Boosting of action potential backpropagation by neocortical network activity in vivo.

Action potentials backpropagate into the dendritic trees of pyramidal neurons, reporting output activity to the sites of synaptic input and provoking long-lasting changes in synaptic strength. It is unclear how this retrograde signal is modified by neural network activity. Using whole-cell recordings from somata, apical trunks, and dendritic tuft branches of layer 2/3 pyramidal neurons in vivo, we show that network-driven subthreshold membrane depolarizations ("up states") occur simultaneously throughout the apical dendritic tree. This spontaneous synaptic activity enhances action potential-evoked calcium influx into the distal apical dendrite by promoting action potential backpropagation. Hence, somatic feedback to the dendrites becomes stronger with increasing network activity.