Neurohistological analysis of the lateral lobe in a weakly electric fish,Gymnotus carapo (Gymnotidae,Pisces)

Summary The cyto- and fiber architecture of the lateral lobe (LL) of Gymnotus carapo was investigated using Nissl, Golgi and reduced silver stains as well as 1 semi-thin sections. The neurons and fiber tracts are distributed in six layers. The first layer is subdivided into two sublayers: 1A where the primary afferent fibers run in rostro-caudal direction and 1B where these fibers terminate and the large multipolar neurons can be found. The 2nd layer consists of a single row of adendritic, pear-shaped neurons. The axons of these neurons enter the 4th layer and leave the lateral lobe in medial direction. The 3rd layer contains the granular cells of two different types: granular cells with two dendritic trunks directed into dorsal and ventral directions respectively, and granular cells in which, instead of the dendritic trunk, the axon emerges from the dorsal pole of the perikaryon. The axon can be followed up to the 6th layer. The 4th layer consists of bundles of nerve fibers. Beside the axons of the pear-shaped neurons the bundles contain also the axons of the pyramidal neurons (5th layer) leaving the lateral lobe. The 5th layer contains the perikarya of the pyramidal neurons. They have two separate dendritic arborizations, one directed ventrally and entering layer 1B and another directed dorsally and penetrating into the 6th layer. Their axons join the 4th layer and run in medial, rostro-medial or rostral direction depending upon the localization of the neurons in the lateral lobe. The 6th layer (crista cerebellaris) consists of three sublayers. Sublayer 6A contains fine myelinated fibers of unknown origin; sublayer 6B contains fine, mainly non-myelinated fibers originating from the mesencephalon, sublayer 6C is built up of non-myelinated fibers originating from the cerebellum. — A preliminary diagram of the neuron circuits and of the synaptic arrangements involved in the relay of lateral-line organ impulses is suggested.Supported by Research Grant No. 659440 accorded to Dr. T. Szabo by the Direction de Recherches et Moyens d'Essais (D. R. M. E.).Visiting investigator, aided by Institut national de la Santé et de la Recherche Médical (I. N. S. E. R. M.).Address: Groupe des Laboratoires du C. N. R. S. 91190 Gif-sur-Yvette, France.