The dendritic tree of spinal neurons

The size of the dendritic tree was determined from the number, length and diameter of primary dendrites and number and length of their branches in 360 unselected neurons from all areas of the spinal gray of L7 segment in Golgi‐Kopsch preparations (dog). A direct proportionality was demonstrated between the size of the dendritic tree of a neuron and the number of its primary dendrites; a basic dependence of the entire dendritic complex upon the number of initial dendrite extensions from cell body. The existence of two dendrite parameter levels, and consequently two size levels of the whole dendritic tree, was also established; one for the large neurons and one for the small ones. The parameter and tree size differences between the two classes of neurons, the latter distinguished by different cell body‐size ranges, were all statistically highly significant. Very little of the variability of the parameters or of the whole tree could be accounted for by the variability of the specific cell body‐size within each of the two groups of neurons or within the entire sample. The morphological distinction is between two classes of neurons. The mean dendrite surface area for the entire population sample of large neurons was some five times greater than the mean for the small ones. With commensurate cell body‐size differences, and thus total receptive surface area, this implies a 5‐fold difference in mean number of synaptic contacts/neuron. The evidence, in addition, strongly pointed to motoneurons as having larger dendritic trees, on the average, than interneurons with comparably‐sized cell bodies. It was previously estimated that only 2% of interneurons in L7 segment, which outnumber motoneurons 30:1, are large cells.