Associations between the morphology and physiology of ventral-horn neurons in the adult turtle.

This study compared some morphologic and physiological properties of adult turtle spinal motoneurons (MNs) vs. interneurons (INs). Reconstructions were made of 20 biocytin-stained cells, which had been previously studied physiologically in 2-mm-thick slices of lumbosacral spinal cord. The intracellularly measured physiological properties included resting potential, input resistance (R(N)), threshold (rheobase, I(Rh)), and slope of the stimulus current (I) -spike frequency (f) relation. The seven morphologic properties that were quantified for each cell included three indices of somal size (diameter, area, volume), and four of dendritic size: the number of first- and last-order branches, rostrocaudal extent, and sigma individual lengths. Significant differences were shown between all seven morphologic parameters for MNs vs. INs. Despite the small sample size, significant differences were also shown for five of seven parameters for high-threshold vs. low-threshold MNs, and three of seven for low-threshold MNs vs. INs. These latter three parameters were the number of terminal dendritic branches, their rostrocaudal extent, and the sigma dendritic lengths. Linear associations for the MN + IN and the MN samples were stronger between the four dendritic parameters than between soma-dendritic ones. Exponential associations between morphologic and physiological properties were mostly significant (28 of 30), and their strength was in the order I(Rh) < R(N) < f/I slope for the MN +IN sample and I(Rh) < R(N) = f/I slope for the MN sample. There is discussion of the relevance of the above findings to the provisional classification of turtle ventral-horn neurons on the basis of electrophysiology alone.