Figure 5. Dendritic regions differ in their spiking properties.

A spot of light (60 µm dia) excited a small subregion of the On layer of the DSGC, under control conditions and during simulated focal application of TTX to the soma, which blocked Na-channels in the soma, thin segment, and proximal dendrites. Three local regions were examined in this figure. (a) Region 1 was selected for having high conductance thresholds. Excitation with a small spot produced local dendritic spike propagation, but an impedance mismatch caused spike failure upon reaching the primary dendrite. Recording that generated the color-map was taken at 80 ms, just before the first spikelet, to show the effect of the synaptic excitation. (b) Recording at soma, showing that excitation of region 1 in the model under control conditions (gray trace) produced dendritric spikes attenuated by propagation to the soma, seen as 10mV “spikelets”. When the spikelets sufficiently depolarized the soma, it initiated a full-blown spike. With somatic TTX application (black trace), no dendritic spikes were initiated because depolarization was insufficient. The reason was that somatic TTX application hyperpolarized the soma and proximal dendrites. (c) Region 2 was selected for having low-conductance thresholds and high propagation efficiencies. The recording was taken at 55 ms. (d) Excitation of region 2 produced large PSPs, dendritic spike initiation, and successful propagation to the soma, which initiated full-blown spiking. (gray trace). During somatic TTX application (black trace), dendritic spikes were initiated and appeared at the soma as small spikelets, attenuated in thick proximal dendrites that lacked active Na-channels. (e) Region 3 was selected for having slightly higher conductance thresholds than region 2, which therefore were less excitable. The recording was taken at 55 ms. (f) When stimulated, region 3 produced the same number of spikes as region 2, however the spike trains differed slightly in impulse shape and amplitude, and ISIs (gray trace). Simulated somatic TTX application (black trace) showed that dendritic spikes from region 3 were attenuated more than from region 2. The reason was that region 3 is farther from and therefore more isolated from the soma. (g,h) In a separate simulation, moving a bar from left-to-right across the dendritic arbor elicited spikes. Dendrites in region 2 were the first to spike, however when the leading edge of the bar crossed into region 3, the location of dendritic spike initiation moved to region 3. (g) Voltage recorded at the soma for the bar stimulus. The lack of spiking between ∼150ms and ∼170ms occurred when the leading edge of the bar was between region 2 and region 3, where not enough dendrites were depolarized sufficiently in either region to initiate a dendritic spike. (h) Voltage traces from the soma (black) and two locations in the dendritic tree, marked with asterisks in (a,c,e). The blue trace is from region 2, the red trace is from region 3. The relative timing of the spikes clearly shows that the region of dendritic spike initiation moved from region 2 to region 3 as the bar moved across the dendritic field. For each region, the smaller spikes were initiated first, and propagated to the other region as larger spikes which had a much faster rise from Vrest. The somatic spikes (black) arrived later because the soma was further along the path from the initiation site. The supplementary material contains a movie that shows spike initiation at a distal site, propagation to the soma, followed by backpropagation to the remainder of the dendritic tree (Video S1). Note that the cell is “winner-take-all”, i.e. whichever region has the strongest response will prevent other regions from spiking because each spike propagates to the entire cell, resetting its membrane voltage.