Synapses from the high-order thalamic nucleus and motor cortex are co-clustered spatially in the distal tuft dendrites of the somatosensory cortex
The posterior medial nucleus of the thalamus (POm) and vibrissal primary motor cortex (vM1) convey a set of critical information regarding whisker position and movement to the barrel cortex (S1BF), and integration of these inputs is essential for whisker-based object localization. A considerable portion of these inputs locates on the distal tuft dendrites of layer (L) 5 pyramidal neurons, where regenerative dendritic activity determines the successful activity propagation. Therefore,
... erefore, understanding the relative spatial relationship and distribution of the inputs are critical prerequisites to acquire insight into how S1 synthesizes information to understand the location of an object. Using array tomography (AT), a high-resolution wide-field microscopy imaging technique that can accurately resolve synapses, we detected the locations of synapses from vM1 and POm on 18 distal tuft dendrites of L5 pyramidal neurons. We found that synapses from M1 and POm impinge on the dendrites with unusually high density and spatial clustering judged by various independent clustering analysis. We believe the exhaustively high density of synaptic inputs, as well as spatial clustering, could enhance the chance of successful dendritic spikes to mitigate the electrotonic disadvantages of the distal inputs. Furthermore, we found that the synaptic clusters of vM1 and POm locate close to each other on the same set of branches, suggesting that synaptic clusters but not dendritic branches, act as functional units cooperatively contribute to the nonlinear dendritic responses.