Quantitative Ultrastructural Analysis of Hippocampal Excitatory Synapses

Abstract

From three-dimensional reconstructions of CA1 excitatory synapses in the rodent hippocampus and in culture, we have estimated statistical distributions of active zone and postsynaptic density (PSD) sizes (average area ∼0.04 μm²), the number of active zones per bouton (usually one), the number of docked vesicles per active zone (∼10), and the total number of vesicles per bouton (∼200), and we have determined relationships between these quantities, all of which vary from synapse to synapse but are highly correlated. These measurements have been related to synaptic physiology. In particular, we propose that the distribution of active zone areas can account for the distribution of synaptic release probabilities and that each active zone constitutes a release site as identified in the standard quantal theory attributable to Katz (1969).