Periaqueductal grey matter
Periaqueductal grey (PAG) matter is a column of grey matter that surrounds the cerebral aqueduct of Sylvius in the midbrain. The periaqueductal grey matter extends from the superior border of the midbrain (which forms the posterior aspect of the floor of the third ventricle), inferiorly towards the fourth ventricle and the superior medullary velum. The mesencephalic nucleus and the mesencephalic root of the trigeminal nerve are found lateral to the periaqueductal grey matter. The oculomotor nucleus and the Edinger–Westphal nucleus are found anterior to the periaqueductal grey matter.
The periaqueductal grey matter participates in the spinomesencephalic tract by way of descending modulation of nociceptive transmission. Recent research has demonstrated that the periaqueductal grey matter is involved in the primary-process emotional networks (those subcortical functions that constitute the primary affective processes in the mammalian brain) and contributes to neural circuits for empathy.