Brain tissue comprises an extremely complex arrangement of cells that together make up the information- processing network enabling brain function. The connectivity of neurons underlies the unparalleled capabilities of our mind. Thus, mapping brain structure, which underlies brain function, has become a central focus in neuroscience.Electron microscopy provides extremely high resolution and comprehensive visualization of brain structure but requires correlative workflows to access molecular information. Light microscopy holds tremendous potential to analyze the ultrastructure of brain tissue together with its molecular makeup. However, conventional light microscopy (LM) provides limited resolution (~ 200 nm laterally and ~ 1000 nm axially), far too coarse to precisely locate specific molecular players within sub-micrometer-sized structures, such as synapses.We have developed an optical imaging approach based on high-fidelity hydrogel expansion to visualize even the finest of neuronal structures, such as axons and dendritic spines when imaging with diffraction- limited confocal microscopy, unveiling neuronal structure together with molecular information. This enables dense segmentation of neuronal structures using deep-learning based segmentation and determining neuronal connectivity at single-synapse resolution, which we showcase in cortex and hippocampus. Our technology will help shed light on brain structure, connectivity and molecular composition in a readily adoptable manner.