CAROTID SINUS REFLEXES AND CONVULSIONS

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Baroreceptor stimulation: pain perception and sensory thresholds

Baroreceptor activity has been implicated in the modulation of pain. Sensory detection thresholds and pain ratings were measured in a group of 28 men during carotid baroreceptor manipulation with the PRES (phase-related external suction) neck suction technique. Brief, cardiac phase-related electrical impulses were delivered intracutaneously to the finger. The results indicate that minimum baroreceptor activity was associated with more severe pain, but had no effect on sensory detection threshold. The results are discussed in terms of the learned model of hypertension.

Cigarette smoking, blood lipids, and baroreceptor-modulated nociception

Activation of arterial blood pressure has been shown to influence higher central nervous activity. In animals, induction of sleep-like states and increases of seizure and pain thresholds in response to baroreceptor stimulation have been reported. In certain human groups, mechanical stimulation of the carotid baroreceptors also increases pain thresholds. The present paper examines the hypothesis that smokers show baroreceptor dependent antinociception as compared to non-smokers. It is speculated that one effect which rewards smoking is the nicotine induced phasic blood pressure increase which leads to baroreceptor stimulation and dampens pain perception. One hundred and twenty subjects were investigated using a recently developed mechanical baroreceptor stimulation technique and an electrical pain stimulus. The group of heavy smokers showed the predicted effect: their pain thresholds were enhanced during conditions of increased baroreceptor activity as compared to the control condition. The group of medium, light and non-smokers, however, did not show this effect. Neither blood lipid levels nor diastolic or systolic blood pressure paralleled the group differences on baroreceptor dependent antinociception. In heavy smokers, the nicotine induced phasic blood pressure increases might have baroreceptor dependent pain dampening effects, which might be among the reinforcing qualities of smoking.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Anaesthesia and cardiovascular regulation

Cardiovascular homeostasis is dependent on the efficient performance of the effector organs, i.e. the vascular smooth muscle and the heart. Besides inherent activity and local control mechanisms, these effector organs are regulated by circulatory control centres within the central nervous system, which in turn receives information from receptors inside and outside the cardiovascular system. All these components of the circulatory systems, i.e. receptors, afferent and efferent pathways, control centres and effector organs, are possible sites for interactions by anaesthetics. Since different anaesthetics have different potencies and special predilections, there are a large variety of interaction patterns, as is discussed in the paper. Another way of evaluating circulatory effects of drugs used in anaesthesia is to analyse how these drugs may modify circulatory reflexes associated with surgery and trauma. For example, pain, hypoxia and/or hypovolaemia may evoke circulatory adjustments which correspond to and are functionally related to, from experimental physiology, well-known reflex patterns such as the somatosympathetic reflex, the chemoreceptor reflex and the baroreceptor reflex. These reflex adjustments are liable to modification by anaesthetics, as exemplified in the paper. Due to the complexity of circulatory control and the varying effects of different anaesthetic agents, it is difficult to draw general conclusions. It can, however, be stated that most general anaesthetics depress cardiovascular reflexes in proportion to the depth of anaesthesia, and that suprabulbar centres are more easily depressed than bulbar ones. Opiates seem to have a specific inhibitory effect on circulatory adjustments induced by noxious stimuli. Transmission in efferent and afferent pathways is liable to modification by local anaesthetics, ganglionic blockers or alpha- and beta-receptor antagonists.

Effects of baroreceptor activation on spontaneous activity in the sweat glands and nictitating membrane of the cat

(1) In chloralose-anesthetized cats, elevation of carotid sinus pressure caused blood pressure, sweat gland potentials and nictitating membrane tension to decrease. (2) The onset and recovery of the sweat gland and nictitating membrane responses usually preceded the respective phases of the blood pressure (depressor) response; the latencies of the sweat gland and nictitating membrane responses agreed with the latencies predicted for neural reflex pathways. (3) The sweat gland and nictitating membrane responses were evoked less consistently than the depressor response. (4) In experiments where only the sweat gland potentials and blood pressure were studied: (a) the sinus pressure threshold for inhibition of sweat gland activity was similar to the threshold for the depressor response; (b) cutting the sinus nerves, or blocking the efferent neural activity to the sweat glands, eliminated the effects of sinus pressure elevation on the sweat gland potentials; (c) with carotid sinus pressure held constant, decreases in blood pressure, produced by stimulating the peripheral end of the vagus nerve, did not affect the sweat gland potentials. (5) These results indicate that baroreceptors can reflexly modulate activity in sympathetic neurons whose target organs are not fundamentally involved in blood pressure regulation.