Sensory innervation of the heart

Interoceptive signals from the heart and coronary circulation in health and disease

This review considers interoceptive signalling from the heart and coronary circulation. Vagal and cardiac sympathetic afferent sensory nerve endings are distributed throughout the atria, ventricles (mainly left), and coronary artery. A small proportion of cardiac receptors attached to thick myelinated vagal afferents are tonically active during the cardiac cycle. Dependent upon location, these mechanoreceptors detect fluctuations in atrial volume and coronary arterial perfusion. Atrial volume and coronary arterial signals contribute to beat-to-beat feedback control and physiological homeostasis. Most cardiac receptors are attached to thinly myelinated or nonmyelinated C fibres, many of which are unresponsive to the cardiac cycle. Of these, there are many chemically sensitive cardiac receptors which are activated during myocardial stress by locally released endogenous substances. In contrast, some tonically inactive receptors become activated by irregular ventricular wall mechanics or by distortion of the ischaemic myocardium. Furthermore, some are excited both by chemical mediators of ischaemia and wall abnormalities. Reflex responses arising from cardiac receptors attached to thinly myelinated or nonmyelinated are complex. Impulses that project centrally through vagal afferents elicit sympathoinhibition and hypotension, whereas impulses travelling in cardiac sympathetic afferents and spinal pathways elicit sympathoexcitation and hypertension. Two opposing cardiac reflexes may provide a mechanism for fine-tuning a composite haemodynamic response during myocardial stress. Sympathetic afferents provide the primary pathway for transmission of cardiac nociception to the central nervous system. However, activation of sympathetic afferents may increase susceptibility to life-threatening arrhythmias. Notably, the cardiac sympathetic afferent reflex predominates in pathophysiological states including hypertension and heart failure.

The autism-associated Meis2 gene is necessary for cardiac baroreflex regulation in mice

Recent understanding of Autism Spectrum Disorder (ASD) showed that peripheral primary mechanosensitive neurons involved in touch sensation and central neurons affected in ASD share transcriptional regulators. Mutant mice for ASD-associated transcription factors exhibit impaired primary tactile perception and restoring those genes specifically in primary sensory neurons rescues some of the anxiety-like behavior and social interaction defects. Interestingly, peripheral mechanosensitive sensory neurons also project to internal organs including the cardiovascular system, and an imbalance of the cardio-vascular sympathovagal regulation is evidenced in ASD and intellectual disability. ASD patients have decreased vagal tone, suggesting dysfunction of sensory neurons involved in cardio-vascular sensing. In light of our previous finding that the ASD-associated Meis2 gene is necessary for normal touch neuron development and function, we investigated here if its inactivation in mouse peripheral sensory neurons also affects cardio-vascular sympathovagal regulation and baroreflex. Combining echocardiography, pharmacological challenge, blood pressure monitoring, and heart rate variability analysis, we found that Meis2 mutant mice exhibited a blunted vagal response independently of any apparent cardiac malformation. These results suggest that defects in primary sensory neurons with mechanosensitive identity could participate in the imbalanced cardio-vascular sympathovagal tone found in ASD patients, reinforcing current hypotheses on the role of primary sensory neurons in the etiology of ASD.

Does stroke volume influence heartbeat evoked responses?

We know surprisingly little on how heartbeat-evoked responses (HERs) vary with cardiac parameters. Here, we measured both stroke volume, or volume of blood ejected at each heartbeat, with impedance cardiography, and HER amplitude with magneto-encephalography, in 21 male and female participants at rest with eyes open. We observed that HER co-fluctuates with stroke volume on a beat-to-beat basis, but only when no correction for cardiac artifact was performed. This highlights the importance of an ICA correction tailored to the cardiac artifact. We also observed that easy-to-measure cardiac parameters (interbeat intervals, ECG amplitude) are sensitive to stroke volume fluctuations and can be used as proxies when stroke volume measurements are not available. Finally, interindividual differences in stroke volume were reflected in MEG data, but whether this effect is locked to heartbeats is unclear. Altogether, our results question assumptions on the link between stroke volume and HERs.

