Cardiovascular sympathetic afferent fibers

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.

Clinical potential of sensory neurites in the heart and their role in decision-making

The process of decision-making is quite complex involving different aspects of logic, emotion, and intuition. The process of decision-making can be summarized as choosing the best alternative among a given plethora of options in order to achieve the desired outcome. This requires establishing numerous neural networks between various factors associated with the decision and creation of possible combinations and speculating their possible outcomes. In a nutshell, it is a highly coordinated process consuming the majority of the brain's energy. It has been found that the heart comprises an intrinsic neural system that contributes not only to the decision-making process but also the short-term and long-term memory. There are approximately 40,000 cells present in the heart known as sensory neurites which play a vital role in memory transfer. The heart is quite a mysterious organ, which functions as a blood-pumping machine and an endocrine gland, as well as possesses a nervous system. There are multiple factors that affect this heart ecosystem, and they directly affect our decision-making capabilities. These interlinked relationships hint toward the sensory neurites which modulate cognition and mood regulation. This review article aims to provide deeper insights into the various roles played by sensory neurites in decision-making and other cognitive functions. The article highlights the pivotal role of sensory neurites in the numerous brain functions, and it also meticulously discusses the mechanisms through which they modulate their effects.

Psychophysiological data-driven multi-feature information fusion and recognition of miner fatigue in high-altitude and cold areas

Fatigue-induced human error is a leading cause of accidents. The purpose of this exploratory study in China was to perform field tests to measure fatigue psychophysiological parameters, such as electrocardiography (ECG), electromyography (EMG), pulse, blood pressure, reaction time and vital capacity (VC), in miners in high-altitude and cold areas and to perform multi-feature information fusion and fatigue identification. Forty-five miners were randomly selected as subjects for a field test, and feature signals were extracted from 90 psychophysiological features as basic signals for fatigue analysis. Fatigue sensitivity indices were obtained by Pearson correlation analysis, t-test and receiver operating characteristic (ROC) curve performance evaluation. The ECG time-domain, ECG frequency-domain, EMG, VC, systolic blood pressure (SBP), and pulse were significantly different after miner fatigue. The support vector machine (SVM) and random forest (RF) techniques were used to classify and identify fatigue by information fusion and factor combination. The optimal fatigue classification factors were ECG-FD (CV Accuracy = 85.0%) and EMG (CV Accuracy = 90.0%). The optimal combination of factors was ECG-TD + ECG-FD + EMG (CV accuracy = 80.0%). Furthermore, SVM machine learning had a good recognition effect. This study shows that SVM and RF can effectively identify miner fatigue based on fatigue-related factor combinations. ECG-FD and EMG are the best indicators of fatigue, and the best performance and robustness are obtained with three-factor combination classification. This study on miner fatigue identification provides a reference for research on clinical medicine and the identification of human fatigue under high-altitude, cold and low-oxygen conditions.

Algogen-induced vasosensory reflexes modulate short-term heart rate variability parameters in experimental rat models

OBJECTIVES

The present work was designed to study the modulatory effects of algogen-induced vasosensory reflex responses on short-term heart rate variability (HRV) parameters in naïve and vagotomized rat models.

METHODS

In this study, vasosensory reflex responses were elicited by instilling algogens (bradykinin/histamine), a component of inflammatory mediators into a local segment of medium-sized peripheral blood vessel (femoral artery) while a continuous electrocardiogram (ECG) was recorded. Short-term (5 min) ECG segments obtained from original recordings were examined in detail and relevant data of HRV parameters were pooled. Time domain and frequency domain analyses were performed using dedicated software (LabChart 8, AD Instruments^®, Australia) and results were analyzed.

RESULTS

Bradykinin-induced vasosensory reflexes caused significant alterations in both time domain and frequency domain HRV parameters as compared to the time-matched saline control group. Instillation of bradykinin caused a transient increase in NN interval, RMSSD, TSP, HF power (HFP) along with a decrease in the standard deviation of all normal NN intervals (SDNN), SDNN/RMSSD, LF power (LFP), LFP/HFP. Histamine produced a similar pattern of responses, but HRV alterations were less pronounced compared to those with bradykinin. Further analysis revealed that algogen-induced vasosensory reflex responses caused an increase in the parasympathetic influence on the heart accompanied by a decrease in sympathetic influence. In addition, HRV modulation by algogen-induced vasosensory reflexes was significantly attenuated in vagotomized rats, illustrating the principal role of vagus in the reflex HRV modulation.

CONCLUSIONS

The present study proposes a novel hypothesis regarding the cardio-protective role of inflammatory mediators during acute stress, by potentiating the vagal impact and attenuating the sympathetic impact on the heart.

Linear and nonlinear analyses of normal and fatigue heart rate variability signals for miners in high-altitude and cold areas

BACKGROUND AND OBJECTIVE

Fatigue is an important cause of operational errors, and human errors are the main cause of accidents. This study is an exploratory study in China. Field tests were conducted on heart rate variability (HRV) parameters and physiological indicators of fatigue among miners in high-altitude, cold and low-oxygen areas. This paper studies heart activity patterns during work fatigue in miners.

METHODS

Fatigue affects both the sympathetic and parasympathetic nervous systems, and it is expressed as an abnormal pattern of HRV parameters. Thirty miners were selected as subjects for a field test, and HRV was extracted from 60 groups of electrocardiography (ECG) datasets as basic signals for fatigue analysis. Then, we analyzed the HRV signals of the miners using linear (time domain and frequency domain) and nonlinear dynamics (Poincaré plot and sample entropy (SampEn)), and a Pearson's correlation coefficient analysis and t-tests were performed on the measured indices.

