TIME CODING IN THE MIDBRAIN OF MORMYRID ELECTRIC FISH - I - PHYSIOLOGY AND ANATOMY OF CELLS IN THE NUCLEUS EXTEROLATERALIS PARS ANTERIOR

In mormyrid electric fish, species-specific electric organ discharge w aveforms are thought to be analyzed by the Knollenorgan electrorecepto r subsystem. The midbrain anterior and posterior exterolateral nuclei (ELa and ELp) are thought to be the sites of this analysis, This paper is an electrophysiological study of the properties of the neurons in ELa. We recorded intracellularly from three classes of cells within EL a: the afferent axons from the nucleus of the electrosensory lateral l ine lobe (NELL), the large interstitial cells of ELa and an unidentifi ed cell type. The large cells and the NELL axons were identified by in tracellular injection of biocytin and are physiologically similar. Cel ls in ELa responded to square pulse stimuli with one or more time-lock ed action potentials with 2.8-3.0 ms latency. Both large cells and NEL L axons arborized extensively in ELa and contacted numerous small cell s. Based on the pattern of arborizations, we constructed a countercurr ent flow model of temporal coding by the small cells of ELa. We postul ate that individual small cells are not selectively tuned for specific stimulus durations, but rather, the firing patterns of groups of smal l cells must be analyzed by neurons further up in the sensory hierarch y to determine the stimulus duration.