THE PERIAQUEDUCTAL GRAY ROSTRAL MEDULLA CONNECTION IN THE DEFENSE REACTION - EFFERENT PATHWAYS AND DESCENDING CONTROL MECHANISMS

Neuronal systems controlling cardiovascular components of emotional re sponses must have the capacity to generate different patterns of respo nse and must also be able to modify those patterns in response to chan ges in environmental circumstances. Using the cardiovascular ''defence '' response as a model, evidence is presented to show that sympathetic premotor neurones 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 periaque ductal gray matter (dPAG) which integrates the characteristic ''defens ive'' pattern of cardiovascular response that accompanies activation o f the midbrain aversive system. Activity in this pathway can be modula ted, at the level of the RVLM, by a descending pathway which originate s in the ventrolateral PAG. It is suggested that both the dorsolateral and the ventrolateral control systems in the PAG become activated dur ing periods of physical or emotional stress, particularly those which involve sustained motor activity. Activity in the dorsal system initia tes cardiovascular componens of aversive/defensive behaviour whilst th e ventrolateral system plays an important role in initiating the recup erative phase of behaviour characterised by sympathoinhibition, muscul ar relaxation and immobility which follows a stressful encounter.