Abstract
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 (~2.50 μm2), with a single AZ (~0.13 µm2); preferentially established on spines. SBs had a total pool of ~1800 SVs with strikingly large readily releasable (~20), recycling (~80) and resting pools (~850). Thus, human L4 SBs may act as 'amplifiers' of signals from the sensory periphery, integrate, synchronize and modulate intra- and extracortical synaptic activity.
Highlights
The neocortex of various animal species including non-human primates (NHPs) and humans is characterized by its six-layered structure, the organization into vertical oriented functional slabs so-called cortical columns and a system of long-range horizontal axonal collaterals that connect neurons in the same and in different cortical layers of a given brain area and with trans-regional projections to different brain regions
The density of synaptic contacts was measured since such data are rare for humans for the TLN
These measurements provide the basis to further gain information regarding the synaptic organization of the neuropil, rate of connectivity as well as possible inter-individual and gender-specific differences in humans aCC-BY 4.0 International license
Summary
The human TLN representing ~17% of the total volume of the neocortex (Kiernan 2012) is regarded as a highly specialized associative brain region roughly subdivided into a superior, medial and inferior gyrus that is highly interconnected with the limbic and various sensory systems. The temporal lobe is involved in several neurological diseases most importantly as the area of origin and onset of temporal lobe epilepsy (TLE; reviewed by Allone et al 2017; Tai et al 2018). TLE is the most common form of refractory epilepsy characterized by recurrent, unprovoked focal seizures that may, with progressing disease, spread to other areas of the brain. The TLN represents an important region in the normal and pathologically altered brain in humans. Relatively little is known about its neural
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