Modern neuroscience, for all its complexity, can trace its roots directly to a series of pen-and-paper sketches rendered by Nobel laureate Santiago Ramón y Cajal in the late 19th and early 20th centuries. His observations and drawings exposed the previously hidden composition of the brain, revealing neuronal cell bodies and delicate projections that connect individual neurons together into intricate networks. As he explored the nervous systems of various organisms under his microscope, a natural question arose: What makes a human brain different from the brain of any other species? At least part of the answer, Ramón y Cajal hypothesized, lay in a specific class of neuron--one found in a dazzling variety of shapes and patterns of connectivity, and present in higher proportions in the human brain than in the brains of other species. He dubbed them the "butterflies of the soul." Known as interneurons, these cells play critical roles in transmitting information between sensory and motor neurons, and, when defective, have been linked to diseases such as schizophrenia, autism, and intellectual disability. Despite more than a century of study, however, it remains unclear why interneurons are so diverse and what specific functions the different subtypes carry out. Now, in a study published in the March 22, 2018 issue of Nature, researchers from Harvard Medical School (HMS), the New York Genome Center, New York University, and the Broad Institute of MIT and Harvard have detailed for the first time how interneurons emerge and diversify in the brain.
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