RESULTS

The results showed that the time-domain indices (SDNN, RMSSD, SDSD, pNN50, RRn, heart rate (HR), R-wave humps (RH)) and the coefficient of variation (CV)) and the frequency-domain indices (low frequency/high frequency (LF/HF), LFnorm and HFnorm) clearly changed after fatigue. These features were selected using a Poincaré plot, sample entropy, Pearson's correlation coefficient and a t-test for further analysis. The fatigue characteristics and sensitivity parameters of miners in a high-altitude, cold and hypoxic environment were obtained.

CONCLUSIONS

This study provides deep insight into the use of linear and nonlinear fatigue characteristics to effectively and reliably identify miner fatigue. Furthermore, the study provides a reference for clinical studies of acute mountain sickness in high-altitude, cold and hypoxic environments.

Spinal genesis of Mayer waves

Variability in cardiovascular spectra was first described by Stephan Hales in 1733. Traube and Hering initially noted respirophasic variation of the arterial pressure waveform in 1865 and Sigmund Mayer noted a lower frequency oscillation of the same in anesthetized rabbits in 1876. Very low frequency oscillations were noted by Barcroft and Nisimaru in 1932, likely representing vasogenic autorhythmicity. While the origins of Traube Hering and very low frequency oscillatory variability in cardiovascular spectra are well described, genesis mechanisms and functional significance of Mayer waves remain in controversy. Various theories have posited baroreflex and central supraspinal mechanisms for genesis of Mayer waves. Several studies have demonstrated the persistence of Mayer waves following high cervical transection, indicating a spinal capacity for genesis of these oscillations. We suggest a general tendency for central sympathetic neurons to oscillate at the Mayer wave frequency, the presence of multiple Mayer wave oscillators throughout the brainstem and spinal cord, and possible contemporaneous genesis by baroreflex and vasomotor mechanisms.

Jaw pain and myocardial ischemia: A review of potential neuroanatomical pathways

Cardiac pain is usually manifested as a crushing, squeezing, or sensation of pressure in the center of the chest. The pain can be referred to the left shoulder, neck, jaw, and epigastric region as well as the temporomandibular region, paranasal sinuses, and head in general. Although not well understood, during myocardial ischemia, the process of cardiac referred pain to craniofacial structures can be explained by the convergence of visceral and somatic relays at the trigeminal nucleus in the brain stem. The goal of this article is to review the possible pathways for referred jaw pain due to myocardial ischemia. Clin. Anat. 32:476-479, 2019. © 2019 Wiley Periodicals, Inc.

Contribution of Autonomic Reflexes to the Hyperadrenergic State in Heart Failure

Heart failure (HF) is a complex syndrome representing the clinical endpoint of many cardiovascular diseases of different etiology. Given its prevalence, incidence and social impact, a better understanding of HF pathophysiology is paramount to implement more effective anti-HF therapies. Based on left ventricle (LV) performance, HF is currently classified as follows: (1) with reduced ejection fraction (HFrEF); (2) with mid-range EF (HFmrEF); and (3) with preserved EF (HFpEF). A central tenet of HFrEF pathophysiology is adrenergic hyperactivity, featuring increased sympathetic nerve discharge and a progressive loss of rhythmical sympathetic oscillations. The role of reflex mechanisms in sustaining adrenergic abnormalities during HFrEF is increasingly well appreciated and delineated. However, the same cannot be said for patients affected by HFpEF or HFmrEF, whom also present with autonomic dysfunction. Neural mechanisms of cardiovascular regulation act as “controller units,” detecting and adjusting for changes in arterial blood pressure, blood volume, and arterial concentrations of oxygen, carbon dioxide and pH, as well as for humoral factors eventually released after myocardial (or other tissue) ischemia. They do so on a beat-to-beat basis. The central dynamic integration of all these afferent signals ensures homeostasis, at rest and during states of physiological or pathophysiological stress. Thus, the net result of information gathered by each controller unit is transmitted by the autonomic branch using two different codes: intensity and rhythm of sympathetic discharges. The main scope of the present article is to (i) review the key neural mechanisms involved in cardiovascular regulation; (ii) discuss how their dysfunction accounts for the hyperadrenergic state present in certain forms of HF; and (iii) summarize how sympathetic efferent traffic reveal central integration among autonomic mechanisms under physiological and pathological conditions, with a special emphasis on pathophysiological characteristics of HF.

Acute effects of water immersion on heart rate variability in participants with heart disease

BACKGROUND

Water immersion and aquatic exercise can be an important therapeutic tool in patients suffering from heart disease (HD). However, the effects of water immersion on heart rate variability (HRV) in HD participants remain unknown.

METHODS

Twenty-eight volunteers in sinus rhythm within the same age range took part in this study: 18 HD and ten healthy controls (HC). Heart rhythm was collected with a heart rate monitor (sampling rate 1000 Hz) for periods of 10 min at rest in the supine position on land, standing on land (STL) and standing in water (STW) to the xiphoid process.

RESULTS

Heart disease participants had the same response as HC participants to the three experimental conditions (no significant between-group differences in all HRV variables). STW (immersion) caused in both groups to increase HRV when compared to supine and STL.

CONCLUSION

Heart disease participants demonstrate similar beneficial adaptations as HC participants to the effects of immersion, reinforcing the concept that immersion can be a valuable aquatic cardiac rehabilitation tool to acutely increase HRV. Approaches that improve HRV in both healthy and cardiac patients may have a positive impact on the reduction of morbidity and mortality.

