Structure and function of electrosensory neurons in the torus semicircularis of Eigenmannia: morphological correlates of phase and amplitude sensitivity.

Structure-function relations in the electrosensory system of Eigenmannia were examined by labeling physiologically characterized neurons of the dorsal torus semicircularis. The sensitivity of cells to modulations in amplitude (AMs) and modulations in differential phase was determined. Approximately half of the 48 cell types defined by Golgi studies (Carr, C.E., and L. Maler (1985) J. Comp. Neurol. 235: 207-240) were identified in this manner. The majority of the neurons located in laminae (8a, b, c, and d, and 9 exhibited sensitivity to differential phase. In laminae 5 and 7, however, in addition to neurons which were sensitive to differential phase, many cells were found that were purely AM sensitive. Differential phase sensitivity originates in the small cells of lamina 6 (Heiligenberg, W., and G. Rose (1985) J. Neurosci. 5: 515-531), the exclusive termination site of phase-coding afferents from the electrosensory lateral line lobe. Cells that had dendritic extensions into the neuropil of lamina 6 exhibited sensitivity to differential phase, whereas neurons lacking dendrites in this lamina were only excited by AMs. These findings support the notion of a relationship between the morphology and laminar position of a neuron and its function.