Professor Bruce McEwen describes the blood-brain barrier, which prevents most proteins from accessing the brain. Selective proteins can cross the barrier, instigating processes such as neurogenesis.

The blood-brain barrier is an interesting concept because it originally really was thought to exclude many molecules that we now know are even selectively transported into the brain. In general, cells do not get into the brain, except under some circumstances where immune cells in damage will actually get into the brain. Protein molecules, large ones, generally don’t get into the brain, but smaller proteins, like insulin, actually are selectively transported in the brain. Perhaps one of the most dramatic examples is that circulating insulin-like growth factor, which is a hormone produced by the liver, actually is actively taken up by the brain, and stimulates the neurogenesis that occurs in the dentate gyrus and the hippocampus, caused by voluntary exercise. So here, when we start exercising – go out and run or something – this hormone is actually helping to generate new nerve cells in the hippocampus.

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