Thalamus enables active dendritic coupling of inputs arriving at different cortical layers

Dendritic calcium action potentials (APs) enable the main output neurons of the cerebral cortex - pyramidal tract neurons (PTs) - to associate inputs that arrive at different cortical layers. How synaptic inputs evoke calcium APs during in vivo conditions is yet unclear. We combine in vivo recordings in male rats with synaptic input reconstructions, multi-scale modelling and optogenetic manipulations. We find that thalamocortical (TC) synapses, which provide sensory input to cortex, target specifically and most densely the dendritic domain that initiates calcium APs in PTs. Sensory input from thalamus is hence a reliable, but weak source for activating the dendritic calcium domain. Because it is fast and local, this activation enables active dendritic coupling of sensory input with multiple sensory-evoked and ongoing input streams that arrive during and surprisingly before the stimulus. This 'TC coupling' mechanism accounts for the modulation of the first sensory responses that leave the cortex with bursts of APs.