An easy-to-use technique to characterize cardiodynamics from first-return maps on ΔRR-intervals

Heart rate variability analysis using 24-h Holter monitoring is frequently performed to assess the cardiovascular status of a patient. The present retrospective study is based on the beat-to-beat interval variations or ΔRR, which offer a better view of the underlying structures governing the cardiodynamics than the common RR-intervals. By investigating data for three groups of adults (with normal sinus rhythm, congestive heart failure, and atrial fibrillation, respectively), we showed that the first-return maps built on ΔRR can be classified according to three structures: (i) a moderate central disk, (ii) a reduced central disk with well-defined segments, and (iii) a large triangular shape. These three very different structures can be distinguished by computing a Shannon entropy based on a symbolic dynamics and an asymmetry coefficient, here introduced to quantify the balance between accelerations and decelerations in the cardiac rhythm. The probability P111111 of successive heart beats without large beat-to-beat fluctuations allows to assess the regularity of the cardiodynamics. A characteristic time scale, corresponding to the partition inducing the largest Shannon entropy, was also introduced to quantify the ability of the heart to modulate its rhythm: it was significantly different for the three structures of first-return maps. A blind validation was performed to validate the technique.

The Autonomic Nervous System and Hypertension

Physiological studies have long documented the key role played by the autonomic nervous system in modulating cardiovascular functions and in controlling blood pressure values, both at rest and in response to environmental stimuli. Experimental and clinical investigations have tested the hypothesis that the origin, progression, and outcome of human hypertension are related to dysfunctional autonomic cardiovascular control and especially to abnormal activation of the sympathetic division. Here, we review the recent literature on the adrenergic and vagal abnormalities that have been reported in essential hypertension, with emphasis on their role as promoters and as amplifiers of the high blood pressure state. We also discuss the possible mechanisms underlying these abnormalities and their importance in the development and progression of the structural and functional cardiovascular damage that characterizes hypertension. Finally, we examine the modifications of sympathetic and vagal cardiovascular influences induced by current nonpharmacological and pharmacological interventions aimed at correcting elevations in blood pressure and restoring the normotensive state.

Relative contributions of the thalamus and the paraventricular nucleus of the hypothalamus to the cardiac sympathetic afferent reflex

The cardiac sympathetic afferent reflex (CSAR) is induced by stimulating the cardiac sympathetic afferents, which evokes increases in sympathetic outflow and arterial pressure. In the present study, we attempted to identify the contribution of thalamic and hypothalamic nuclei involved in the CSAR. First, we observed that there was an increase in the number of c-Fos-labeled cells in the paraventricular nucleus (PVN) (190 ± 18 vs. 101 ± 15; P < 0.05), the paraventricular nucleus of the thalamus (PVT) (239 ± 23 vs. 151 ± 15; P < 0.05), and the mediodorsal thalamic nucleus (MD) (92 ± 9 vs. 63 ± 6; P < 0.05) following epicardial application of bradykinin (BK) compared with the control group (P < 0.05). Second, using extracellular single-unit recording, we found 25% of spontaneously active neurons in the thalamus were stimulated by epicardial application of BK or capsaicin in intact rats. However, 24% of spontaneously active neurons in the thalamus were still stimulated by epicardial application of BK or capsaicin despite vagotomy and sinoaortic denervation. None of the neurons in the thalamus responded to baroreflex changes in arterial pressure, induced by intravenous injection of phenylephrine or sodium nitroprusside. The CSAR was inhibited by microinjection of muscimol or lidocaine into the PVN. However, it was not inhibited or blocked by microinjection of muscimol or lidocaine into the thalamus. Taken together, these data suggest that the thalamus, while activated, is not critical for autonomic adjustments in response to activation of the CSAR. On the other hand, the PVN is critically involved in the central pathway of the CSAR.

Spinal Cord Stimulation and the Induction of c-fos and Heat Shock Protein 72 in the Central Nervous System of Rats

For more than a decade, spinal cord stimulation (SCS) has been used as an adjuvant treatment for patients who are unresponsive to conventional therapies for angina pectoris. Many studies showed that SCS has both electro-analgesic and anti-ischemic effects. Nonetheless, the biological substrates by which SCS acts have not yet been unraveled, although recently areas in the brain have been described that show changes in blood flow, following SCS, and during provocation of angina. In search of a putative mechanism of action of SCS, we hypothesized that SCS affects processing of nociceptive information within the central nervous system (CNS). Moreover, it may alter the limbic system activity that maintains the balance between sympathetic and parasympathetic activity in the heart. Hence, we have developed a rat model to investigate its suitability for studying the induction of neural activity during SCS. To characterize neural activity, we used the expression of both the immediate early gene c-fos and the heat shock protein 72 (HSP72). c-Fos was used to identify structures in the CNS affected by SCS, and HSP72 was applied in order to ascertain whether SCS might operate as a stressor. In 20 halothane-anesthetized male Wistar rats, two electrodes were placed epidurally, one at the C7 level and the other at the T2 level. Two days after surgery, the rats were either stimulated "treated" animals, n = 10) or used as controls ("unstimulated" = "sham," n = 10) in random order. Furthermore, we studied the effect of SCS on behavior in five treated and five control rats. Three hours after stimulation, the rats were euthanized and the brain and spinal cord were removed. The treated group showed regional increased c-fos expression in regions of the limbic system (periaqueductal gray, paraventricular hypothalamic nucleus, paraventricular thalamic nucleus, central amygdala, agranular and dysgranular insular cortex, (peri)ambiguus, nucleus tractus solitarius, and spinal cord) that are involved in the processing of pain and cardiovascular regulation, among other things. Moreover, in both treated rats and controls, HSP72-expression was found in the endothelium of the enthorhinal cortex, the amygdala, and the ventral hypothalamus, but not in the neurons. Finally, treated animals were significantly more alert and active than controls. In conclusion, the rat model we developed appears to be suitable for studying potential mechanisms through which SCS may act. In addition, SCS affects c-fos expression in specific parts of the brain known to be involved in regulation of pain and emotions. HSP72-expression is limited to the endothelium of certain parts of the CNS and thereby excludes physical stress effects as a potential mechanism of SCS.

