Periaqueductal gray

Periaqueductal Gray (PAG; also called the "central gray") is the midbrain grey matter that is located around the cerebral aqueduct within 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. The spinomesencephalic tract is so-named because the fibers orginate in the spine and terminate in the mesencephalon, another name for the midbrain, which is the part of the brain in which the PAG resides.

Role in Analgesia
Stimulation of the periaqueductal grey matter of the midbrain results in release of serotonin in the dorsal horn of the spinal cord, profound analgesia, activation of interneurons containing enkephalin, and release of norepinephrine (also known as noradrenaline) in the dorsal horn of the spinal cord.

Role in Defensive Behavior
Stimulation of the dorsal and lateral aspects of the PAG [in the rat] can provoke defensive responses characterised by freezing, running, jumping, tachycardia and increases in blood pressure and muscle tonus. Conversely, stimulation of the caudal ventrolateral PAG can result in an immobile, relaxed posture known as quiescence.

Lesions of the caudal ventrolateral PAG can greatly reduce conditioned freezing, while lesions of the dorsal aspect can reduce innate defensive behavior, virtually "taming" the animal.

Role in Reproductive Behavior
Neurons of the PAG are excited by endorphins and by opiate analgesics. It also plays a role in female copulatory behavior (see Lordosis behavior) via a pathway from the ventromedial nucleus of the thalamus.