Figure 7. Axon branch points (bifurcations) are not generally Steiner points.

Distribution of local internal (aperture) angles at neocortical axon arbor bifurcations did not match Steiner point angle condition of 120° (filled bars indicate ±10° range) for either spiny (upper, 12% within ±10° range from n = 1298 nodes) or basket cell classes (lower, 14% out of n = 6192 nodes). Inset (upper) shows schematically how internal branch angle measurements were made from axon arbor reconstructions. Best-fit Gaussian distributions are shown in thick black lines (spiny, μ = 80.3°, sd = 35.7°; basket, μ = 84.8°, sd = 34.5°).