The periaqueductal grey (PAG) is the grey matter that surrounds the cerebral aqueduct within the tegmentum of the midbrain. It is the primary control centre for the brain’s descending modulation of pain, and organises defensive responses to threat. [1]
The 3D brain needs JavaScript. The text below still says where the periaqueductal grey is and what it does.
The midbrain and cerebral aqueduct are lit together; the periaqueductal grey is not drawn on its own. The side (sagittal) slice button cuts through them. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
Not a separate shape in this atlas; lit: the part that holds it. Lit: the whole midbrain and cerebral aqueduct. The periaqueductal grey is the sleeve of grey matter around the cerebral aqueduct inside the midbrain; it has no shape of its own in the BodyParts3D model, so the midbrain that holds it and the aqueduct it surrounds are lit and nothing is drawn at a guessed point. [1][2]
Lit here: the midbrain and the cerebral aqueduct, the parts that hold the periaqueductal grey — see each on its own page.
It is the grey matter located around the cerebral aqueduct, within the tegmentum of the midbrain. [1]
The cerebral aqueduct it surrounds is the narrow canal through the midbrain that carries cerebrospinal fluid from the third ventricle down to the fourth ventricle. [2]
What it does
Pain modulation: it is the primary control centre for descending pain modulation, and has enkephalin-making cells that turn pain signals down. [1]
How the turning-down works: its neurons project to the raphe nuclei of the brainstem, whose serotonin fibres run down to the spinal cord’s dorsal horn and excite small inhibitory neurons there that damp incoming pain signals before they travel up to the thalamus. [1][3]
Defence: its dorsal and lateral parts are active during defensive behaviour (freezing, running, a faster heart, higher blood pressure), while its caudal ventrolateral part is tied to a still, quiet state. [1]
Voice: it is a midbrain hub that starts and gates vocal calls; it enables calling rather than shaping the fine sound of each call. [1]
Connections
Input: pain and temperature fibres of the spinothalamic system reach it by the spinomesencephalic pathway. [1]
Output: it projects to the nucleus raphe magnus in the medulla and carries descending autonomic tracts; it also has direct links with the orbitofrontal cortex. [1]
Other names
Abbreviated PAG; spelled periaqueductal gray in American English, and also called the central grey (central gray). [1]
Size
The model has no shape for the periaqueductal grey itself, so it is not measured here. The midbrain that holds it measures about 29 mm front to back, 17 mm top to bottom and 29 mm side to side on this model (the box it fits in, not its volume). [4]
The model has no shape for the periaqueductal grey itself, so it is not measured here. The cerebral aqueduct that holds it measures about 7 mm front to back, 18 mm top to bottom and 3 mm side to side on this model (the box it fits in, not its volume). [4]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.