All units were spontaneously active, with a mean firing rate of 27spikes s−1. Two units responded to a visual stimulus. No topographical representation of lateral line receptive fields was found in the caudal part of the medulla. The response characteristics of primary afferents reported in the literature differ from those of the medullary units of this study, so we conclude that the latter are higher-order units. Medullary lateral line units stimulated by the vibrating sphere appeared to be less sensitive than units stimulated by the vibrating membrane. The sensitivity of the units and the size of their receptive fields indicate that lateral line input converges in the medial nucleus.