From heart to brain: the genesis and processing of cardiac pain

Angina pectoris is important because of its association with heart disease and risk of death. Historically after Heberden's account of angina in 1772, the association of pain with coronary artery disease quickly followed. Within a few years, Burns suggested an etiological role for ischemia. Subsequently, theories of differential myocardial stretch dominated thinking until Lewis' chemical hypothesis in 1932, in which the local release of chemical substances during ischemia was seen as the cause of pain. This review considers how ischemia at the tissue level triggers activation of afferent nociceptive pain fibres. The afferent projections of sympathetic and vagal afferent fibres are described, with a number of methodologies cited (eg, injection of pseudorabies virus into the heart with mapping of the retrograde viral transport pathways; and elevation of neuronal c-fos synthesis in brain regions activated by capsaicin application to the heart). Our own functional neuroimaging studies of angina are also reviewed. There are 2 intriguing features of angina. The first is the poor correlation between symptoms and extent of coronary disease. The spectrum ranges from entirely silent myocardial ischemia to that of a functional pain syndrome--the 'sensitive heart'--of cardiac syndrome X. An even more difficult aspect is the wide variability in symptoms experienced by an individual patient. A new paradigm is presented which, besides considering myocardial oxygen supply/demand imbalance, also draws insights from the broader field of pain research. Neuromodulation applies at multiple levels of the neuraxis--peripheral nerves, spinal cord, and brain--and it invites exploitation, whether pharmacological or electrical, for the benefit of the cardiac patient in pain.

B-Afferents: A fundamental division of the nervous system mediating homeostasis?

The peripheral nervous system (PNS) has classically been separated into a somatic division composed of both afferent and efferent pathways and an autonomic division containing only efferents. J. N. Langley, who codified this asymmetrical plan at the beginning of the twentieth century, considered different afferents, including visceral ones, as candidates for inclusion in his concept of the “autonomic nervous system” (ANS), but he finally excluded all candidates for lack of any distinguishing histological markers. Langley's classification has been enormously influential in shaping modern ideas about both the structure and the function of the PNS. We survey recent information about the PNS and argue that many of the sensory neurons designated as “visceral” and “somatic” are in fact part of a histologically distinct group of afferents concerned primarily autonomic function. These afferents have traditionally been known as “small dark” neurons or B-neurons. In this target article we outline an association between autonomic and B-neurons based on ontogeny, cell phenotype, and functional relations, grouping them together as part of a common reflex system involved in homeostasis. This more parsimonious classification of the PNS, made possible by the identification of a group of afferents associated primarily with the ANS, avoids a number of confusions produced by the classical orientation. It may also have practical implications for an understanding of nociception, homeostatic reflexes, and the evolution of the nervous system.

Methods of assessing vagus nerve activity and reflexes

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

Neural control of heart rate: the role of neuronal networking

Neural control of heart rate, particularly its sympathetic component, is generally thought to reside primarily in the central nervous system, though accumulating evidence suggests that intrathoracic extracardiac and intrinsic cardiac ganglia are also involved. We propose an integrated model in which the control of heart rate is achieved via three neuronal "levels" representing three control centers instead of the conventional one. Most importantly, in this model control is effected through networking between neuronal populations within and among these layers. The results obtained indicate that networking serves to process demands for systemic blood flow before transducing them to cardiac motor neurons. This provides the heart with a measure of protection against the possibility of "overdrive" implied by the currently held centrally driven system. The results also show that localized networking instabilities can lead to sporadic low frequency oscillations that have the characteristics of the well-known Mayer waves. The sporadic nature of Mayer waves has been unexplained so far and is of particular interest in clinical diagnosis.

Physiological recordings: basic concepts and implementation during functional magnetic resonance imaging

Combining human functional neuroimaging with other forms of psychophysiological measurement, including autonomic monitoring, provides an empirical basis for understanding brain-body interactions. This approach can be applied to characterize unwanted physiological noise, examine the neural control and representation of bodily processes relevant to health and morbidity, and index covert expression of affective and cognitive processes to enhance the interpretation of task-evoked regional brain activity. In recent years, human neuroimaging has been dominated by functional magnetic resonance imaging (fMRI) studies. The spatiotemporal information of fMRI regarding central neural activity is valuably complemented by parallel physiological monitoring, yet such studies still remain in the minority. This review article highlights fMRI studies that employed cardiac, vascular, respiratory, electrodermal, gastrointestinal and pupillary psychophysiological indices to address specific questions regarding interaction between brain and bodily state in the context of experience, cognition, emotion and behaviour. Physiological monitoring within the fMRI environment presents specific technical issues, most importantly related to safety. Mechanical and electrical hazards may present dangers to scanned subjects, operator and/or equipment. Furthermore, physiological monitoring may interfere with the quality of neuroimaging data, or itself be compromised by artefacts induced by the operation of the scanner. We review the sources of these potential problems and the current approaches and advice to enable the combination of fMRI and physiological monitoring in a safe and effective manner.

Early infant diet and the omega 3 fatty acid DHA: effects on resting cardiovascular activity and behavioral development during the first half-year of life

This investigation evaluated variations in resting heart rate (HR) measures during the first half year of life in healthy, full-term infants who were either breast-fed (BF), or fed formula with (milk-based: MF; soy-based: SF) or without (soy-based: SF(-)) commercially supplemented DHA (decosahexaenoic acid). In infants fed the DHA-deficient diet, higher HR and lower values for heart rate variability measures were observed, indicating decreased parasympathetic tone in this group. These effects, appearing at 4 months and continuing for the remainder of the study period, are consistent with suggestions that the 3-5-month postnatal interval may be an important period in the development of cardiovascular regulation. The absence of these effects in SF infants receiving the DHA-supplemented formula suggests that neither soy protein nor the associated phytochemicals in soy formula contribute to these effects to any appreciable extent. In general, the results do not indicate differences in any of the study variables attributable to soy formula per se.