Behavioral and neurophysiological signatures of interoceptive enhancements following vagus nerve stimulation

An accruing body of research has shown that interoception (the sensing of signals from the body's internal milieu) relies on both a direct route (afforded by the vagus nerve) and a secondary route (supported by somatosensory mechanisms). However, no study has causally tested the differential role of these pathways, let alone via direct stimulation. To bridge this gap, we tested whether multidimensional signatures of interoception are modulated by noninvasive vagus nerve stimulation (nVNS). Sixty‐three participants were divided into an nVNS and a sham‐stimulation group. Before and after stimulation, both groups performed a validated heartbeat detection (HBD) task including a genuinely interoceptive condition (monitoring one's own heartbeat) and a control exteroceptive condition (tracking an aurally presented heartbeat). Electroencephalographic signals were obtained during both conditions to examine modulations of the heartbeat‐evoked potential (HEP). Moreover, before and after stimulation, participants were asked to complete a somatosensory heartbeat localization task. Results from the interoceptive condition revealed that, after treatment, only the nVNS group exhibited improved performance and greater HEP modulations. No behavioral differences were found for the exteroceptive control condition, which was nonetheless associated with significant HEP modulations. Finally, no between‐group differences were observed regarding the localization of the heartbeat sensations or relevant cardiodynamic variables (heart rate and or heart rate variability). Taken together, these results constitute unprecedented evidence that the vagus nerve plays a direct role in neurovisceral integration during interoception. This finding can constrain mechanistic models of the domain while informing a promising transdiagnostic agenda for interoceptive impairments across neuropsychiatric conditions.

Clinical Presentation of Sustained Monomorphic Ventricular Tachycardia Without Cardiac Arrest

Background

“Palpitations” are one of the most common complaints prompting medical attention. Textbooks of medicine and cardiology as well as guideline documents and position papers describe palpitations as a common symptom of ventricular tachycardia (VT). However, data to support this description are lacking. The aim of our study was to evaluate the symptomatology of sustained monomorphic VT with emphasis on the prevalence of palpitations.

Methods and Results

Consecutive patients presenting to our center with a first event of a regular sustained monomorphic VT (n=59) or a regular supraventricular tachycardia (SVT; n=109) between January 2012 and September 2019 were interviewed regarding their symptoms during the arrhythmic event. We included only patients with a first arrhythmic event to avoid the influence of previous medical encounters on our patients’ terminology. As expected, patients with VT were older (age 68.8±13.6 versus 52.6±16.8 years; P<0.001), more often of male sex (94.9% versus 37.6%; P<0.001), had lower left ventricular ejection fraction (37±11% versus 59±2%, P<0.001) and more comorbidities (87.6% versus 40.5%; P<0.001) compared with patients with SVT. Importantly, even though the heart rate upon presentation did not differ between the 2 groups (165±26 beats/min during VT versus 171±32 beats/min during SVT; P=0.16), symptomatology differed significantly; specifically, palpitations were reported in only 8.8% of VT patients, compared with 90.7% of SVT patients (P<0.001). Common symptoms in the VT group included chest pain (64%), dyspnea (21%), and dizziness (26%).

Conclusions

Despite similar heart rate, patients with VT rarely report having palpitations, whereas patients with SVT do so commonly. This finding may assist with decision making in patients reporting palpitations in whom an ECG tracing is not available.

