FIG. 3.

Developmental reduction of EPSP temporal summation. A and B: somatic voltage responses evoked by the injection of identical patterns of simulated EPSCs (bottom traces) at somatic and apical dendritic sites. Note in the P10 layer 5 pyramidal neuron (A) that the somatic voltage response evoked by EPSCs injected at somatic and dendritic sites are similar in amplitude. In contrast, in the P27 neuron (B) dendritic EPSCs only weakly depolarized the soma. Barrages of simulated EPSCs of different frequency and the corresponding voltage responses are delineated by gray scale for clarity. Inset: photomicrographs show the morphology of the illustrated neurons (scale bars = 100 μm). Summary graphs show the relationship between the magnitude of injected simulated synaptic current and evoked somatic voltage responses, when identical barrages of simulated EPSCs were generated at somatic (○) or apical dendritic sites (•). Data has been pooled (means ± SE) for neurons from the indicated age groups. Values of injected current and evoked voltage responses represent the average amplitude throughout the barrage. Note the similar slope of these relationships in P9-10 neurons (n = 6), but the weak somatic impact of dendritic input in P26-29 neurons (n = 8). Relationships were fit for subthreshold voltage responses by linear regression (lines).