The periaqueductal gray (PAG) (also known as the central gray) is the primary control center for descending pain modulation. It has enkephalin-producing cells that suppress pain.

The periaqueductal grey matter is the grey matter located around the cerebral aqueduct within the tegmentum of the midbrain. It projects to the nucleus raphe magnus, and also contains descending autonomic tracts. The ascending pain and temperature fibers of the spinothalamic tract send information to the PAG via the spinomesencephalic tract (so-named because the fibers originate in the spine and terminate in the PAG, in the mesencephalon or midbrain).