"Lacking warmth": Alexithymia trait is related to warm-specific thermal somatosensory processing

Alexithymia is a personality trait involving deficits in emotional processing. The personality construct has been extensively validated, but the underlying neural and physiological systems remain controversial. One theory suggests that low-level somatosensory mechanisms act as somatic markers of emotion, underpinning cognitive and affective impairments in alexithymia. In two separate samples (total N=100), we used an established Quantitative Sensory Testing (QST) battery to probe multiple neurophysiological submodalities of somatosensation, and investigated their associations with the widely-used Toronto Alexithymia Scale (TAS-20). Experiment one found reduced sensitivity to warmth in people with higher alexithymia scores, compared to individuals with lower scores, without deficits in other somatosensory submodalities. Experiment two replicated this result in a new group of participants using a full-sample correlation between threshold for warm detection and TAS-20 scores. We discuss the relations between low-level thermoceptive function and cognitive processing of emotion.

The man who feels two hearts: the different pathways of interoception

Recent advances in neuroscience have provided new insights into the understanding of heart-brain interaction and communication. Cardiac information to the brain relies on two pathways, terminating in the insular cortex (IC) and anterior cingulate cortex (ACC), along with the somatosensory cortex (S1-S2). Interoception relying on these neuroanatomical pathways has been shown to modulate social cognition. We report the case study of C.S., a patient with an 'external heart' (an extracorporeal left-univentricular cardiac assist device, LVAD). The patient was assessed with neural/behavioral measures of cardiac interoception complemented by neuropsychological and social cognition measures. The patient's performance on the interoception task (heartbeat detection) seemed to be guided by signals from the artificial LVAD, which provides a somatosensory beat rather than by his endogenous heart. Cortical activity (HEP, heartbeat-evoked potential) was found decreased in comparison with normal volunteers, particularly during interoceptive states. The patient accurately performed several cognitive tasks, except for interoception-related social cognition domains (empathy, theory of mind and decision making). This evidence suggests an imbalance in the patient's cardiac interoceptive pathways that enhances sensation driven by the artificial pump over that from the cardiac vagal-IC/ACC pathway. A patient with two hearts, one endogenous and one artificial, presents a unique opportunity to explore models of interoception and heart-brain interaction.

Cardiac sympathetic afferent activation provoked by myocardial ischemia and reperfusion. Mechanisms and reflexes

Cardiac sympathetic afferents are known to reflexly activate the cardiovascular system, leading to increases in blood pressure, heart rate, and myocardial contractile function. During myocardial ischemia, these sensory nerves also transmit the sensation of pain (angina pectoris) and cause tachyarrhythmias. The authors' laboratory has been interested in defining the mechanisms of activation of this neural system during ischemia and reperfusion. During these periods, reactive oxygen species, particularly hydroxyl radicals, are produced from the breakdown of purine metabolites and lead to stimulation of sympathetic (and vagal) ventricular chemosensitive nerve endings. For example, stimulation with hydrogen peroxide leads to a small reflex increase in blood pressure from the predominant sympathetic afferent activation that is reduced by simultaneous activation of cardiac vagal afferents (known to exert predominantly depressor reflexes). Central integration of these two opposing reflexes likely occurs at several regions of the brain stem, including the nucleus tractus solitarii, where neural occlusion occurs during simultaneous cardiac sympathetic and vagal-afferent stimulation. Activation of platelets also appears to play a role during myocardial ischemia, leading to local release of serotonin (5HT), which, through a 5HT3 mechanism, stimulates sympathetic afferents. Finally, regional changes in pH from lactic acid (but not hypercapnia), stimulate ventricular afferents and may activate kallikrein to increase bradykinin (BK), which, in turn, breaks down arachidonic acid to form prostaglandins. Prostaglandins sensitize cardiac sympathetic afferents to BK. Thus, stimulation of cardiac sympathetic afferents during ischemia and reperfusion and the resulting reflex events form a multifactorial process resulting from activation of a number of chemical pathways in the myocardium.

