Structural organization of the mormyrid electrosensory lateral line lobe.

The electrosensory lateral line lobe (ELL) of mormyrid teleosts is the first central stage in electrosensory input processing. It is a well-developed structure with six main layers, located in the roof of the rhombencephalon. Its main layers are, from superficial to deep, the molecular, ganglionic, plexiform, granular, intermediate and deep fiber layers. An important input arises from electroreceptors, but corollary electromotor command signals and proprioceptive, mechanosensory lateral line and descending electrosensory feedback inputs reach the ELL as well. The ELL input is processed by at least 14 cell types, which frequently show plastic responses to different inputs. The large ganglionic and large fusiform cells are the ELL projection cells. They are glutamatergic and project to the isthmic preeminential nucleus and the midbrain lateral toral nucleus. Interneurons are located in all ELL layers and are mostly GABAergic. The most remarkable interneurons are large multipolar cells in the intermediate layer, which have myelinated dendrites making presynaptic terminals contacting granular cells. With respect to the synaptic organization and microcircuitry of the ELL, a number of qualitative and quantitative aspects have been elucidated using electron microscopical and intracellular labeling techniques. However, the pathways by which primary afferent input influences the ELL projection cells are still undetermined: primary afferents do not seem to contact large fusiform or large ganglionic cells directly, but seem to terminate exclusively on granular cells, the axonal properties of which are not known. Consequently, more information of the structural organization of the ELL is still necessary for a detailed understanding of the neural basis of the plastic electrosensory input processing in mormyrids.