Psychiatry faces the most difficult problem in medicine, because its organ of interest, the brain, is by orders of magnitude the most complex of the body. The human brain is composed of an intricate network of perhaps 200 billion cells (neurons and glia) and 30 trillion synaptic connections.

Moreover, brain cells are diverse and vary markedly both within and between brain regions; the anatomical complexity of other organs, such as the kidney, liver, or lung, pales in comparison. Diversity of structure is accompanied by diversity of function; the myriad functions of the brain are associated with specialized anatomical structures and circuits that are each composed of many different cell types.

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