The nucleus praeeminentialis: a Golgi study of a feedback center in the electrosensory system of gymnotid fish.

The cytoarchitecture of the dorsal nucleus praeeminentialis in two families of weakly electric fish (Eigenmannia viriscens and Apteronotus albifrons) was examined in both Nissl and Golgi material, and an attempt was made to correlate this information with our data from HRP studies on the afferent and efferent connections of this nucleus. The n. praeeminentialis is an isthmic structure located dorsolateral to the lateral lemniscus and anterior to the eminentia granularis--a subdivision of the archicerebellum of fish. The n. praeeminentialis can be divided into a large dorsal portion concerned with electroreception and a small ventral portion involved with the lateral line mechanoreception. The dorsal n. praeeminentialis consists of three parts: a pars medialis, a large pars principalis (p.P.) and a narrow pars lateralis. The p.P. presents three zones: a dorsal, a central, and a ventral zone, which are reciprocally and topographically connected with the zones of the electroreceptive lateral line lobe (ELLL), medial ELLL with ventral zone, central ELLL with central zone, and lateral ELLL with dorsal zone. Several types of projection cells are present in the n. praeeminentialis: (a) neurons that show preferential orientation of their long dendrites in relation to the afferent fiber systems, (b) cells with wide dendritic fields radiating in all directions, and (c) cells with small polarized dendritic fields toward the incoming ELLL afferents. Interneurons are also identified, showing different axonal ramification patterns. The afferent and efferent fiber systems linking the n. praeeminentialis to ELLL, lobus caudalis (L.C.), and torus semicircularis (T.S.d.) point to the important position of this nucleus in the feedback loop of the electrosensory pathway. The complex processing within this nucleus is reminiscent of the feedback loops in the auditory system.