The organization of the midbrain periaqueductal grey and the integration of pain behaviours

This chapter shows that analgesia evoked from the periaqueductal grey matter (PAG) is best viewed as a component of one or more complex adaptive emotional coping responses. The PAG is organized functionally into four longitudinal columns of neurons. Two distinct forms of analgesia arise from activation specifically of the dorsolateral/lateral PAG columns or the ventrolateral PAG column. A long-acting, opioid-mediated analgesia is a component of a vlPAG-mediated passive coping or conservation-withdrawal reaction that promotes recovery and healing, typically as a response to extreme, inescapable physical stress, including traumatic injury. In contrast, a short-acting, non-opioid-mediated analgesia represents a component of a dlPAG- or lPAG-mediated active coping or defensive reaction to an escapable threat or stress, including acute pain. Anatomical data indicate that each PAG column lies embedded within a distinct forebrain circuit that includes select medial and orbital PFC, hypothalamic and amygdaloid areas.