PT - JOURNAL ARTICLE AU - Alexander Shapson-Coe AU - Michał Januszewski AU - Daniel R. Berger AU - Art Pope AU - Yuelong Wu AU - Tim Blakely AU - Richard L. Schalek AU - Peter H. Li AU - Shuohong Wang AU - Jeremy Maitin-Shepard AU - Neha Karlupia AU - Sven Dorkenwald AU - Evelina Sjostedt AU - Laramie Leavitt AU - Dongil Lee AU - Luke Bailey AU - Angerica Fitzmaurice AU - Rohin Kar AU - Benjamin Field AU - Hank Wu AU - Julian Wagner-Carena AU - David Aley AU - Joanna Lau AU - Zudi Lin AU - Donglai Wei AU - Hanspeter Pfister AU - Adi Peleg AU - Viren Jain AU - Jeff W. Lichtman TI - A connectomic study of a petascale fragment of human cerebral cortex AID - 10.1101/2021.05.29.446289 DP - 2021 Jan 01 TA - bioRxiv PG - 2021.05.29.446289 4099 - http://biorxiv.org/content/early/2021/06/16/2021.05.29.446289.short 4100 - http://biorxiv.org/content/early/2021/06/16/2021.05.29.446289.full AB - We acquired a rapidly preserved human surgical sample from the temporal lobe of the cerebral cortex. We stained a 1 mm3 volume with heavy metals, embedded it in resin, cut more than 5000 slices at ∼30 nm and imaged these sections using a high-speed multibeam scanning electron microscope. We used computational methods to render the three-dimensional structure of 50,000 cells, hundreds of millions of neurites and 133.7 million synaptic connections. The 1.4 petabyte electron microscopy volume, the segmented cells, cell parts, blood vessels, myelin, inhibitory and excitatory synapses, and 100 manually proofread cells are available to peruse online. Despite the incompleteness of the automated segmentation caused by split and merge errors, many interesting features were evident. Glia outnumbered neurons 2:1 and oligodendrocytes were the most common cell type in the volume. The E:I balance of neurons was 69:31%, similar to the ratio of excitatory versus inhibitory synapses in the volume, which was 63:37%. The E:I ratio of synapses was significantly higher on pyramidal neurons than inhibitory interneurons. We found that deep layer excitatory cell types can be classified into subsets, based on structural and connectivity differences, that chandelier interneurons not only innervate excitatory neuron initial segments as previously described, but also each other’s initial segments. Furthermore, among the thousands of weak connections established on each neuron, there exist rarer highly powerful axonal inputs that establish multi-synaptic contacts (up to ∼20 synapses) with target neurons. Our analysis indicates that these strong inputs are specific, and allow small numbers of axons to have an outsized role in the activity of some of their postsynaptic partners.Competing Interest StatementThe authors have declared no competing interest.