The periaqueductal gray-rostral medulla connection in the defence reaction: efferent pathways and descending control mechanisms.

Neuronal systems controlling cardiovascular components of emotional responses must have the capacity to generate different patterns of response and must also be able to modify those patterns in response to changes in environmental circumstances. Using the cardiovascular "defence" response as a model, evidence is presented to show that sympathetic premotor neurons of the rostral ventrolateral medulla (RVLM) possess such properties. Neurones in the RVLM act as relays in the descending efferent pathway to the sympathetic outflows from the dorsal periaqueductal gray matter (dPAG) which integrates the characteristic "defensive" pattern of cardiovascular response that accompanies activation of the midbrain aversive system. Activity in this pathway can be modulated, at the level of the RVLM, by a descending pathway which originates in the ventrolateral PAG. It is suggested that both the dorsolateral and the ventrolateral control systems in the PAG become activated during periods of physical or emotional stress, particularly those which involve sustained motor activity. Activity in the dorsal system initiates cardiovascular components of aversive/defensive behaviour whilst the ventrolateral system plays an important role in initiating the recuperative phase of behaviour characterised by sympathoinhibition, muscular relaxation and immobility which follows a stressful encounter.