Figure 8. Distribution of hotspots.

(A) Average dendrogram of all reconstructed neurons (n= 53), with soma at left. Line length represents average straightened length of structures, e.g. tertiary branches that later bifurcate. Numbers indicate percentage of branches in each category (e.g., 2^nd-order terminal branches represent 13.4% of all branches). More distal branches, representing < 1% of all branches, are not shown. Each diamond represents the location of a Ca hotspot (n = 85). Hotspots (diamonds) were placed on the dendrogram according to their structural location (e.g., halfway along a tertiary dendrite that later bifurcated), not according to their absolute distance from soma. Colored diamonds indicate pairs or triplets of hotspots generated by the activity of a single thalamic axon. The example neuron of Fig. 3A is in red.

(B) Hotspots were detected more frequently in the proximal dendritic arbor. Normalized density of distribution of hotspots (red) and dendritic tree (hotspot-bearing dendrites only). Inset, magnified view of the first 200 μm.

(C) Hotspots occur preferentially close to dendritic branch points. (Left) Bimodal distribution of hotspots in a plot of normalized inter-node distance. (Right) Cumulative percentage of hotspots that fell within the given distance from a dendritic branch point, with inset for distances < 20 μm. Black with gray shading indicates mean ± 2 × SD from repeated simulations of random hotspot distribution (see Methods).

(D) Lack of correlation between uEPSC amplitude and hotspot proximity to soma (left; R² = 0.04, p = 0.11; n = 70) or with hotspot intensity (middle, R² = 0.02, p = 0.41; two large ΔF/F responses not shown). Large-amplitude uEPSCs are associated with higher numbers of Ca hotspots (right; means in black; * p = 0.03, Wilcoxon unpaired). (See also Fig. S3).