Dendritic locations and dendritic spine morphology determine effectiveness of thalamocortical pathways in the auditory cortex

Afferents from the thalamus efficiently deliver acoustic information to the auditory cortex (ACx). Although very reliable, they are substantially outnumbered by afferents from other brain regions. We hypothesized that thalamic afferents may achieve effectiveness by forming synapses at optimal locations on dendritic trees of cortical neurons. Here we identified synaptically connected dendritic spines to construct maps of active thalamic inputs on dendritic trees of cortical neurons in layers 3/4 of the ACx. These maps revealed that thalamic afferents preferentially synapse on stubby dendritic spines located within 100 mum of the soma. Selective activation of these dendritic spines generated stronger postsynaptic responses than activation of nearby dendritic spines of other morphological types. In addition, spines activated closer to the soma produced greater responses than those further away; the attenuation constant was 45 mum. These results suggest a novel mechanism of thalamocortical effectiveness in the sensory cortices-the positioning of crucial thalamic inputs at optimal synaptic locations.