Quantitative analyses of anatomical and electrotonic structures of crayfish nonspiking interneurons by three-dimensional morphometry.

Abstract

The three-dimensional structure of premotor nonspiking interneurons in the terminal abdominal ganglion of crayfish have been studied quantitatively by using a confocal laser-scanning microscope. Their passive membrane properties have also been studied electrophysiologically to analyze their electrotonic structure. In either one of the two major morphological types, anterolateral (AL) and posterolateral (PL), that are characterized by different locations of cell bodies in the ganglion, the monopolar cell body is connected with a fine primary process to a thick main segment projecting numerous fine secondary processes. These two types of cells share a common dendritic field in the neuropil, showing similar anatomical characteristics of dendrites. Electrotonic analyses based on the present anatomical and physiological measurements have revealed that the steady-state voltage-attenuation factors for the secondary processes were not statistically different between the AL- and PL-type cells. Comparison between the premotor nonspiking interneurons and an identified sensory nonspiking interneuron, which was studied previously, has revealed that voltage attenuation over secondary processes in both the centripetal and the centrifugal directions was significantly greater in the sensory than in the premotor interneurons, although the anatomical length of each secondary process from its terminal to the main segment was not different between them. Differences in the electrotonic structure between sensory and premotor nonspiking interneurons indicate their different modes of synaptic integration in the control of postsynaptic nerve cells.