Connectomics is the area of neuroscience that aims to collect and curate the entirety of the connections made by all neurons in a brain (the product being called a “connectome”). For the human brain, that would be a data set of 100 billion neurons, each of which is estimated to make 1000-10000 synapses with other neurons (on the order of 1017 connections). The roll-up-your-sleeves-this-will-get-really-messy way of collecting that kind of data is to slice the brain into nanometers-thick sections and to image each slice with an electron-microscope, which has resolution below the nanometer range, and can reveal the structure of cells on a fine scale. In the image from Kristen Harris's lab below you can see a part of a neuron’s dendrite making a synapse with an axon filled with neurotransmitter vesicles; EM images however cannot show individual proteins or molecules).