Figure 6. Spine Ca²⁺ cooperativity correlates with dendritic Na⁺ spike strength.

(a) Partial 2P z-stack of a CA1PC, with stimulation sites on terminal sister dendrites (D1 and D2) branching off from a common parent dendrite (P) indicated. (b) Representative voltage traces (upper) and corresponding dV/dt (lower) of dendritic Na⁺ spikes (pointed at by arrowheads) evoked at the stimulation sites in the three dendrites indicated in a (see Methods). Note that parent branch spike is much stronger than daughter branch spikes. All dendritic families used in the analysis had parent branch spike with dV/dt>2 V s⁻¹. (c) Comparison of dendritic Na⁺ spike strength between terminal sister branches. Black: dV/dt_{stronger branch}/dV/dt_{weaker branch}<1.7 (n=9); green: dV/dt_{stronger branch}/dV/dt_{weaker branch}>1.7 (n=9). (d) Stimulated spine set in D1 shown in a and b. (e) 4S protocol on the four spines in D1 shown in d. (f) Somatic EPSP nonlinearity corresponding to the experiment in e. (g–i) same as d–f but in D2 shown in a and b. (j,k) Ratio of spine Ca²⁺ nonlinearity (j, Spearman R=0.671, P<0.05, n=18) and somatic EPSP nonlinearity (k, Spearman R=−0.003, P=0.990, n=18) by the 4S protocol in connected sister branches, as a function of the ratio of their dV/dt (Na⁺ spike strength). Black and green dots indicate homogenous and heterogeneous pairs, respectively.