Periaqueductal gray:

area of grey matter in the midbrain that is involved in a descending pain control pathway. The periaqueductal gray, or PAG, receives pain information via the spinomescencephalic tract and then can activate the endogenous opiate system to suppress pain.

Learn more: Know your brain - Periaqueductal gray

2-Minute Neuroscience: Periaqueductal gray