A dendrite-resolved, in vivo transfer function from spike patterns to dendritic Ca(2).

Dendrites transform local electrical activity into intracellular Ca(2+) signals that drive plasticity(1,2), yet the voltage→Ca(2+) mapping during natural behavior remains poorly defined. Here, we measure this transfer function via simultaneous voltage and Ca(2+) imaging throughout the dendritic arbors of hippocampal CA2 pyramidal neurons in behaving mice. Dendritic Ca(2+) exhibited a hierarchical activation pattern dominated by back-propagating action potentials: simple spikes primarily drove somatic and proximal Ca(2+), whereas complex spikes produced larger somatic Ca(2+) signals and propagated farther into distal dendrites, sometimes in a branch-selective manner. Dendrite-restricted co-activation of voltage and Ca(2+) without concurrent somatic events was rare. A biophysics-inspired model accurately predicted local Ca(2+) transients from local voltage waveforms. Our data and model provide a quantitative understanding of when - and why - dendritic Ca(2+) signals in CA2 pyramidal cells arise during behavior.