HINDBRAIN CIRCUITRY MEDIATING COMMON MODE SUPPRESSION OF VENTILATORY REAFFERENCE IN THE ELECTROSENSORY SYSTEM OF THE LITTLE SKATE RAJA ERINACEA

Elasmobranch fish have an electrosensory system which they use for prey detection and for orientation. Sensory inputs to this system are corrupted by a form of reafference generated by the animal9s own ventilation, but this noise is reduced by sensory processing within the medullary nucleus of the electrosensory system. This noise cancellation is achieved, at least in part, by a common mode rejection mechanism. In this study we have examined characteristics of neurones within the medullary nucleus in an attempt to understand the neural circuitry responsible for common mode suppression. Our results are in accord with previous indications that ascending efferent neurones of the medullary nucleus are monosynaptically activated from the ipsilateral electrosensory nerves and project to the midbrain. We demonstrate that in Raja erinacea, as has been previously shown in one other species (Cephaloscyllium isabella), ascending efferent neurones typically have a discrete focal excitatory receptive field and an inhibitory receptive field which may be discrete or diffuse and which often includes a contralateral component. We identify a group of interneurones within the medullary nucleus which are driven monosynaptically from the electrosensory nerves, have simple discrete excitatory receptive fields and respond vigorously to imposed common mode signals. The simplest model of the circuitry underlying common mode rejection that is consistent with the evidence is that direct afferent input impinges onto the basal dendrites of the ascending efferent neurones and onto interneurones within the nucleus, and the interneurones in turn inhibit the ascending efferents. The pattern of this projection, including commissural inputs, determines the nature and extent of ascending efferents9 inhibitory surrounds and mediates the suppression of common mode signals.