Fig. 2: Diversity of synaptic covariates and mapping degeneracy.

Examples of synaptic signals mined from peer-reviewed studies (solid lines: blue, GABAergic; gold, glutamatergic) and corresponding digitized reconstruction (dotted lines). Each signal is mapped to the possible presynaptic and postsynaptic neuron types (schematic morphologies; SL, lucidum). We only illustrated the most likely neuron types. The green arrows point to all possible mappings for every signal into the matrix of 3,120 potential connections (rows: presynaptic, columns: postsynaptic) among 122 neuron types. Blue and gold brightness in the connectivity matrix indicate the number of available experimental recordings. Light pink entries are potential connections with missing synaptic data. Black entries mark the absence of potential connection. The icons illustrate a sample of experimental covariates: species, age, sex, recording temperature and modality, and relative intracellular anionic concentrations. a Recording between a pair of dentate gyrus (DG) MOLAX or DG Total Moleculare Layer interneurons⁷⁵. b signal from a DG Granule cell (or CA3 Granule, DG Semilunar Granule, or DG Hilar Ectopic Granule cell) to a CA3 Basket CCK+ cell⁷⁶. c signal from a CA3 Pyramidal (or CA3c Pyramidal cell) to a CA3 Trilaminar (or CA3 Interneuron Specific Oriens) cell⁷⁷. d signal from a CA1 Basket CCK+ (or CA1 Radial Trilaminar, CA1 Oriens/Alveus, or CA1 Schaffer Collateral-Associated) cell to a CA1 Pyramidal cell⁷⁸. e signal from a CA1 O-LM cell to a CA1 Neurogliaform cell⁷⁹. f signal from an entorhinal cortex (EC) LII Basket-Multipolar (or EC LII Axo-Axonic or medial EC LII Basket) cell to a medial EC LII Stellate neuron⁸⁰. g signal from a CA1 Pyramidal cell to a CA1 O-LM (or CA1 Recurrent O-LM, or CA1 O-LMR) cell⁸¹.