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 waveforms are thought to be analyzed by the Knollenorgan electroreceptor 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 ELa: the afferent axons from the nucleus of the electrosensory lateral line lobe (NELL), the large interstitial cells of ELa and an unidentified cell type. The large cells and the NELL axons were identified by intracellular injection of biocytin and are physiologically similar. Cells in ELa responded to square pulse stimuli with one or more time-locked action potentials with 2.8-3.0 ms latency. Both large cells and NELL axons arborized extensively in ELa and contacted numerous small cells. Based on the pattern of arborizations, we constructed a countercurrent flow model of temporal coding by the small cells of ELa. We postulate that individual small cells are not selectively tuned for specific stimulus durations, but rather, the firing patterns of groups of small cells must be analyzed by neurons further up in the sensory hierarchy to determine the stimulus duration.