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

Synapses are fundamental building blocks controlling and modulating the 'behavior' of brain networks. How their structural composition, most notably their quantitative morphology underlie 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. Biopsies from epilepsy surgery were used for fine-scale and tomographic electron microscopy (EM) to generate 3D-reconstructions of SBs. Particularly, the size of active zones (AZs) and that of the three functionally defined pools of synaptic vesicles (SVs) were quantified. SBs were comparatively small (similar to 2.50 mu m(2)), with a single AZ (similar to 0.13 mu m(2)); preferentially established on spines. SBs had a total pool of similar to 1800 SVs with strikingly large readily releasable (similar to 20), recycling (similar to 80) and resting pools (similar to 850). Thus, human L4 SBs may act as 'amplifiers' of signals from the sensory periphery, integrate, synchronize and modulate intra- and extracortical synaptic activity.