Neural autonomic control in orthostatic intolerance

Inability to maintain the upright position is manifested by a number of symptoms shared by either human pathophysiology and conditions following weightlessness or bed rest. Alterations of the neural sympathetic cardiovascular control have been suggested to be one of the potential underlying etiopathogenetic mechanisms in these conditions. We hypothesize that the study of the autonomic profile of human orthostatic intolerance syndromes may furnish a valuable insight into the complexity of the sympathetic alterations leading to a reduced gravitational tolerance. In the present paper we describe abnormalities both in the magnitude and in the pattern of the sympathetic neural firing observed in patients affected by orthostatic intolerance, attending the upright position. Also, we discuss similarity and differences in the neural sympathetic mechanisms regulating the cardiovascular system during the gravitational stimulus both in clinical syndromes and in subjects returning from space.

Clinical relevance of heart rate variability

Heart rate variability (HRV) has recently become a popular noninvasive research tool in cardiology. Clinical assessment of HRV is frequently based on standard long-term ambulatory electrocardiograms, whereas physiologic studies employ spectral analysis of short-term recordings under controlled conditions. From a general point of view, HRV can be used in clinical practice to estimate (1) the integrity of cardiac autonomic innervation, (2) the physiologic status of cardiac autonomic activity, and (3) the vulnerability to various cardiac arrhythmias resulting from autonomic imbalance. Clinical relevance of HRV has been clearly demonstrated in only two clinical conditions: (1) impaired HRV can be used alone or in a combination with other factors to predict risk of arrhythmic events after acute myocardial infarction, and (2) decrease in HRV is a useful clinical marker for evolving diabetic neuropathy. Substantial advances of our knowledge are required to establish and promote clinical applications in other areas of clinical medicine. To accomplish this task, proper hypotheses should be studied and appropriate techniques selected.

Neurochemical diversity of afferent neurons that transduce sensory signals from dog ventricular myocardium

While much is known about the influence of ventricular afferent neurons on cardiovascular function in the dog, identification of the neurochemicals transmitting cardiac afferent signals to central neurons is lacking. Accordingly, we identified ventricular afferent neurons in canine dorsal root ganglia (DRG) and nodose ganglia by retrograde labeling after injecting horseradish peroxidase (HRP) into the anterior right and left ventricles. Primary antibodies from three host species were used in immunohistochemical experiments to simultaneously evaluate afferent somata for the presence of HRP and markers for two neurotransmitters. Only a small percentage (2%) of afferent somata were labeled with HRP. About half of the HRP-identified ventricular afferent neurons in T(3) DRG also stained for substance P (SP), calcitonin gene-related peptide (CGRP), or neuronal nitric oxide synthase (nNOS), either alone or with two markers colocalized. Ventricular afferent neurons and the general population of T(3) DRG neurons showed the same labeling profiles; CGRP (alone or colocalized with SP) being the most common (30-40% of ventricular afferent somata in T(3) DRG). About 30% of the ventricular afferent neurons in T(2) DRG displayed CGRP immunoreactivity and binding of the putative nociceptive marker IB(4). Ventricular afferent neurons of the nodose ganglia were distinct from those in the DRG by having smaller size and lacking immunoreactivity for SP, CGRP, and nNOS. These findings suggest that ventricular sensory information is transferred to the central nervous system by relatively small populations of vagal and spinal afferent neurons and that spinal afferents use a variety of neurotransmitters.

Sleep-dependent changes in the coupling between heart period and blood pressure in human subjects

We investigated whether in human subjects, the pattern of coupling between the spontaneous fluctuations of heart period (HP) and those of systolic blood pressure (SBP) differs among wake-sleep states. Polysomnographic recordings and finger blood pressure measurements were performed for 48 h in 15 nonobese adults without sleep-disordered breathing. The cross-correlation function (CCF) between the fluctuations of HP and SBP at frequencies <0.15 Hz was computed during quiet wakefulness (QW), light (stages 1 and 2) and deep (stages 3 and 4) nonrapid-eye-movement sleep (NREMS), and rapid-eye-movement sleep (REMS). A positive correlation between HP and the previous SBP values, which is the expected result of baroreflex feedback control, was observed in the sleep states but not in QW. In deep NREMS, the maximum CCF value was significantly higher than in any other state, suggesting the greatest baroreflex contribution to the coupling between HP and SBP. A negative correlation between HP and the subsequent SBP values was also observed in each state, consistent with the mechanical feed-forward action of HP on SBP and with central autonomic commands. The contribution of these mechanisms to the coupling between HP and SBP, estimated from the minimum CCF value, was significantly lower in deep NREMS than either in light NREMS or QW. These results indicate that the pattern of coupling between HP and SBP at low frequencies differs among wake-sleep states in human subjects, with deep NREMS entailing the highest feedback contribution of the baroreflex and a low effectiveness of feed-forward mechanisms.

Potential clinical relevance of the 'little brain' on the mammalian heart

It is hypothesized that the heart possesses a nervous system intrinsic to it that represents the final relay station for the co-ordination of regional cardiac indices. This 'little brain' on the heart is comprised of spatially distributed sensory (afferent), interconnecting (local circuit) and motor (adrenergic and cholinergic efferent) neurones that communicate with others in intrathoracic extracardiac ganglia, all under the tonic influence of central neuronal command and circulating catecholamines. Neurones residing from the level of the heart to the insular cortex form temporally dependent reflexes that control overlapping, spatially determined cardiac indices. The emergent properties that most of its components display depend primarily on sensory transduction of the cardiovascular milieu. It is further hypothesized that the stochastic nature of such neuronal interactions represents a stabilizing feature that matches cardiac output to normal corporal blood flow demands. Thus, with regard to cardiac disease states, one must consider not only cardiac myocyte dysfunction but also the fact that components within this neuroaxis may interact abnormally to alter myocyte function. This review emphasizes the stochastic behaviour displayed by most peripheral cardiac neurones, which appears to be a consequence of their predominant cardiac chemosensory inputs, as well as their complex functional interconnectivity. Despite our limited understanding of the whole, current data indicate that the emergent properties displayed by most neurones comprising the cardiac neuroaxis will have to be taken into consideration when contemplating the targeting of its individual components if predictable, long-term therapeutic benefits are to accrue.

