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The morphology of excitatory central synapses: from structure to function

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Sätzler,  Kurt
Department of Cell Physiology, Max Planck Institute for Medical Research, Max Planck Society;

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Bollmann,  Johann H.
Department of Cell Physiology, Max Planck Institute for Medical Research, Max Planck Society;
Department of Biomedical Optics, Max Planck Institute for Medical Research, Max Planck Society;

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Borst,  J. Gerard G.
Department of Cell Physiology, Max Planck Institute for Medical Research, Max Planck Society;

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Sakmann,  Bert
Department of Cell Physiology, Max Planck Institute for Medical Research, Max Planck Society;

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Citation

Sätzler, K., Söhl, L. F., Bollmann, J. H., Borst, J. G. G., Frotscher, M., Sakmann, B., et al. (2002). The morphology of excitatory central synapses: from structure to function. The Journal of Neuroscience: the Official Journal of the Society for Neuroscience, 22(24), 10567-10579. doi:0270-6474/02/2210567-13.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0029-7958-C
Abstract
The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted ∼35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised ∼5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 μm2, and the nearest neighbors were separated, on average, by 0.59 μm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of ∼60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were “anatomically docked.” Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) ∼35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.