Quantitative analyses of intracellularly characterized and labeled thalamocortical projection neurons in the ventrobasal complex of primates.

Abstract

This study describes the architecture of neurons and individual dendritic arbors of thirteen intracellularly labeled thalamocortical projection neurons that respond to non-noxious stimuli from the primate (Macaca fascicularis or Macaca mulatta) ventrobasal complex (VB). The neurons compose a homogeneous morphological class with total dendritic lengths from 10,169 microns to 21,711 microns (mean 17,615 microns +/- 3,705). The labeled neurons were remarkably similar in most measured parameters including the number of dendrites (7.5 +/- 1.2), percentage of dichotomous branching (89.8% +/- 3.4), and contribution of terminal branches to total dendritic length (88.4% +/- 2.0). The individual dendrites ranged in total length from 443 microns to 7,657 microns with a mean of 2,346 microns (+/- 137, n = 98). There was a positive correlation between stem dendrite diameter and total dendrite length, making it possible to estimate the total size of an individual dendrite by measuring the stem dendrite diameter. There was only a small increase in mean path distance with increasing dendritic size at the whole neuron and individual dendritic levels, so that for individual dendrites the mean path distance of a dendrite consisting of only two segments was 199 microns, while the mean path distance for a dendrite with eight segments was only 45 microns longer. Analysis of dendrite diameter, segment order, and path distance shows that dendritic diameter is not reliable for determining the location of synaptic contacts viewed by electron microscopy onto dendritic trees. The small variation of measured parameters between these neurons presents a powerful tool for future developmental, plasticity and comparative studies of VB neurons.