Figure 7. Estimating Rneck using biophysical models. A, A morphologically realistic multicompartmental model of a L5 pyramidal neuron was used (Mainen and Sejnowski, 1996). The passive electrical properties of the model were as follows: Rm
= 30,000 Ω cm2, Cm = 0.75 μF cm−2, and Ra = 150 Ω cm. The resulting somatic input resistance was 43 MΩ. B, Basal dendrite region, which was sampled by moving a spine head and neck with passive properties sequentially between 117 different locations to test the role of the spine neck resistance Rneck. C, With the test spine at a single dendritic location, Rneck increases the spEPSP and values depend on the maximal synaptic conductance Gsyn (right side color bar for families of curves). D, At the same time, attenuation ratio of spine–somatic EPSP amplitude appears very linear and insensitive to Gsyn. E, For each Rneck value, EPSPs were generated at all 117 basal dendritic locations, and the spine–soma attenuation ratio was plotted versus the distance of each model site (open circles) along with a quadratic fit (blue line) and superimposed with the experimental attenuation values (N = 19, closed circles with error bars). F, For each experimental attenuation data point (distance, spine–soma attenuation), the model’s Rneck value that led to the quadratic fit (blue line) coinciding with that point was determined and plotted in a histogram (mean 179 ± 25 SE MΩ).