Light and electron microscopical studies on the spherical neurons in the electrosensory lateral line lobe of the gymnotiform fish, Sternopygus

Spherical cells are a principal cell type of the electrosensory lateral line lobe (ELLL) and play a crucial role in the jamming avoidance response (JAR) behavior. Since Sternopygus, a low frequency gymnotiform genus, does not display a JAR we searched for spherical cells in its ELLL. While present in Sternopygus, spherical cells differed remarkedly from those in the high-frequency gymnotiforms, Eigenmannia and Apteronotus. This study reveals species-characteristic differences in the morphology and synaptology of the spherical cell, a projection neuron located in the deep neuropil layer (DNL) of the ELLL. In contrast to the adendritic spherical cell of other species, the spherical neuron in Sternopygus exhibits an extensive basilar dendrite that extends into the primary electroreceptive afferent zone, the deep fiber layer (DFL). In Sternopygus, these neurons are distributed evenly across the full length of each tuberous subdivision, with cell densities highest in the centrolateral subdivision. At the ultrastructural level, the contacts on the soma, proximal, and distal dendrite of the spherical neuron in Sternopygus are asymmetrical chemical synapses, quite distinct from the electrotonic gap junctions found on the spherical neurons of other species.