The brain is the basis of the self. We may imagine it to be a relatively unchanging structure, but recent research has shown that the brain is in fact continuously changing its microstructure, and it does so by ‘eating’ itself. The processes of eating things outside the cell, including other cells, is called phagocytosis. In the brain, phagocytosis is performed by a particular type of cell called microglia, which can ‘eat’ neurons (nerve cells) or the connections between neurons (synapses). Microglia engulf neurons and synapses during development in order to sculpt the neural circuits of the brain. Into adulthood, they continue to eat synapses, pathogens and debris in order to shape memory, stop infections and clear accumulating rubbish from the brain. However, too little synaptic pruning by microglia during development may lead to autism; whilst too much eating of synapses during adolescence may contribute to schizophrenia. Too little phagocytosis of debris and protein aggregates or too much eating of synapses and neurons may cause neurodegeneration. A whole host of problems have recently been blamed on excessive phagocytosis in the brain, including aspects of obesity, aging and even sleep deprivation. In this article we will review the evidence, outline what biochemistry can contribute and discuss how to stop the brain eating too much of itself.

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