8 . Baroreceptor activity and nociception

The arterial baroreceptors contribute to stabilize the arterial blood pressure: Increases in blood pressure lead to enhanced baroreceptor activation. This dampens sympathetic arousal and stimulates the n.vagus. As a consequence, cardiac contractility and heart rate will be lowered reflexively and blood pressure will drop to normal levels. Decreases in blood pressure, on the other hand, lead to reduced baroreceptor activity, the sympathetic nervous system is activated while the parasympathetic branch becomes inhibited, cardiac output and in turn blood pressure increases. Apart from these homoestatic effects of baroreceptor activation, accumulating evidence suggests that baroreceptors may have considerable impact on higher brain structures and thus also on behavioral processes. Such extrahomeostatic effects seem to involve more global changes in brain states like changes in the activation level or neural excitability and on activation and processing of aversive environs on the behavioral side. This chapter will concentrate on laboratory experiments investigating the latter aspect, while the fonner has been dealt with in the chapter by Elbert and Rau (this volume). The following is a brief chronological summary of the literafure on the extrahomoeostatic effects of the baroreceptors: Koch [12] detected decreases in the activation level in dogs after mechanical stimulation ofthe arterial carotid baroreceptors. Gellhorn, Yesinick, Kessler, and Hailman [11] demonstrated that convulsions in dogs and cats could be abolished through the activation of arterial baroreceptors. Bonvallet, Dell, and Hiebel [2] reported EEG synchronizing as response to mechanical stimulation ofthe carotid baroreceptors in animals.