FUNCTIONAL-CHARACTERISTICS OF THE MIDBRAIN PERIAQUEDUCTAL GRAY

The major functions of the midbrain periaqueductal gray (PAG), includi ng pain and analgesia, fear and anxiety, vocalization, lordosis and ca rdiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferen ts from nociceptive neurons in the spinal cord and sends projections t o thalamic nuclei that process nociception. The PAG is also a major co mponent of a descending pain inhibitory system. Activation of this sys tem inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety pr oduced by stimulation of amygdala. Stimulation of PAG produces vocaliz ation and its lesion produces mutism. The firing of many cells within the FAG correlates with vocalization. The PAG is a major site for lord osis and this role of PAG is mediated by a pathway connecting the medi al preoptic with the PAG. The cardiovascular controlling network withi n the PAG are organized in columns. The dorsal column is involved in p resser and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the curent information about the anatomy, physiology, and pharmacology of the PAG, a model is propo sed to account for the interactions between these different functional components.