A quantitative study of synaptic patterns in turtle visual cortex

Abstract

The distribution of different synaptic types has been analyzed quantitatively in the molecular and subcellular layers of turtle „vislual”︁ cortex. The density of vesicle‐containing‐profiles (VCPs) is 122.2/400 square μm in the molecular layer and 61.0/400 square μm in the subcellular layer. In both layers, approximately 85% of VCPs conatin round vesicles (RVCPs), 12%, flat vesicles (FVCPs) and 1%, dense core vesicles (DVCPs). Vacuolar invaginations (VIs) are not uncommon in the molecular layer (3.4%) but are rare in the subcellular layer (0.6%). Synaptic conatacts are formed in the plane of section by three out of ten RVCPs and FVCPs, while only one out of the 146 DVCPs sampled in this study was associated with a membrane differentiation. In the molecualr layer, the percentage distribution of the different subgroups of synaptic types is as follows: round‐asymmetrical contacts, 82.5% (77.5% on dendritic spines and 5.0% on dendritic shafts); round‐symmetrical contacts, 3.8% (2.3% on spines and 1.5% on shafts);flat‐symmetrical contacts, 13.1% (8.8% on shafts and 4.3% on spines; flatasymmetrical contacts 0.6% (0.4% on spines and 0.2% on shafts)). In the subcellular layer, the distribution is quite different: round‐asymmetrical contacts. 72.4% (38.6% on shafts and 33.8% on spines); round‐symmertical contacts, 12.8% (8.2% on shafts and 4.6% on spines); flat‐symmetrical contacts, 13.9% (9.3% on shafts and 4.6% on spines); flat‐asymmetrical contacts, 0.9% all on dendritic shafts. The density distribution of VCPs in the molecular layer changes with depth. RVCPs are fewer on the distal third of the apical dendrites, and most numerous in the middle third with a small dercease in number in th proximal third. FVCPs are more homogeneously distributed, perhaps increasing slightly in number towards the proximal parts of the dendritets. RVCPs are most numerous on dendritic spines. These spines are of two types whose relative number varies with depth. Large organelle‐filled spines predominate in the upper third while small „empty”︁ spines are most numerous in the lower third of the molecular layer.