Cardiovascular Receptors. In: Neil E, editor. Enteroceptors. Berlin: Springer Berlin Heidelberg; 1972. pp. 1-45. [DOI: 10.1007/978-3-642-65252-3_1]

The effects of vibrotactile masking on heartbeat detection: Evidence that somatosensory mechanoreceptors transduce heartbeat sensations

The ability to detect heartbeat sensations is the most common basis for inferring individual differences in sensitivity to the interoceptive stimuli generated by the visceral activity. While the sensory sources of heartbeat sensations have yet to be identified, there is a growing consensus that visceral sensation, in general, is supported not only by the interoceptive system but also by the somatosensory system, and even by exteroception. The current experiment sought evidence on this issue by exploring the effects of masking the functions of somatosensory Pacinian and non-Pacinian mechanoreceptors on the ability to detect heartbeat sensations. Twelve verified heartbeat detectors completed a multi-session experiment in which they judged heartbeat-tone and light-tone simultaneity under two vibrotactile masking conditions involving the stimulation of the sternum: (a) using 250 Hz vibrotactile stimuli to mask the Pacinian channel, and (b) using 6 Hz vibrotactile stimuli to mask the non-Pacinian channel. A no-vibration control condition in which no masking stimuli were presented was also implemented. Presentation of both the 250 Hz and the 6 Hz masking stimuli impaired the ability to judge the simultaneity of heartbeats and tones but did not influence the ability to judge the simultaneity of stimuli presented to different exteroceptive modalities (lights and tones). Our findings reinforce the view that the somatosensory system is involved in cardioception and support the conclusion that both Pacinian and non-Pacinian somatosensory mechanoreceptors are implicated in heartbeat detection.

Coronary artery baroreceptor-mediated changes in arterial pressure: a pilot study in conscious and anaesthetized sheep

1. Evidence suggesting the presence of coronary artery baroreceptors on coronary arteries has existed for over 30 years. 2. Evidence that activation of ventricular mechanoreceptors can elicit cardiovascular changes has been challenged, with those changes now thought to be due to coronary artery mechanoreceptors. 3. Studies have suggested that coronary artery mechanoreceptors act as coronary baroreceptors with a role in cardiovascular regulation. However, all evidence to date has been obtained in anaesthetized animal models in physiologically compromised intra-operative states. 4. The purpose of the present study was to design an ovine model that would allow the discrete stretch of coronary arteries without causing ischaemia or changing flow or intra-arterial pressure and that would confirm results seen in previous studies. In addition, the possibility that the technique could be used for studies of coronary artery baroreflexes in conscious sheep was investigated. 5. Controlled stretch of the proximal left anterior descending coronary artery elicited decreases in arterial pressure without changes in heart rate or electrocardiographic activity in halothane-anaesthetized sheep. Similar results were demonstrated in conscious sheep after surgical recovery of up to 2 weeks. 6. The present study supports the possibility that coronary artery baroreceptors exist and likely have a role in cardiovascular regulation. The results of the present study in anaesthetized sheep are in agreement with previous results in anaesthetized animals, but also provide the first demonstration of coronary baroreceptor activity in a conscious animal model, underscoring the potential use of the model in the study of coronary artery baroreceptors in the intact animal.

Topography and morphology of heart action-related EEG potentials

Joint ECG and EEG measurements were performed in 22 healthy subjects under standardized laboratory conditions. Averaged EEG potentials were computed using the R-peaks in the ECG as reference events. Spatio-temporal potential patterns of heart action-related EEG activity were obtained from 26 scalp channels. A heart action-related positive potential was found, peaking over the parietal scalp regions. Its independence from the cardiac electrical field, the source of an EEG artifact that may be confounded with heart action-related brain potentials, is demonstrated. The potential reaches its maximum amplitude of about 0.5 microV at a latency of about 500 ms after the R-peak. Its topography, with peak amplitudes at the parietal electrode locations, is different from the topography of potentials observed in the few comparable experimental studies published so far. This suggests the presence of somatosensory-evoked components in heart action-related potentials and indicates that a renewed discussion of the underlying neuronal processes is necessary.

