Short-latency interneurons in the dorsal nucleus of the little skate, Raja erinacea.

The elasmobranch electrosensory system is the most thoroughly understood of the non-teleost electrosensory systems and is useful for studying central nervous system mechanisms for the separation of behaviorally relevant signals from self-generated noise. In the little skate, Raja erinacea, the electrosensory primary afferents are responsive to electrical potentials created during the animal's own ventilation, while second-order neurons in the dorsal nucleus of the medulla suppress responses to ventilatory potentials (self-generated noise) but retain their extreme sensitivity to electric signals in the environment. The selective suppression of ventilatory noise in second-order cells is due in part to the fact that ventilatory potentials stimulate all receptors equally and simultaneously. The neuronal circuitry mediating rejection of such 'common mode' signals in the dorsal nucleus likely includes inhibitory interneurons. This study describes physiological and anatomical characteristics of a group of dorsal nucleus interneurons that are distinguished from previously described interneurons by their shorter orthodromic activation latencies and higher spontaneous firing rates. The interneurons show sustained responses to an external dipole stimulus and respond well during simultaneous activation of all afferents by a 'common mode' stimulus. Intracellular labeling indicates that the short-latency interneurons are located in the central and peripheral zones of the dorsal nucleus and the extent of their labeled processes is limited to the projection area of afferents from a single ampullary cluster. These features are consistent with a hypothesized role for these interneurons in inhibiting second-order cells, including inhibition which contributes to common mode rejection.