Ultrastructural heterogeneity of human layer 4 excitatory synaptic boutons in the adult temporal lobe neocortex

Synapses are fundamental building blocks that control and modulate the behavior of brain networks. How their structural composition, most notably their quantitative morphology underlies their computational properties remains rather unclear, particularly in humans. Here, excitatory synaptic boutons (SBs) in layer 4 (L4) of the temporal lobe neocortex (TLN) were quantitatively investigated.\n\nBiopsies from epilepsy surgery were used for fine-scale and tomographic electron microscopy to generate 3D-reconstructions of SBs. Particularly, the size of active zones (AZs) and of the three functionally defined pools of synaptic vesicles (SVs) were quantified.\n\nSBs were comparably small ([~]2.50 m2), with a single AZ ([~]0.13 {micro}m2) and preferentially established on spines. SBs had a total pool of [~]1800SVs with strikingly large readily releasable ([~] 20), recycling ([~] 80) and resting pools ([~]850).\n\nThus, human L4 SBs may act as amplifiers of signals from the sensory periphery and integrate, synchronize and modulate intra- and extra-cortical synaptic activity.