New look to an old symptom: angina pectoris

At the turn of this century, it was proposed that ischemic cardiac pain might be related to distension of the ventricular wall ("mechanical hypothesis"). Three decades later, it was hypothesized that ischemic pain might be elicited by the intramyocardial release of pain-producing substances induced by ischemia ("chemical hypothesis"). Studies carried out in the past 10 years have given strong support to the chemical hypothesis, because they have consistently shown that adenosine is a mediator of ischemic cardiac pain. Adenosine-induced ischemic cardiac pain is mediated primarily by stimulation of A1 receptors located in cardiac nerve endings and is potentiated by substance P. Conversely, the magnitude and rate of left ventricular dilation during ischemia do not predict the severity of angina. It is worth noting, however, that stretching of epicardial coronary arteries appears to potentiate the severity of angina caused by myocardial ischemia. The nervous activity generated by myocardial ischemia is modulated in intrinsic cardiac, mediastinal, and thoracic ganglia. Then it is further modulated in the central nervous system and projects bilaterally to the cortex, as demonstrated in humans by positron emission tomography, where it is decoded as a painful sensation. The causes responsible for the lack of angina during myocardial ischemia are probably different in patients who present both pain-free and painful myocardial ischemia, in patients with predominantly painless ischemia, and in diabetic patients.

On the relation between cardiodynamics and heartbeat perception

The mediation of heartbeat perception by cardiodynamic parameters was investigated by experimentally introducing changes in heart performance. Twenty-five subjects (14 women, 11 men) underwent bicycle ergometric exercises (0-75 W) at different tilt angles (90 degrees-0 degree). After each of the 31 tilt/exercise phases, the following measures of cardiac performance were taken using impedance cardiography: stroke volume, Heather Index (as an indicator of contractile force), heart rate, and an estimate of the momentum of the ejected blood mass. Additionally, a heartbeat perception task was performed. The different tilt/exercise combinations induced profound changes in all cardiodynamic measures. The correlation between heartbeat perception and cardiodynamic parameters was highest for "momentum" (r = .67) and for stroke volume (r = .59). From our data, we infer that intraindividually occurring changes in cardiac perception are mediated partially by characteristics of heart dynamics and that the source of these interoceptive signals lies in the heart or in its close vicinity.

Non-adrenergic non-cholinergic (NANC) neural control of the atrial myocardium

1. Current concepts in the regulation of atrial contractility by non-adrenergic non-cholinergic (NANC) sensory nerves are reviewed. 2. There is now evidence that in addition to sympathetic and parasympathetic innervation capsaicin-sensitive sensory nerves contribute to the local control of atrial contractility by releasing NANC transmitters, such as calcitonin gene-related peptide (CGRP). 3. Certain chemical and physical stimuli affect atrial contractility by inducing the release of CGRP from sensory nerves. In addition, as widely recognized for the sympathetic and vagal atrial innervation, NANC neurotransmission is under the inhibitory control of several endogenous modulators. 4. Cardioexcitatory actions of NANC neurotransmission on the atrial myocardium are considered. 5. Pharmacological modulation of NANC neurotransmission and functional evidence for cross-talk between NANC and sympathetic neurones are also discussed.

Some intrinsic neurons of the guinea-pig heart contain substance P

Whole-mount preparations of the posterior wall of the atria of the guinea pig heart containing intrinsic ganglion cells and nerve plexuses were stained for substance P-like immunoreactivity by the peroxidase-antiperoxidase method. Substance P-like nerve fibres are present as pericellular baskets around most, but not all, of the neuronal cell bodies, and are also found in the connecting nerve bundles, as perivascular nerve plexuses and in the myocardium and pericardium. The majority of ganglion cell bodies are negative for substance P, as reported previously, but we describe for the first time, a small subpopulation of intrinsic neuronal cell bodies which show immunoreactivity for substance P. Therefore, not all cardiac substance P nerves are extrinsic afferent fibres. At present, the physiological role of intrinsic substance P neurones is not clear.