Central and baroreflex control of heart period during the wake-sleep cycle in spontaneously hypertensive rats

We investigated whether the relative contribution of the baroreflex and central commands to the control of heart period differs between spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) during physiological behavior. Rats were instrumented with an arterial catheter and with electrodes for discriminating wakefulness, nonrapid eye movement sleep (NREMS), and rapid eye movement sleep (REMS). The cross-correlation function (CCF) between spontaneous fluctuations of heart period and mean arterial pressure was computed at frequencies <0.2 Hz. The baroreflex determines a positive correlation between heart period and previous pressure values. This pattern was observed in the CCF during quiet wakefulness (QW) and NREMS, and in QW, it was accompanied by a pronounced negative correlation between heart period and subsequent pressure values. The relative baroreflex contribution to the control of heart period, estimated from the positive peak value of the CCF, was lower in SHR than in WKY during QW but not during NREMS. During REMS, the CCF showed a negative correlation between heart period and both previous and subsequent pressure values, reflecting the prevalence of central autonomic commands. The relative contribution of central commands to the control of heart period, estimated from the negative peak value of the CCF, was lower in SHR than in WKY during REMS. These results suggest that during QW and REMS, the control of heart period exerted by the baroreflex and central commands, respectively, is less effective in SHR than in WKY. This difference is not apparent in a behavioral state of autonomic stability such as NREMS.

Nonuniformity in the von Bezold-Jarisch reflex

The von Bezold-Jarisch reflex (BJR) is a vagally mediated chemoreflex from the heart and lungs, causing hypopnea, bradycardia, and inhibition of sympathetic vasomotor tone. However, cardiac sympathetic nerve activity (CSNA) has not been systematically compared with vasomotor activity during the BJR. In 11 urethane-anesthetized (1-1.5 g/kg iv), artificially ventilated rats, we measured CSNA simultaneously with lumbar sympathetic activity (LSNA) while the BJR was evoked by right atrial bolus injections of phenylbiguanide (0.5, 1.0, 1.5, and 2 microg). Nerve and heartbeat responses were analyzed by calculating normalized cumulative sums. LSNA and heartbeats were always reduced by the BJR. An excitatory "rebound" component often followed the inhibition of LSNA but never outweighed it. For CSNA, however, excitation usually (in 7 of 11 rats) outweighed any initial inhibition, such that the net response to phenylbiguanide was excitatory. The differences in net response between LSNA, CSNA, and heartbeats were all significant (P < 0.01). A second experimental series on seven rats showed that methyl atropine (1 mg/kg iv) abolished the bradycardia of the BJR, whereas subsequent bilateral vagotomy substantially reduced LSNA and CSNA responses, both excitatory and inhibitory. These findings show that, during the BJR, 1) CSNA is often excited, 2) there may be coactivation of sympathetic and parasympathetic drives to the heart, 3) divergent responses may be evoked simultaneously in cardiac vagal, cardiac sympathetic, and vasomotor nervous pathways, and 4) those divergent responses are mediated primarily by the vagi.

The strength of QT-RR coupling decreases during graded head-up tilt

A cross-conditional entropy approach was applied to evaluate the degree of coupling between the beat-to-beat series of heart period (RR interval) and ventricular repolarization duration (QT interval). The strength of the QT-RR coupling was measured during graded head-up tilt test with table inclination randomly chosen in the set {0,15,30,45,60,75,90} in 17 healthy subjects. We found that RR and QT variabilities are significantly coupled during the entire experimental protocol and the strength of the QT-RR variability interactions progressively decreases as a function of the tilt angles. Results suggest that the fraction of QT interval variability independent of RR interval changes is not constant but depends on the level of sympathetic activity and/or amplitude of sympathetic modulation.

Skipping breakfast: gender effects on resting heart rate measures in preadolescents

The cardiovascular response in children to morning nutrition has received little attention, and associated gender-related effects are virtually uninvestigated. This study evaluated resting heart-rate (HR) and heart-rate variability (HRV) in preadolescents after overnight fasting and again after eating a standardized breakfast or continuing to fast. HR increased slightly after eating and decreased significantly with continued fasting. These effects were present for both sexes. Relative to children who ate, those who continued fasting showed increases in HRV-particularly for inter-beat-interval and low frequency component (LF: 0.04-0.15 Hz) measures. Analyses revealed significant increases across variability measures for fasting children, but a selective LF decrease in those who were fed-an effect most prominent in females. Otherwise, males and females showed similar treatment-related changes in HRV. While within-gender comparisons showed similar results for HR, i.e., faster HR in fed compared with fasting males and females, respectively, fasting females-but not males-showed significantly greater increases in variability relative to their fed counterparts. Together, these findings suggest that extended overnight fasting initiates an increase in parasympathetic activity that attenuates the expected increase in cardiovascular output following a mid-morning meal. Observed gender differences were related to greater parasympathetic activity in males and to the apparent emphasis on parasympathetic regulation of LF variability. The implications of these findings for health concerns, the nature of responses to physiological and cognitive stressors, and how such differences may influence performance variables-particularly early in development when cardiovascular responses to these stressors may be more sensitive to nutritional factors-are discussed.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Sympathetic overactivity in active ulcerative colitis: effects of clonidine

