Quantitative morphological analysis of deep superior colliculus neurons stained intracellularly with HRP in the cat.

Neurons of the deep collicular layers were identified electrophysiologically, stained intracellularly with horseradish peroxidase and reconstructed from serial sections. Three neurons located in the stratum griseum intermedium were selected for detailed, light microscopic analyses. 7-10 dendritic stems arose from the polygonally shaped perikarya; they branch out in up to 8 successive bifurcations, giving rise to a mean of 9.1 tips per dendrite, or equivalently 78.7 tips per neuron. Dendrites extended up to 700 microns from soma, with tip diameters below 1 micron. Dendritic lengths were shown to be independent on branch order; mean branch length amounted to 72.3 microns, 96.3 microns and 99.4 microns. In each of the neurons, intermediate branches were significantly shorter than terminating branches. By excluding all of the end-branches, an inverse length-diameter relation could be demonstrated in each neuron to exist. Dendritic membrane surface area constituted more than 90% of the total soma-dendritic surface; dendritic-to-somatic surface area ratios were 9.8, 13.2 and 21.4, respectively. Measurements at 92 bifurcations of first to fourth order led to branch power n = 1.47 showing that the 3/2 power relationship is fulfilled on the average. All collicular neurons exhibited drastic arborizational taper due to disappearance of terminal branches, not to dendritic thinning within the tree. In spite of some differences among these deep collicular neurons in their general morphology, on many of the feature characterizing their dendritic trees they proved to be clearly similar confirming a previous description as single class of isodendritic, collicular neurons.