Dendritic Architecture of Rat Somatosensory Thalamocortical Projection Neurons

This study examines dendrites from physiologically characterized and intracellularly labelled thalamocortical projection (TCP) neurons from the rat ventrobasal complex (VB) and posterior nucleus (POm). The goals were to provide quantitative descriptions of TCP neuron dendrites, examine underlying design principles of dendritic morphology, and determine correlations between dendritic size parameters. Forty-four dendrites from seven VB neurons and 21 dendrites from three POm TCP neurons that responded to low-threshold mechanical stimuli were reconstructed and quantitatively analyzed at the light microscopic level. The dendritic architecture of the neurons was remarkably similar in most parameters studied, including the percentage of dichotomous branching, contribution of terminal branches to total dendritic length, and branching symmetry. There was a positive correlation between stem dendrite diameter and the length of the entire dendrite arbor, making it possible to estimate the total length of a dendritic arbor by measuring the stem dendrite diameter. The correlations of the VB and POm dendrites had different slopes. The path distance (the distance from the soma to a dendritic end point) of individual dendrites showed only a small variation with large differences in the total dendritic length of an arbor. The constant diameter of distal dendrites shows that dendrite diameter is a poor predictor of synaptic location on the dendritic tree. Although the morphology of neurons and their individual dendrites varied considerably in overall size and qualitative appearance, when examined qualitatively, many aspects of dendritic structure were similar within and between groups. We suggest that the rat somatosensory TCP neurons have a stereotyped dendritic architecture and present data which provide a base for future comparative, developmental, and plasticity studies.