Extracellular HRP injections into the nucleus praeeminentialis dorsalis (NPd) of Apteronotus leptorhynchus retrogradely labeled a population of electrosensory lateral line lobe (ELL) efferent cells, deep basilar pyramidal cells, that differ morphologically from the previously described basilar and nonbasilar pyramidal cells. These neurons are found deep in the ELL cellular layers; they have small cell bodies and very short sparsely branching apical dendritic trees. The previously described basilar and nonbasilar pyramidal cells are larger, have extensive apical dendrites and are found more superficially. 2. Axon terminals of the deep basilar pyramidal cells were recorded from in the NPd and labeled with lucifer yellow. These NPd afferents have high, regular spontaneous firing rates, and respond tonically to changes in electric organ discharge amplitude. 3. Deep basilar pyramidal cell bodies were recorded from and labeled in the ELL, and these showed the same physiological responses as did the NPd afferent fibers. 4. In addition, basilar pyramidal cells were found which had spontaneous activity patterns and adaptation characteristics intermediate to those typical of the superficial basilar pyramidal cells and the deep basilar pyramidal cells. The size of the pyramidal cells' apical dendritic trees and the placement of their somata within the dorsoventral extent of the ELL cellular layers are highly correlated with the neurons' physiological properties.
