Inescapable and escapable pain is represented in distinct hypothalamic-midbrain circuits:: specific roles for Aδ- and C-nociceptors

The affective responses to pain arising from deep somatic and visceral tissues differ markedly from those evoked by brief cutaneous insults. Deep pain evokes passive emotional coping that includes quiescence and vasodepression. In contrast, cutaneous pain evokes an active emotional coping: the fight or flight response. There is now considerable evidence to support the notion that nociceptive inputs arising from different peripheral domains drive the different functional columns of the periaqueductal grey (PAG) that co-ordinate either active or passive coping strategies. Nociceptive inputs from deep structures drive neurones in the ventrolateral columns that co-ordinate passive emotional coping whereas brief cutaneous insults activate the dorsolateral/lateral columns that co-ordinate active coping strategies. An emerging concept, as presented in the preceding article by Keay & Bandler, is that it is the behavioural significance of the nociceptive input, rather than its organ of origin per se, that determines the characteristics of the affective response. These authors provide evidence that brief, escapable stimuli activate neurones in the dorsolateral/lateral columns of the PAG and that inescapable, persistent pain, irrespective of its organ of origin, activates the ventrolateral column. This review will present recent evidence that differential representation of escapable and inescapable pain in the PAG extends to distinct representations of 'first' and 'second' pain, as indicated by the columnar distribution of neurones activated by inputs from Adelta- and C-nociceptors. Furthermore, the functional organisation of projections from circumscribed regions of the hypothalamus to the different columns of the PAG indicates that the behavioural significance of the pain signal is represented in brain regions other than the PAG.