title: Three-dimensional reconstruction of a calyx of Held and its postsynaptic principal neuron in the medial nucleus of the trapezoid body.
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 approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. 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 approximately 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) approximately 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.
authors.affiliation: Department of Cell Physiology, Max Planck Institute for Medical Research, D-69120 Heidelberg, Germany.
authors.forename: Kurt
authors.initials: K
authors.lastname: Sätzler
authors.forename: Leander F
authors.initials: LF
authors.lastname: Söhl
authors.forename: Johann H
authors.initials: JH
authors.lastname: Bollmann
authors.forename: J Gerard G
authors.initials: JG
authors.lastname: Borst
authors.forename: Michael
authors.initials: M
authors.lastname: Frotscher
authors.forename: Bert
authors.initials: B
authors.lastname: Sakmann
authors.forename: Joachim H R
authors.initials: JH
authors.lastname: Lübke
mesh_terms: D000818:Animals; D001933:Brain Stem / Q000166:cytology* / Q000502:physiology / Q000648:ultrastructure*; D048429:Cell Size; D017626:Cochlear Nucleus / Q000166:cytology / Q000648:ultrastructure*; D019706:Excitatory Postsynaptic Potentials; D021621:Imaging, Three-Dimensional; D008854:Microscopy, Electron / Q000379:methods; D009474:Neurons / Q000648:ultrastructure*; D009847:Olivary Nucleus / Q000166:cytology; D051381:Rats; D017208:Rats, Wistar; D017470:Receptors, Glutamate / Q000378:metabolism; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure*; D013570:Synaptic Membranes / Q000648:ultrastructure; D013572:Synaptic Vesicles / Q000648:ultrastructure
chemical_list: D017470:Receptors, Glutamate