Probing Action Potential Generation and Timing under Multiplexed Basal Dendritic Computations Using Two-photon 3D Holographic Uncaging

Basal dendrites of layer 5 cortical pyramidal neurons exhibit Na+ and NMDAR spikes, and are uniquely poised to influence somatic output. Nevertheless, due to technical limitations, how multibranch basal dendritic integration shapes action-potential output remains poorly mapped. Here, we combine 3D two-photon holographic transmitter-uncaging, whole-cell dynamic-clamp, and biophysical modeling, to reveal how synchronously activated synapses (distributed and clustered) across multiple basal dendritic branches impacts action-potential generation - under quiescent and in vivo like conditions. While dendritic Na+ spikes promote milli-second precision, distributed inputs and NMDAR spikes modulate firing rates via axo-somatic persistent sodium channel amplification. Action-potential precision, noise-enhanced responsiveness, and improved temporal resolution, were observed under high conductance states, revealing multiplexed dendritic control of somatic output amidst noisy membrane-voltage fluctuations and backpropagating spikes. Our results unveil a critical multibranch integration framework in which a delicate interplay between distributed synapses, clustered synapses, and axo-somatic subthreshold conductances, dictates somatic spike precision and gain.