Differentiated Interaction between the Hypothalamic Defence Reaction and Baroreceptor Reflexes

Abstract As the interrelationship of autonomic patterns involving cortico‐hypothalamic and bulbar levels is of general interest, the interaction between the hypothalamic defence reaction and the homeostatic baroreceptor reflexes was explored in cats with respect to cardiac as well as vascular effects. The results suggest, in conformity with Hilton's observations (1963), that defence area stimulation suppresses the baroreceptor reflex inhibition of the heart. On the other hand, the baroreceptor influence on vasoconstrictor fibre discharge was largely the same, whether defence area stimulations were performed or not. This had important haemodynamic consequences, especially for muscle blood flow, since the reflex inhibition of regional constrictor fibre tone could greatly enhance the cholinergic vasodilatation. Therefore such a differentiated interaction between oppositely directed central and reflex autonomic mechanisms has the consequence that the baroreceptor reflexes, rather than damping the defence reaction, act in synergism with it with respect to net haemodynamic effects; some further implications of this are dealt with in the subsequent paper (Kylstra and Lisander 1969).