CELL BODY AND DENDRITIC TREE SIZE OF INTRACELLULARLY LABELED THALAMOCORTICAL PROJECTION NEURONS IN THE VENTROBASAL COMPLEX OF CAT

Fifteen thalamocortical projection (TCP) neurons from the adult cat ve ntrobasal complex (VB) were intracellularly labeled with horseradish p eroxidase or neurobiotin and examined quantitatively. We find that cat TCP neurons share key morphological features and form one neuronal ty pe. Previously reported variations in dendritic appearance cannot be s upported by the present quantitative data. The number of dendrites var ied between 4 and 13 (mean 9.1; +/-4.0) and the total dendritic length of adult cat VB neurons varied between 9,421 and 19,646 mu m (mean 13 ,120 mu m; +/-2,605). Linear regression analyses showed that soma diam eter or cross-sectional area measurements provide a poor estimate for total dendritic length in TCP neurons. In contrast, the number of firs t order dendrites or the sum of first order dendrite diameters do prov ide a good estimate of overall TCP neuron size. This relationship is u seful in predicting total dendritic length when it is not possible to reconstruct the entire dendritic tree. The mean dendritic path distanc e (distance from soma to the dendritic tip measured along the dendrite ) was relatively constant for all neurons regardless of differences in total dendritic length or the number of branches that form the path d istance.