Cardiac receptors: their function in health and disease

Cardiac receptors include both mechanically and chemically sensitive receptors located in atria and in ventricles. Atrial receptors innervated by myelinated vagal afferent fibers reflexly regulate heart rate and intravascular volume. On the other hand, stimulation of ventricular receptors can cause either reflex bradycardia and hypotension or, alternatively, excitation of the cardiovascular system. The former response is mediated by vagal afferents, whereas the latter is mediated by sympathetic (spinal) afferents. Under normal circumstances, cardiac receptors sense changes in wall motion or diastolic pressure and perhaps provide a fine tuning of the cardiovascular system. However, under certain pathological conditions such as coronary ischemia, which cause release of substances such as bradykinin and prostaglandins, there is an exaggerated response of the ventricular receptors. Because these receptors cause a reflex depression of the cardiovascular system and, in particular, induce renal vasodilation, they may protect the heart and kidney by lessening myocardial oxygen requirements and by increasing renal blood flow. In the situation of heart failure both atrial and ventricular receptors are reset and therefore provide for an exaggerated neurohumoral discharge. Finally, patients with aortic stenosis may demonstrate a paradoxical vasodilation and syncope during exercise when there likely is excessive stimulation of left ventricular receptors by the high transmural pressure.

Behavior of left ventricular mechanoreceptors with myelinated and nonmyelinated afferent vagal fibers in cats

The purpose of this study was to determine the behavior of left ventricular mechanoreceptors with myelinated vagal afferents and to compare them with endings with nonmyelinated vagal afferents. Single unit activity was recorded from 13 endings with nonmyelinated vagal afferents (conduction velocity 2.1 +/- 0.3 m/sec) and from 16 endings with myelinated vagal afferents (conduction velocity 7.3 +/- 1.3 m/sec). Resting discharge frequencies of nonmyelinated afferents and of myelinated vagal afferents were 1.7 +/- 0.3 and 2.7 +/- 0.5 imp/sec (P less than 0.1), respectively (at left ventricular end diastolic pressure of 6 mm Hg for both groups). Ten of 16 myelinated vagal afferents had pulse synchronous discharge under basal condition, whereas only 3 of 13 nonmyelinated vagal afferents had such activity. During aortic occlusion, the discharge of myelinated vagal afferents increased 1.7 +/- 0.3 imp/sec per mm Hg, whereas nonmyelinated vagal afferents increased significantly (P less than 0.05) less (0.5 +/- 0.1 imp/sec per mm Hg). Discharge for both groups was linearly related to left ventricular end-diastolic pressure but not to left ventricular systolic pressure. Increases in left ventricular systolic pressure alone did not increase firing for either group. During aortic occlusion, the maximum discharge rates of myelinated vagal afferents (43 +/- 7 imp/sec) were significantly higher than those of nonmyelinated vagal afferents (14 +/- 3 imp/sec) at left ventricular end-diastolic pressure of 30 +/- 2 and 24 +/- 2 mm Hg, respectively. Both groups increased their discharge during volume expansion with myelinated vagal afferents showing greater sensitivity than nonmyelinated vagal afferents. All endings studied were in the inferoposterior wall of the left ventricle. All nonmyelinated vagal afferents were in or near the epicardium. In contrast, myelinated vagal afferents were equally distributed between the endocardium and the epicardium. Myelinated vagal afferents had discrete receptive fields (1-2 mm2) whereas those of nonmyelinated vagal afferents were much larger (1 cm2). In conclusion, the discharge of left ventricular endings with nonmyelinated vagal afferents and myelinated vagal afferents both appear to be determined mainly by changes in left ventricular end-diastolic pressure. They may be located at different depths in the left ventricular wall. Myelinated vagal afferents have greater sensitivity and maximum firing frequencies than nonmyelinated vagal afferents.

Cardiopulmonary vagal afferents in the monkey: a survey of receptor activity

A survey was made of vagal afferents whose endings originated in cardiopulmonary areas of the Rhesus monkey. Recordings of action potentials from single fiber preparations of the left cervical vagus were made in both open and closed chest monkeys. A total of 425 receptors were identified in sixteen animals. These consisted of 347 pulmonary stretch receptors (one of which increased its discharge during expiration), 42 aortic baroreceptors, 4 ventricular pressure receptors, 1 epicardial ventricular receptor, 7 type A atrial receptors and 24 type B atrial receptors. The response of each cardiovascular receptor was tested by altering the stimulus for their discharge. Aortic and ventricular baroreceptors increased their discharge in response to an increase in blood pressure induced by intravenous norepinephrine. Type A atrial receptors did not increase their discharge in response to an increase in atrial pressure during intravenous administration of isotonic saline, while type B atrial receptors did. The discharge of the latter became continuous following the intravenous administration of veratridine sulphate (20 microgram). It is concluded that the basic types of cardiopulmonary afferents exist in the non-human primate and that they respond similarly to those which have been demonstrated in lower species.

