Abstract
Layer (L)1, beside receiving massive cortico-cortical, commissural and associational projections, is the termination zone of tufted dendrites of pyramidal neurons and the area of Ca2+ spike initiation. However, its synaptic organization in humans is not known. Quantitative 3D-models of synaptic boutons (SBs) in L1 of the human temporal lobe neocortex were generated from non-epileptic neocortical biopsy tissue using transmission electron microscopy, 3D-volume reconstructions and TEM tomography. Particularly, the size of active zones (AZs) and the readily releasable, recycling and resting pool of synaptic vesicles (SVs) were quantified.
The majority of SBs contained numerous mitochondria comprising ∼7% of the total volume of SBs, had a large macular, non-perforated AZs (∼0.20 µm2) and were predominantly located on dendritic spines. SBs had a total pool of ∼3500 SVs, a comparably large readily releasable (∼4 SVs), recycling (∼470 SVs) and resting (∼2900 SVs) pool. Astrocytic coverage of SBs suggests both synaptic cross talk or removal of spilled glutamate by astrocytic processes at synaptic complexes.
The structural composition of SBs in L1 may underlie the function of L1 networks that mediate, integrate and synchronize contextual and cross-modal information, enabling flexible and state-dependent processing of feedforward sensory inputs from other layers of the cortical column.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Revised version according to the reviewer's comments without track changes.
