Functional anatomy of nonspiking interneuron dendrites

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The structure of sensory and premotor nonspiking interneurons in the terminal abdominal ganglion of the crayfich, Procambarus clarkii Girard, has been studied quantitatively by 3-dimensional morphometry, using a confocal laser scanning microscopic system. Their passive membrane properties have also been studied electrophysiologically to analyze their electrotonic structure. Two types of premotor nonspiking interneurons, AL- and PL-types that are characterized by different structure of major dendrites and different location of cell bodies in the ganglion, have been found to share common dendritic field in the neuropil. They are different from each other both morphologically and physiologically : the PL-type cells show a larger number of branching, anatomically longer fine (<3 mm) dendrites and shorter time and length constants (p<0.05). The total electrotonic length of fine dendrites is accordingly significantly longer in PL- than in AL-type cells. The total anatomical and electrotonic lengths of whole dendrites were not statistically different between them (P>0.05). An identified sensory interneuron had significantly shorter membrane time constant and length constant. Comparison of morphological and electrophysiological properties between premotor and sensory nonspiking interneurons has revealed that the total anatomical length of thick dendrites (>3 mm) is greater in the sensory than in premotor cells whereas the time and length constants are smaller in the former (P<0.05). The total electrotonic lengths of both thick and fine dendrites of the sensory interneuron were significantly greater than those of premotor cells (P<0.05). The difference in electrotonic structure between sensory and premotor nonspiking interneurons seems to reflect their different manners of synaptic integration in the control of postsynaptic nerve cells.