Relationship between morphoelectrotonic properties of motoneuron dendrites and their trajectory

The distribution and geometry of the dendritic trees of spinal motoneurons obey several well-established rules. Some of these rules are based on systematic relationships between quantitative geometrical features (e.g., total dendritic length) and the three-dimensional trajectory followed by dendrites from their origin to their termination. Because dendritic geometry partially determines the transmission of current and voltage signals generated by synapses on the dendritic tree, our goal was to compare the efficacy of signal transmission by dendritic trajectories that followed different directions. To achieve this goal, we constructed detailed compartmental models of the dendritic trees of three intracellularly stained biventer cervicis/complexus (BCCM) motoneurons and calculated the electronic properties of 361 dendritic paths. Each trajectory was classified according to its orientation, e.g., rostral, rostral-dorsal-lateral. The attenuation of current and voltage signals en route to the soma was strongly related to trajectory orientation. Trajectories with similar attenuation factors formed functional subunits that were arranged in distinct domains within the ventral horn. Changes in R(m) or R(i) had little effect on which trajectories belonged to each functional subunit. However, differences in the efficacy of signal transmission between subunits increased during high network activity (mimicked by decreases in R(m)). The most efficient subunit delivered two times more current and four times more voltage to the soma than the least efficient subunit. These results indicate that the input-output properties of motoneurons depend on the direction of the path taken by dendrites from their origin at the cell body to their terminals.