FIGURE 1.

Nervous system properties serving cell classification criteria throughout time. This historical timeline presents properties of nervous system cells that served as classification criteria throughout the 20th and 21st centuries. Initial classifications followed morphological features. These were paired with electrophysiological recordings of neurons, but not glia. Soon, the revolution of molecular biology and genetics in model organisms uncovered effector genes of functional modules and regulatory programs of neural cells. Electron microscopy (EM) tracing enabled mapping synaptic connections, providing circuit connectomes. Recently, calcium activity was described in glial cells, implicated in regulating neuronal function. In the last two decades, transcriptomics and specifically single-cell transcriptomics enables classification of transcriptomic cell clusters through dimensionality reduction analysis (UMAP). Combination of these approaches pave the way to multimodal analysis of properties for integrated classifications. Details and relevant citations are found in the text. EM figure kindly provided by G. Rapti, Y. Lu, S. Shaham (unpublished data).