A comparison of aortic baroreceptor discharge in normotensive and spontaneously hypertensive rats

Electrophysiological characteristics of individual aortic baroreceptors from normotensive rats (NTR) and spontaneously hypertensive rats (SHR) were compared. Impulse activity of afferent fibers was examined following the application of pressure steps to an in vitro aortic arch-aortic nerve preparation. Thirty-one fibers including seven unmyelinated fibers were studied completely over the range of 0-260 mm Hg, using steps 1-30 seconds in duration. The pressure threshold for peak transient discharge of baroreceptors of SHR's was 88.4 +/- 10.1 (mean +/- SE) mm Hg, whereas for baroreceptors of NTR's it was 77.5 +/- 9.3 mm Hg. The pressure threshold for steady state discharge was 137.3 +/- 5.2 mm Hg for SHR baroreceptors and 103.5 +/- 7.1 for NTR baroreceptors. These values compare favorably to measurements in vivo in the rat. The relationship between peak transient impulse frequency and pressure was linear, whereas the relationship between steady state impulse frequency and pressure was hyperbolic. The curvature of the steady state frequency-pressure curves was significantly reduced for baroreceptors of SHR's. The steady state pressure-volume curves of the aortas of SHR's and NTR's were similar, so that a reduction in distensibility could not account for the larger, significant differences in threshhold and sensitivity. Therefore resetting cannot be attributed simply to reduced vascular compliance.

A study of compounds which initiate and block nerve impulses in the perfused rabbit liver

1. A technique for the perfusion of the rabbit liver with Krebs solution is described: the hepatic nerves were monitored for centripetal action potentials. Spontaneous afferent activity, and a good response to stimulating agents persisted for 7 h or longer.2. Action potentials were elicited by injections or infusions of acetylcholine, 5-hydroxytryptamine, bradykinin, phenyldiguanide, adrenaline and various other compounds. From comparison with experiments carried out in vivo by other authors, it would appear that most of the nerves stimulated were afferent.3. Inhibition of action potentials, either those occurring spontaneously or those elicited by injections, was produced by aspirin, paracetamol, mepyramine, chlorpheniramine, procaine and lignocaine but not by morphine or atropine. Pentolinium produced some inhibition, as did hexamethonium in concentrations of about 7.5 mug/ml.4. Three methods for using the preparation for the assessment of local anaesthetic action are described.5. Lobeline, potassium cyanide and 2,4-dinitrophenol stimulated nerve bundles which were not stimulated by anoxia or hypoxia.

Integrative reflex control of heart rate in the rabbit during hypoxia and hyperventilation

Baroreceptor mean arterial blood pressure-heart period curves were obtained for unanesthetized sham-operated, thalamic, or pontine rabbits at an arterial Po 2 of 100 or 30 mm Hg. Each curve was S-shaped: it had a lower heart period plateau below threshold pressure, a part where heart period increased monotonically with rising mean arterial blood pressure, and an upper heart period plateau above upper saturation pressure. In spontaneously breathing rabbits, the pressure-dependent parameters of the curves were altered during hypoxia; in addition, there were baroreflex-independent shifts in heart period plateau levels. Suprapontine centers participated in both of these changes. An afferent analysis performed in rabbits treated with atropine and subjected to controlled ventilation showed that the changes in curve parameters were due to afferent interactions in cardiac sympathetic motoneurons between (1) arterial baroreceptor and chemoreceptor inputs which increased threshold, (2) arterial baroreceptor and cardiopulmonary baroreceptor inputs which lowered threshold and increased gain, and (3) cardiopulmonary baroreceptor and chemoreceptor inputs which increased gain and heart period range. Baroreflex-independent shifts in plateau levels were due to increased chemoreceptor and lung inflation receptor activity. The baroreflex-dependent and the baroreflex-independent components of the heart period response were mediated (as in spontaneously breathing rabbits) through both suprapontine centers and pontine centers.