Parasympathetic neurons in the cranial medial ventricular fat pad on the dog heart selectively decrease ventricular contractility

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.

The Intrinsic Cardiac Nervous System: From Pathophysiology to Therapeutic Implications

The cardiac autonomic nervous system (CANS) plays a pivotal role in cardiac homeostasis as well as in cardiac pathology. The first level of cardiac autonomic control, the intrinsic cardiac nervous system (ICNS), is located within the epicardial fat pads and is physically organized in ganglionated plexi (GPs). The ICNS system does not only contain parasympathetic cardiac efferent neurons, as long believed, but also afferent neurons and local circuit neurons. Thanks to its high degree of connectivity, combined with neuronal plasticity and memory capacity, the ICNS allows for a beat-to-beat control of all cardiac functions and responses as well as integration with extracardiac and higher centers for longer-term cardiovascular reflexes. The present review provides a detailed overview of the current knowledge of the bidirectional connection between the ICNS and the most studied cardiac pathologies/conditions (myocardial infarction, heart failure, arrhythmias and heart transplant) and the potential therapeutic implications. Indeed, GP modulation with efferent activity inhibition, differently achieved, has been studied for atrial fibrillation and functional bradyarrhythmias, while GP modulation with efferent activity stimulation has been evaluated for myocardial infarction, heart failure and ventricular arrhythmias. Electrical therapy has the unique potential to allow for both kinds of ICNS modulation while preserving the anatomical integrity of the system.

From Mice to Mainframes: Experimental Models for Investigation of the Intracardiac Nervous System

The intracardiac nervous system (IcNS), sometimes referred to as the "little brain" of the heart, is involved in modulating many aspects of cardiac physiology. In recent years our fundamental understanding of autonomic control of the heart has drastically improved, and the IcNS is increasingly being viewed as a therapeutic target in cardiovascular disease. However, investigations of the physiology and specific roles of intracardiac neurons within the neural circuitry mediating cardiac control has been hampered by an incomplete knowledge of the anatomical organisation of the IcNS. A more thorough understanding of the IcNS is hoped to promote the development of new, highly targeted therapies to modulate IcNS activity in cardiovascular disease. In this paper, we first provide an overview of IcNS anatomy and function derived from experiments in mammals. We then provide descriptions of alternate experimental models for investigation of the IcNS, focusing on a non-mammalian model (zebrafish), neuron-cardiomyocyte co-cultures, and computational models to demonstrate how the similarity of the relevant processes in each model can help to further our understanding of the IcNS in health and disease.

Location of Parasympathetic Innervation Regions From Electrograms to Guide Atrial Fibrillation Ablation Therapy: An in silico Modeling Study

The autonomic nervous system (ANS) plays an essential role in the generation and maintenance of cardiac arrhythmias. The cardiac ANS can be divided into its extrinsic and intrinsic components, with the latter being organized in an epicardial neural network of interconnecting axons and clusters of autonomic ganglia called ganglionated plexi (GPs). GP ablation has been associated with a decreased risk of atrial fibrillation (AF) recurrence, but the accurate location of GPs is required for ablation to be effective. Although GP stimulation triggers both sympathetic and parasympathetic ANS branches, a predominance of parasympathetic activity has been shown. This study aims was to develop a method to locate atrial parasympathetic innervation sites based on measurements from a grid of electrograms (EGMs). Electrophysiological models representative of non-AF, paroxysmal AF (PxAF), and persistent AF (PsAF) tissues were developed. Parasympathetic effects were modeled by increasing the concentration of the neurotransmitter acetylcholine (ACh) in randomly distributed circles across the tissue. Different circle sizes of ACh and fibrosis geometries were considered, accounting for both uniform diffuse and non-uniform diffuse fibrosis. Computational simulations were performed, from which unipolar EGMs were computed in a 16 × 1 6 electrode mesh. Different distances of the electrodes to the tissue (0.5, 1, and 2 mm) and noise levels with signal-to-noise ratio (SNR) values of 0, 5, 10, 15, and 20 dB were tested. The amplitude of the atrial EGM repolarization wave was found to be representative of the presence or absence of ACh release sites, with larger positive amplitudes indicating that the electrode was placed over an ACh region. Statistical analysis was performed to identify the optimal thresholds for the identification of ACh sites. In all non-AF, PxAF, and PsAF tissues, the repolarization amplitude rendered successful identification. The algorithm performed better in the absence of fibrosis or when fibrosis was uniformly diffuse, with a mean accuracy of 0.94 in contrast with a mean accuracy of 0.89 for non-uniform diffuse fibrotic cases. The algorithm was robust against noise and worked for the tested ranges of electrode-to-tissue distance. In conclusion, the results from this study support the feasibility to locate atrial parasympathetic innervation sites from the amplitude of repolarization wave.

