Quantitative Ultrastructural Analysis of Hippocampal Excitatory Synapses Thomas Schikorski and Charles F. Stevens This study presents a quantitative analysis of hippocampal excitatory synapses in rodents, focusing on the statistical distribution of active zone and postsynaptic density (PSD) sizes, the number of active zones per bouton, and the number of docked vesicles per active zone. The average area of active zones is approximately 0.04 µm², with one active zone per bouton and about 10 docked vesicles per active zone. The total number of vesicles per bouton is approximately 200. These measurements are related to synaptic physiology, with the distribution of active zone areas accounting for the distribution of synaptic release probabilities. The study builds on the work of Katz and early electron microscopists, who identified the structural basis for synaptic transmission. The synaptic vesicle is generally accepted as corresponding to Katz's quantum, although definitive evidence is still lacking. The number of release sites (N_s) associated with an axon has been linked to the total number of releasable vesicles, the number of boutons, and the number of active zones. With the development of optical techniques for studying synaptic transmission and the extension of these techniques to hippocampal neurons in culture, many synaptic properties can now be investigated in culture at the level of single central synapses. Although individual hippocampal synapses differ greatly in various properties, a variety of these properties are highly correlated with the release probability of the synapse. The study characterized rodent hippocampal excitatory synapses both in brain and in culture. Using electron microscopy and quantitative analysis, the researchers determined the statistical distribution of presynaptic properties, including the size of active zones, the number of docked vesicles, and the volume of boutons. The active zone area was found to be linearly related to the volume of the bouton bearing the active zone. The study found that the majority of hippocampal boutons in both brain and culture exhibit only a single active zone. This suggests that the study of single boutons can be reduced to the study of single active zones. The number of release sites (N_s) corresponds to the number of active zones and boutons. The pool of morphologically docked vesicles is approximately 10, and these docked vesicles correspond to the readily releasable pool determined physiologically. The distribution of active zone areas was found to be highly correlated with the number of docked vesicles. The study also found that the release probability is proportional to the number of docked vesicles, which is proportional to the active zone area. This hypothesis was tested and found to be consistent with physiological data. The study provides a detailed quantitative analysis of hippocampal excitatory synapses, revealing the statistical distribution of presynaptic properties and their relationship to synaptic physiology. TheQuantitative Ultrastructural Analysis of Hippocampal Excitatory Synapses Thomas Schikorski and Charles F. Stevens This study presents a quantitative analysis of hippocampal excitatory synapses in rodents, focusing on the statistical distribution of active zone and postsynaptic density (PSD) sizes, the number of active zones per bouton, and the number of docked vesicles per active zone. The average area of active zones is approximately 0.04 µm², with one active zone per bouton and about 10 docked vesicles per active zone. The total number of vesicles per bouton is approximately 200. These measurements are related to synaptic physiology, with the distribution of active zone areas accounting for the distribution of synaptic release probabilities. The study builds on the work of Katz and early electron microscopists, who identified the structural basis for synaptic transmission. The synaptic vesicle is generally accepted as corresponding to Katz's quantum, although definitive evidence is still lacking. The number of release sites (N_s) associated with an axon has been linked to the total number of releasable vesicles, the number of boutons, and the number of active zones. With the development of optical techniques for studying synaptic transmission and the extension of these techniques to hippocampal neurons in culture, many synaptic properties can now be investigated in culture at the level of single central synapses. Although individual hippocampal synapses differ greatly in various properties, a variety of these properties are highly correlated with the release probability of the synapse. The study characterized rodent hippocampal excitatory synapses both in brain and in culture. Using electron microscopy and quantitative analysis, the researchers determined the statistical distribution of presynaptic properties, including the size of active zones, the number of docked vesicles, and the volume of boutons. The active zone area was found to be linearly related to the volume of the bouton bearing the active zone. The study found that the majority of hippocampal boutons in both brain and culture exhibit only a single active zone. This suggests that the study of single boutons can be reduced to the study of single active zones. The number of release sites (N_s) corresponds to the number of active zones and boutons. The pool of morphologically docked vesicles is approximately 10, and these docked vesicles correspond to the readily releasable pool determined physiologically. The distribution of active zone areas was found to be highly correlated with the number of docked vesicles. The study also found that the release probability is proportional to the number of docked vesicles, which is proportional to the active zone area. This hypothesis was tested and found to be consistent with physiological data. The study provides a detailed quantitative analysis of hippocampal excitatory synapses, revealing the statistical distribution of presynaptic properties and their relationship to synaptic physiology. The