Distribution of contacts from vestibulospinal axons on the dendrites of splenius motoneurons.

Current descriptions of the organization of synapses on the dendritic trees of spinal motoneurons indicate that the inputs are arranged in several patterns: some are widely distributed; some are distributed to proximal dendrites; others are distributed based on the trajectory of the dendrites. However, the principles governing the organization of synapses on spinal motoneurons remain poorly defined. Our goal was to extend the descriptions of the distribution of synapses, identified by their source, on the dendritic trees of spinal motoneurons. We combined anterograde and intracellular staining techniques in cats to determine the distribution of contacts between excitatory axons from the rostral aspect of the descending vestibular nucleus and the dendrites of motoneurons supplying a dorsal neck muscle, splenius. In five of five motoneurons, the contacts were preferentially distributed on dendrites medial to the soma. This qualitative observation was confirmed by using Monte Carlo methods. The results from this analysis showed that the distribution of contacts can be explained not by the overall distribution of the dendritic membrane area but rather by a systematic innervation of the medial regions of the dendritic trees (P < 0.02). Despite this selectivity, there was no additional bias in the distribution of contacts to proximal vs. distal dendrites. By concentrating excitatory synapses in a restricted region of the dendritic tree, the actions of vestibulospinal connections on neck motoneurons may be increased as a result of a greater probability of activating persistent inward currents on the dendrites.