Like bouncers at a night club, an assembly of cells in the brain's blood system, called the blood-brain barrier, lets only a few molecules into the nervous system's inner sanctum - the brain.

就像夜总会门口的保镖一样，在大脑的血液系统中也有这样一组细胞叫做血-脑屏障，它们只允许少数分子进入神经系统的密室——大脑。