The term periaqueductal gray (PAG) refers to the densely cellular core-structure surrounding the dorsal and lateral aspects of the cerebral aqueduct (AQU) in the midbrain (MBR). Defined histologically, it is the part of the central gray (CGR) that extends from the epithalamus and posterior hypothalamic area rostrally to the pontine central gray caudally ( Swanson-2004 ). Relative to surrounding structures, it extends approximately from the posterior commissure rostrally to the locus ceruleus caudally. It is bounded dorsally by the commissure of the inferior colliculus and commissure of the superior colliculi; and is bounded laterally by the tectospinal tract and the mesencephalic tract of the trigeminal nerve ( Carrive-2012 ). The PAG is distinguished from cellular structures ventral to the AQU by its homogeneity. It is found in the human ( Carpenter-1983 ), macaque ( Martin-2000 ), rat ( Paxinos-2009b ) and the mouse ( Franklin-2008 ). Indistinct boundaries within PAG have been identified on the basis of the shapes and sizes of cells. That has resulted in several different segmentations within and across species. The simplest segmentation reported to show a similar pattern across species is based on multiple criteria ( Carrive-2012 ). By that criterion, areas stained with greater or lesser intensity are grouped into four longitudinal columns. In cross-section each column is narrowest at the aqueductal surface and widens toward the outer surface. The surface abutting the AQU is lined by ependyma ( Paxinos-2012). The four columns are the dorsomedial periaqueductal gray, dorsolateral periaqueductal gray, lateral periaqueductal gray, and ventrolateral periaqueductal gray. Other substructures include the intercalated periaqueductal gray, supraoculomotor cap, pleioglia periaqueductal gray, and the nucleus of Darkshevich; some authors consider the supraoculomotor nucleus a component ( Paxinos-2001; Paxinos-2009a; Paxinos-2009b; Paxinos-2012 ). Authors differ as to whether these other structures are to be regarded as parts of the longitudinal columns or as independent structures ( Carrive-2012; Paxinos-2012 ). Functionally the periaqueductal gray belongs to the behavior control column of the motor system ( Swanson-2004 ) and the brainstem motor control system (BSM) of the motivational control system and the instinctive control system ( Bowden-2021). See also: motor periaqueductal gray. Updated 6 Nov 2025.