Previous reports suggest that inflammatory bowel diseases may be accompanied by abnormalities in the neural autonomic profile. We tested the hypotheses that 1) an exaggerated sympathetic activity characterizes active ulcerative colitis (UC) and 2) a reduction of sympathetic activity by clonidine would be associated with clinical changes of UC. In 23 patients with UC and 20 controls, muscle sympathetic nerve activity (MSNA), ECG, blood pressure, and respiration were continuously recorded, and plasma catecholamine was evaluated both at rest and during a 75 degrees head-up tilt. Autonomic profile was assessed by MSNA, norepinephrine, epinephrine, spectral markers of low-frequency (LF) cardiac sympathetic (LF(RR); normalized units) and high-frequency (HF) parasympathetic (HF(RR); normalized units) modulation and sympathetic vasomotor control (LF systolic arterial pressure; LF(SAP)), obtained by spectrum analysis of the R-R interval and systolic pressure variability. Among UC patients, 16 agreed to be randomly assigned to 8-wk transdermal clonidine (15 mg/wk, 9 subjects), or placebo (7 patients). An autonomic profile, Disease Activity Index (DAI), and endoscopic pattern were compared before and after clonidine/placebo. At rest, MSNA, heart rate (HR), LF(RR), LF/HF, and LF(SAP) were higher and HF(RR) was lower in patients than in controls. Tilt decreased HF(RR) and increased MSNA and LF(RR) less in patients than in controls. Clonidine decreased HR, MSNA, epinephrine, LF(RR), and increased HF(RR), whereas placebo had no effects. Changes of the autonomic profile after clonidine were associated with reduction of DAI score. An overall increase of sympathetic activity characterized active UC. Normalization of the autonomic profile by clonidine was accompanied by an improvement of the disease.

Early detection of cardiac dysfunction in thalassemic patients by radionuclide angiography and heart rate variability analysis

BACKGROUND

Cardiac dysfunction remains the major cause of death in beta-thalassemia. Aim of this study was to assess early myocardial damage in thalassemic patients with no symptoms or echocardiographic evidence of dysfunction at routine monitoring.

METHODS

Twenty patients (seven females; median 25 yr [first quartile 22,third quartile 28]) with beta-thalassemia underwent radionuclide angiography (RNA) at rest and during low-dose dobutamine infusion (5-10 gamma/kg/min). Right and left ventricular ejection fractions (EF) were determined by first-pass method and gated equilibrium acquisition, respectively. Twenty-four-hour Holter monitoring with time-domain heart rate variability (HRV) assessment and echocardiographic follow-up (21 months [5,27]) were performed.

RESULTS

Eleven patients showed regional wall motion abnormalities at RNA; left ventricular EF, HR and diastolic measurements significantly increased after dobutamine infusion. Patients with abnormal RNA right ventricular EF (n = 8, <0.45) showed lower echocardiographic left ventricular EF at the enrollment (0.54 [0.50,0.61] vs. 0.62 [0.56,0.67], P = 0.02) than those with a normal right ventricular EF. Patients with reduced standard deviation of the averages of RR intervals in all 5-minute periods of entire recording (SDANN) (n = 6, <100 ms), a measure of HRV, had lower echocardiographic left ventricular EF (0.53 [0.49,0.62] vs. 0.62 [0.56,0.66], P = 0.03) and lower fractional shortening (0.28 [0.25,0.32] vs. 0.36 [0.30,0.39], P = 0.003) at the enrollment than those with normal SDANN. No significant association was found between RNA and HRV measurements and follow-up left ventricular function.

CONCLUSIONS

Right ventricular dysfunction and abnormal HRV may represent the early features of cardiac disease in thalassemic patients with no evidence of ventricular dysfunction at routine evaluation.

Mechanisms of neurohormonal activation in chronic congestive heart failure: pathophysiology and therapeutic implications

Patients with chronic congestive heart failure have a sequential and incessant activation of those neurohormonal systems, which control body fluids, cardiac output and systemic blood pressure. Neurohormonal activation is initially selective and regional. Generalized activation is a late event in the natural history of congestive heart failure. Although the ultimate stimulus responsible for the activation of these neurohormonal systems is unknown, a decreased cardiac output and diminished effective blood volume have been proposed as the responsible mechanisms. However, extensive clinical and experimental research suggest that cardiac remodeling and loading of low-pressure cardiac receptors with sympathetic afferents could be the triggering events followed by unloading of high-pressure carotid receptors by decreased cardiac output and diminished effective blood volume.

Augmented input from cardiac sympathetic afferents inhibits baroreflex in rats with heart failure

It has been established that the baroreflex is markedly decreased in chronic heart failure (CHF). Our recent study has indicated that activation of the cardiac sympathetic afferent reflex (CSAR) inhibits the baroreflex in normal rats, and in the rats with CHF the CSAR is significantly enhanced, which is related to augmented central angiotensin II (Ang II) mechanism. Therefore, the hypothesis is that the augmented CSAR in the CHF state tonically inhibits the baroreflex via central AT1 receptor. To test the hypothesis, the rats with myocardial infarction-induced CHF or sham surgery were anesthetized with alpha-chloralose and urethane, vagotomized, and recordings were made of the mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). We found: (1) left ventricular epicardial application of capsaicin or electrical stimulation of the central end of the left cardiac sympathetic nerve blunted the baroreflex in both sham and CHF rats; (2) left ventricular epicardial application of lidocaine had no significant effects on the baroreflex in sham rats but improved the baroreflex in CHF rats (maximum slope, 1.7+/-0.3 to 2.9+/-0.2%/mm Hg; P<0.01); and (3) intracerebral ventricular injection of losartan had no significant effect on baroreflex in sham rats but improved the baroreflex in CHF rats (maximum slope 1.9+/-0.2 to 3.1+/-0.2%/mm Hg; P<0.01). These results suggest that tonic cardiac sympathetic afferent input plays an important role in the blunted baroreflex associated with CHF, which is mediated by central AT1 receptors.