Morphological study of the innervation pattern of the rabbit sinoatrial node

The pattern of nerves, ganglia, and fine nerve processes in the adult rabbit sinoatrial node, identified by microelectrode recording, was defined by staining histochemically for cholinesterase followed by silver impregnation. A generalized repeatable pattern of innervation was recognized, including 1) a large ganglionic complex inferior to the sinoatrial node; 2) two or three moderately large nerves traversing the sinoatrial node parallel to the crista terminalis; 3) nerves entering the region from the atrial septum, the superior vena cava, and the inferior vena cava; and 4) a fine network of nerve processes, particularly extensive in the morphologically dense small-cell part of the sinoatrial node. When the site of initial depolarization in the node was located and marked by a broken-off electrode tip, it was found, after cholinesterase staining, to be characterized by a cluster of cells enclosed in a nest or basket of fine nerves. Similar nested cell clusters were observed elsewhere in the sinoatrial node in this same preparation and in other hearts. A complex interweaving of atrial muscle fibers was observed medial and inferomedial to the sinoatrial node, which may form the anatomical basis for the lack of conduction through this region. The morphological pattern of nerves, ganglia, and myocardial cells described in this study emphasizes the complexity of innervation of the sinoatrial node, including its intrinsic neural elements. Cholinesterase/silver staining can be useful in the definition and comparison of electrophysiologically identified sites within the sinoatrial node.

The distribution of nerve fibers showing substance P-like immunoreactivity in the conduction system of the bovine heart: dense innervation in the atrioventricular bundle

There is limited information on the distribution of nerve fibers containing substance P (SP) in the heart conduction system. Therefore, in the present study, the various parts of the conduction system of the bovine heart were examined by the use of an SP-antiserum and immunohistochemistry. Nerve fibers showing SP-like immunoreactivity (SP-LI) occurred in the proximities of conduction cells in all parts of the conduction system, but were present in greatly larger numbers in the AV bundle than in the other parts. The nerve fibers showed a predilection for certain regions of the bundles of conduction cells (Purkinje fiber bundles) in the AV bundle and the bundle branches and their ramifications. Nerve fibers showing SP-LI also occurred in the walls of the arteries and in association with some the ganglionic cells located in the regions of the conduction system. None of the ganglionic cells exhibited SP-LI. The observations are discussed in relation to what is known of the function of SP in the heart and of the distribution of sympathetic and parasympathetic nerve fibers in the conduction system. As SP is regarded as a marker of afferent fibers the observations support the view that afferent nerve fibers are present throughout the conduction system. It is likely that the existence of a significant SP-innervation in the conduction system is of importance for the function of this part of the heart.

[Pathophysiology of ischemic cardiac pain.]

Cardiac pain is a conscious experience that can be explored only indirectly with experimental approaches. The exact machanisms eliciting cardiac pain still remain obscure. The afferent fibres running in the cardiac sympathetic nerves are regarded as the essential pathway for the transmission of cardiac pain. Atria and ventricle are abundantly supplied with sympathetic sensory innervation. In the spinal cord, impulses transmitted by the sympathetic pathway converge with impulses from somatic thoracic structures onto the same ascending spinothalamic neuron which probably explains the mechanism of referred pain (=projection of pain to another organ). Two hypotheses have been put forward to explain the peripheral mechanism for nociception. The intensity mechanism assumes that pain results from an excessive stimulation of receptive structures normally stimulated at lower levels whereas a specific sensation is considered to result from an excitation of a well defined nociceptive apparatus. Ventricular sympathetic afferent fibres whether myelinated or unmyelinated, always possess some mechanosensitivity and respond to normal chemical and mechanical stimuli, thus displaying properties of polymodal receptors. Afferent vagal fibres may contribute to the mechanisms of cardiac nociception by modulating the threshold and characteristics of pain. Experimental studies identified three main mechanisms, which may be responsible for eliciting cardiac pain during ischemic periods in humans: a) nonphysiological motion of the ischemic left ventricular wall (bulging) and an excitation of mechanical receptors by passive stretching. b) The excitation of free sensory nerve endings by chemicals such as bradykinin, PGE(2), adenosin, histamin or potassium. c) A combination of a and b: algogenic chemicals may sensitize mechanical receptors and therefore lower their threshold for nociception.