FIG. 4.

Developmental refinement of the electrotonic properties of layer 5 pyramidal neurons. A: subthreshold dendritic efficacy decreases with postnatal age. Subthreshold dendritic efficacy was calculated as the fraction of somatic depolarization produced by identical barrages of synaptic inputs generated at somatic and distal apical dendritic sites across a broad range of injected synaptic current (Fig. 3). Data have been pooled for the indicated age groups (means ± SE). —, a fit to grouped data by a single-exponential function. B: simultaneous somato-dendritic recording of voltage responses evoked by the injection of a current step at the soma in P10 and P27 layer 5 pyramidal neurons. Note the minimal somato-dendritic voltage attenuation at the negative peak (○) and at steady state (•) in the P10 neuron. In contrast in the P27 neuron, dramatic somato-dendritic voltage attenuation is evident. In each neuron, a similar amplitude somatic voltage response was generated in response to a step of negative current, the smaller current step was injected in the P10 neuron (bottom traces). C: summary data describing the age-dependent increase in somato-dendritic voltage attenuation when measured at peak and steady state. The grey symbols show the age-dependent increase in apparent input conductance. Data are pooled for 6 age groups (means ± SE). Lines represent fits with single-exponential functions.