Comparative Analysis of Geometry and Electrical Properties of Simulated Normal and Genetically Modified Motoneurons

In computer models of spinal motoneurons with reconstructed dendritic arborizations, we performed a comparative study of the features of the dendritic morphology and those of the output discharge patterns generated in response to stepwise or triangular ramp depolarizing currents applied to the soma; the modeled motoneurons were taken from wild-type (WT) and mutant Cu/Zn superoxide dismutase 1 (SOD1) animals. Six dendrites of the SOD1 motoneuron formed a more complex arborization than a 10-dendrite branching of the WT cell. Although both arborizations had similar maximum path distances of about 600 μm, a maximum dendritic complexity of the SOD1 cell was found at path distances of about 300 μm from the soma (about 100 μm farther than that of the WT cell). A greater number of dendritic paths terminating at longer path distances from the soma formed also different morphometrical asymmetry patterns of SOD1 arborizations, as compared with those of WT ones. The model neurons were capable of reproducing several types of experimentally recorded discharge patterns described in the literature. Main differences between the two cells were observed in the hysteretic frequency-current relations characterizing the discharges caused by the ramp current. Compared to the WT motoneuron, the SOD1 cell (i) had a wider dynamic range of the current intensities at which the discharge was initiated and terminated, (ii) this range was shifted to greater current intensities, and (iii) the maximum discharge frequency was lower. The nature of the found structure-related differences still has to be explored and interpreted considering that SOD1 mutants mimic some cases of amyotrophic lateral sclerosis.