Microglial cells in normal mature brains have long been considered to be cells that are resting until pathological events take place, activating the microglial cells. However, it is currently well known that the microglia that have resting ramified morphology in normal mature brains move actively in the brain parenchyma and phagocytose synapses, thus forming and maintaining neural circuits. This review summarizes recent findings on the roles of microglia in mature brains, with special reference to phagocytosis of synapses and higher brain functions. Phagocytic elimination of synapses by microglia may affect the balance between excitatory and inhibitory synaptic transmission, termed the E/I balance. When impaired synaptic elimination by microglia leads to disturbed E/I balance, various problems may follow in brain functions: in memory and cognitive functions, sleep, movement, social behaviors, and thinking. In addition to the roles of microglia in normal developing and mature brains, impaired microglial phagocytosis functions also correlate with disturbances to these higher brain functions that are caused by neurological, mental, and developmental disorders; Parkinson's and Alzheimer's diseases, autism spectrum disorder, attention deficit/hyperactivity disorder, and schizophrenia.

长期以来，正常成熟大脑中的小胶质细胞一直被认为是在病理事件发生之前处于休息状态的细胞，从而激活小胶质细胞。然而，目前众所周知，正常成熟大脑中具有静息分枝形态的小胶质细胞在脑实质和吞噬突触中活动活跃，从而形成和维持神经回路。这篇综述总结了最近关于小胶质细胞在成熟大脑中的作用的研究结果，特别提到了突触的吞噬作用和更高的大脑功能。小胶质细胞对突触的吞噬作用可能会影响兴奋性和抑制性突触传递之间的平衡，称为 E/I 平衡。当小胶质细胞的突触消除受损导致 E/I 平衡紊乱时，大脑功能可能会出现各种问题：在记忆和认知功能、睡眠、运动、社交行为和思维方面。除了小胶质细胞在正常发育和成熟大脑中的作用外，受损的小胶质细胞吞噬功能还与由神经、精神和发育障碍引起的这些高级脑功能障碍有关；帕金森病和阿尔茨海默病、自闭症谱系障碍、注意力缺陷/多动障碍和精神分裂症。