Fig. 4.

Thinner branches experience greater $\mathrm{AP}\text{-}{\mathrm{Ca}}^{2+}$ amplitudes. (a) A layer 5 pyramidal cell filled with OGB1, AF594, and biocytin was reconstructed after the calcium imaging session. (b) Upper: two dendrites (black) were in the same focal plane allowing simultaneous recordings. Lower: $100\times$ magnification lens used to measure dendritic diameters. (c) Current injections into the soma were used to initiate three APs with interspike interval of 20 ms. Simultaneous recordings of dendritic $\mathrm{AP}\text{-}{\mathrm{Ca}}^{2+}$ signals at two locations marked in (b). (d) Diameters of sister branches in each pair were normalized against the branch with greater diameter ($x$ axis). $\mathrm{AP}\text{-}{\mathrm{Ca}}^{2+}$ signal amplitudes were normalized against the stronger branch in a pair ($y$ axis). Sister branch pairs are connected by straight lines. Note that $\mathrm{AP}\text{-}{\mathrm{Ca}}^{2+}$ signal amplitudes were regularly greater in thinner branches. (e1) Each dendritic pair ($N=15$) is represented by three values: (i) diameter of thinner branch divided by diameter of thicker branch (dotted bar), (ii) calculated surface-to-volume ratio (SVR) of thinner branch divided by calculated SVR of thicker branch (light gray bar), and (iii) measured ${\mathrm{Ca}}^{2+}$ signal in thinner branch divided by ${\mathrm{Ca}}^{2+}$ signal in thicker branch (dark gray bar). Note that in sister branches with similar dendritic diameters ($\text{diameter ratio}=1$), as determined in dendritic pair #9 or dendritic pair #10, the AP-induced ${\mathrm{Ca}}^{2+}$ transients were very similar, too (${\mathrm{Ca}}^{2+}$ $\text{Ratio}=1$). (e2) The measured value (${\mathrm{Ca}}^{2+}\_\text{thin}/{\mathrm{Ca}}^{2+}\_\text{thick}$) is plotted against the predicted value ($\mathrm{SVR}\_\text{thin}/\mathrm{SVR}\_\text{thick}$) for each dendritic pair ($N=15$). Linear regression (forced through zero) has a negative $R^2$.