Synapses from the high-order thalamic nucleus and motor cortex are co-clustered in the distal tuft dendrites of the mouse somatosensory cortex

The posterior medial nucleus of the thalamus (POm) and vibrissal primary motor cortex (vM1) convey essential information to the barrel cortex (S1BF) regarding whisker position and movement. Therefore, understanding the relative spatial relationships of these two inputs is critical prerequisites to acquire insight into how S1 synthesizes information to interpret the location of an object. Using array tomography, we identified the locations of synapses from vM1 and POm on distal tuft dendrites of L5 pyramidal neurons. We found that synapses from vM1 and POm are spatially clustered on the same set of dendrites with unusually high density. Furthermore, the clusters of vM1 and POm synapses colocalize each other. These findings suggest that synaptic clusters, but not dendritic branches, act as functional units and cooperatively contribute to nonlinear dendritic responses.