periaqueductal gray
Acronym: CGMB
The term periaqueductal gray refers to the densely cellular core-structure surrounding the dorsal and lateral aspects of the cerebral aqueduct of the midbrain. Extending 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; it is bounded laterally by the tectospinal tract and the mesencephalic tract of the trigeminal nerve ( Carrive-2012 ). Identified classically by Nissl stain, it is found in the human ( Carpenter-1983 ), macaque ( Martin-2000 ), rat ( Paxinos-2009b ) and the mouse ( Franklin-2008 ).
     The periaqueductal gray is distinguished from the cellular structures ventral to the aqueduct by its homogeneity. Indistinct internal boundaries have been identified on the basis of the shapes and sizes of cells, but they have 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 including, in particular, segmentation on the basis of differential staining for NADPH-diaphorase (nitric oxide synthase) ( Carrive-2012 ). Areas stained with greater or lesser intensity for that enzyme are grouped alternately 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 aqueduct 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 ). See also: motor periaqueductal gray.

Also known as: central (periaqueductal) gray, central gray substance of midbrain, PAG, periaqueductal gray matter, Griseum centrale, Griseum centrale mesencephali, Substantia grisea centralis, Substantia grisea centralis mesencephali


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