ANG II in the paraventricular nucleus potentiates the cardiac sympathetic afferent reflex in rats with heart failure

Chronic heart failure (CHF) is characterized by sympathoexcitation, and the cardiac sympathetic afferent reflex (CSAR) is a sympathoexcitatory reflex. Our previous studies have shown that the CSAR was enhanced in CHF. In addition, central angiotensin II (ANG II) is an important modulator of this reflex. This study was performed to determine whether the CSAR evoked by stimulation of cardiac sympathetic afferent nerves (CSAN) in rats with coronary ligation-induced CHF is enhanced by ANG II in the paraventricular nucleus (PVN). Under alpha-chloralose and urethane anesthesia, renal sympathetic nerve activity (RSNA) was recorded. The RSNA responses to electrical stimulation (5, 10, 20, and 30 Hz) of the CSAN were evaluated. Bilateral microinjection of the AT1-receptor antagonist losartan (50 nmol) into the PVN had no significant effects in the sham group, but it abolished the enhanced RSNA response to stimulation in the CHF group. Unilateral microinjection of three doses of ANG II (0.03, 0.3, and 3 nmol) into the PVN resulted in dose-related increases in the RSNA responses to stimulation. Although ANG II also potentiated the RSNA response to electrical stimulation in sham rats, the RSNA responses to stimulation after ANG II into the PVN in rats with CHF were much greater than in sham rats. The effects of ANG II were prevented by pretreatment with losartan into the PVN in CHF rats. These results suggest that the central gain of the CSAR is enhanced in rats with coronary ligation-induced CHF and that ANG II in the PVN augments the CSAR evoked by CSAN, which is mediated by the central angiotensin AT1 receptors in rats with CHF.

AT1 receptor mRNA antisense normalizes enhanced cardiac sympathetic afferent reflex in rats with chronic heart failure

Previous studies showed that the cardiac sympathetic afferent reflex (CSAR) is enhanced in dogs and rats with chronic heart failure (CHF) and that central ANG II type 1 receptors (AT(1)R) are involved in this augmented reflex. The aim of this study was to determine whether intracerebroventricular administration and microinjection of antisense oligodeoxynucleotides targeted to AT(1)R mRNA would attenuate the enhanced CSAR and decrease resting renal sympathetic nerve activity (RSNA) in rats with coronary ligation-induced CHF. The CSAR was elicited by application of bradykinin to the epicardial surface of the left ventricle. Reflex responses to epicardial administration of bradykinin were enhanced in rats with CHF. The response to bradykinin was determined every 50 min after intracerebroventricular administration (lateral ventricle) or microinjection (into paraventricular nucleus) of antisense or scrambled oligonucleotides to AT(1)R mRNA. AT(1)R mRNA and protein levels in the paraventricular nucleus were significantly reduced 5 h after administration of antisense. Antisense significantly decreased resting RSNA and normalized the enhanced CSAR responses to bradykinin in rats with CHF. Scrambled oligonucleotides did not alter resting RSNA or the enhanced responses to bradykinin in rats with CHF. No significant effects were found in sham-operated rats after administration of either antisense or scrambled oligonucleotides. These results strongly suggest that central AT(1)R mRNA antisense reduces expression of AT(1)R protein and normalizes the augmentation of this excitatory sympathetic reflex and that genetic manipulation of protein expression can be used to normalize the sympathetic enhancement in CHF.

Cardiac Autonomic Derangement and Arrhythmias in Right-Sided Stroke With Insular Involvement

BACKGROUND AND PURPOSE

The insula of the right cerebral hemisphere may have a major role in cardiac autonomic control. This study was aimed at assessing the effects of acute right insular ischemic damage on heart rate variability (HRV) and arrhythmias.

METHODS

Holter monitoring for 24 hours was performed in 103 consecutive patients with first-ever acute ischemic stroke. Time and frequency domain measures of HRV and arrhythmias were considered in all cases.

RESULTS

Forty-nine patients (47.5%) had a right-sided infarction, whereas 54 (52.5%) had a left-sided infarction. Insular involvement was present in 33 patients with right-sided stroke (67.3%) and in 36 patients with left-sided stroke (66.6%). When compared with all other stroke patients, subjects with right-sided insular damage showed significantly lower values of the standard deviation of all normal-to-normal (SDNN) R wave to R wave (RR) intervals and of the root mean square of differences (rMSSD) of adjacent normal-to-normal RR intervals, and higher low-frequency/high-frequency ratio values (P<0.05). Right insular stroke was also associated with more complex arrhythmias than any other localization (P<0.05). Moreover, in the whole population of stroke patients, lower values of SDNN were associated with the presence of more frequent and complex arrhythmias.

CONCLUSIONS

These findings further support the notion that the right insula is implicated in the autonomic control of cardiac activity and that acute right insular damage may lead to a derangement of cardiac function with potential prognostic implications.

Cardiac neuronal hierarchy in health and disease

The cardiac neuronal hierarchy can be represented as a redundant control system made up of spatially distributed cell stations comprising afferent, efferent, and interconnecting neurons. Its peripheral and central neurons are in constant communication with one another such that, for the most part, it behaves as a stochastic control system. Neurons distributed throughout this hierarchy interconnect via specific linkages such that each neuronal cell station is involved in temporally dependent cardio-cardiac reflexes that control overlapping, spatially organized cardiac regions. Its function depends primarily, but not exclusively, on inputs arising from afferent neurons transducing the cardiovascular milieu to directly or indirectly (via interconnecting neurons) modify cardiac motor neurons coordinating regional cardiac behavior. As the function of the whole is greater than that of its individual parts, stable cardiac control occurs most of the time in the absence of direct cause and effect. During altered cardiac status, its redundancy normally represents a stabilizing feature. However, in the presence of regional myocardial ischemia, components within the intrinsic cardiac nervous system undergo pathological change. That, along with any consequent remodeling of the cardiac neuronal hierarchy, alters its spatially and temporally organized reflexes such that populations of neurons, acting in isolation, may destabilize efferent neuronal control of regional cardiac electrical and/or mechanical events.