The effect of brain transection on the response to forced submergence in ducks.

The effect of brain transection at two levels on cardiovascular responses to forced submergence has been investigated in ducks. Compared with intact ducks, neither decerebration nor brain stem transection at the rostral mesencephalic (RM) level had any effect on development of diving bradycardia, or heart rate at the end of two-min dives. Arterial blood pressure was maintained in brain transected ducks as well as in intact ducks. Furthermore, end-dive arterial blood gases and pH were also similar in intact and brain transected ducks confirming that the oxygen sparing cardiovascular adjustments, involving a massive increase in total peripheral resistance, were unimpaired by brain transection. In this respect, ducks with RM transections tolerated four-min dives. However, the increase in post-dive VE seen in intact and decerebrated ducks was prevented by RM transection. We conclude that control of the circulatory response to diving resides in the lower brainstem, is reflexogenic in nature, and does not depend on the cognitive perception of 'fearful' stimuli.