Periaqueductal gray (PAG; also called the "central gray") is the gray matter located around the cerebral aqueduct within the tegmentum of the midbrain. It plays a role in the descending modulation of pain and in defensive behaviour. The ascending pain and temperature fibers of the spinothalamic tract also 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).
Read more about Periaqueductal Gray: Role in Analgesia, Role in Defensive Behavior, Role in Reproductive Behavior, Role in Consciousness, Additional Images
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