Interplay Between Systemic Metabolic Cues and Autonomic Output: Connecting Cardiometabolic Function and Parasympathetic Circuits

There is consensus that the heart is innervated by both the parasympathetic and sympathetic nervous system. However, the role of the parasympathetic nervous system in controlling cardiac function has received significantly less attention than the sympathetic nervous system. New neuromodulatory strategies have renewed interest in the potential of parasympathetic (or vagal) motor output to treat cardiovascular disease and poor cardiac function. This renewed interest emphasizes a critical need to better understand how vagal motor output is generated and regulated. With clear clinical links between cardiovascular and metabolic diseases, addressing this gap in knowledge is undeniably critical to our understanding of the interaction between metabolic cues and vagal motor output, notwithstanding the classical role of the parasympathetic nervous system in regulating gastrointestinal function and energy homeostasis. For this reason, this review focuses on the central, vagal circuits involved in sensing metabolic state(s) and enacting vagal motor output to influence cardiac function. It will review our current understanding of brainstem vagal circuits and their unique position to integrate metabolic signaling into cardiac activity. This will include an overview of not only how metabolic cues alter vagal brainstem circuits, but also how vagal motor output might influence overall systemic concentrations of metabolic cues known to act on the cardiac tissue. Overall, this review proposes that the vagal brainstem circuits provide an integrative network capable of regulating and responding to metabolic cues to control cardiac function.

The Intrinsic Cardiac Nervous System and Its Role in Cardiac Pacemaking and Conduction

The cardiac autonomic nervous system (CANS) plays a key role for the regulation of cardiac activity with its dysregulation being involved in various heart diseases, such as cardiac arrhythmias. The CANS comprises the extrinsic and intrinsic innervation of the heart. The intrinsic cardiac nervous system (ICNS) includes the network of the intracardiac ganglia and interconnecting neurons. The cardiac ganglia contribute to the tight modulation of cardiac electrophysiology, working as a local hub integrating the inputs of the extrinsic innervation and the ICNS. A better understanding of the role of the ICNS for the modulation of the cardiac conduction system will be crucial for targeted therapies of various arrhythmias. We describe the embryonic development, anatomy, and physiology of the ICNS. By correlating the topography of the intracardiac neurons with what is known regarding their biophysical and neurochemical properties, we outline their physiological role in the control of pacemaker activity of the sinoatrial and atrioventricular nodes. We conclude by highlighting cardiac disorders with a putative involvement of the ICNS and outline open questions that need to be addressed in order to better understand the physiology and pathophysiology of the ICNS.

Cardiac Neuroanatomy for the Cardiac Electrophysiologist

The cardiac neuraxis is integral to cardiac physiology, and its dysregulation is implicated in cardiovascular disease. Neuromodulatory therapies are being developed that target the cardiac autonomic nervous system (ANS) to treat cardiac pathophysiology. An appreciation of the cardiac neuroanatomy is a prerequisite for development of such targeted therapies. Here, we provide a review of the current understanding of the cardiac ANS. The parasympathetic and sympathetic nervous system are composed of higher order cortical centers, brainstem, spinal cord, intrathoracic extracardiac ganglia and intrinsic cardiac ganglia. A series of interacting feedback loops mediates reflex pathways to exert control over the cardiac conduction system and contractile tissue. Further exploration of this complex regulatory system promises to yield neuroscience-based therapeutics for cardiac disease.

Cardiac innervation in acute myocardial ischaemia/reperfusion injury and cardioprotection

Acute myocardial infarction (AMI) and the heart failure (HF) that often complicates this condition, are among the leading causes of death and disability worldwide. To reduce myocardial infarct (MI) size and prevent heart failure, novel therapies are required to protect the heart against the detrimental effects of acute ischaemia/reperfusion injury (IRI). In this regard, targeting cardiac innervation may provide a novel therapeutic strategy for cardioprotection. A number of cardiac neural pathways mediate the beneficial effects of cardioprotective strategies such as ischaemic preconditioning and remote ischaemic conditioning, and nerve stimulation may therefore provide a novel therapeutic strategy for cardioprotection. In this article, we provide an overview of cardiac innervation and its impact on acute myocardial IRI, the role of extrinsic and intrinsic cardiac neural pathways in cardioprotection, and highlight peripheral and central nerve stimulation as a cardioprotective strategy with therapeutic potential for reducing MI size and preventing HF following AMI. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

Cardiac Vagus and Exercise

Lower resting heart rate and high autonomic vagal activity are strongly associated with superior exercise capacity, maintenance of which is essential for general well-being and healthy aging. Recent evidence obtained in experimental studies using the latest advances in molecular neuroscience, combined with human exercise physiology, physiological modeling, and genomic data suggest that the strength of cardiac vagal activity causally determines our ability to exercise.

Neurocardiology: Structure-Based Function

Cardiac control is mediated via a series of reflex control networks involving somata in the (i) intrinsic cardiac ganglia (heart), (ii) intrathoracic extracardiac ganglia (stellate, middle cervical), (iii) superior cervical ganglia, (iv) spinal cord, (v) brainstem, and (vi) higher centers. Each of these processing centers contains afferent, efferent, and local circuit neurons, which interact locally and in an interdependent fashion with the other levels to coordinate regional cardiac electrical and mechanical indices on a beat-to-beat basis. This control system is optimized to respond to normal physiological stressors (standing, exercise, and temperature); however, it can be catastrophically disrupted by pathological events such as myocardial ischemia. In fact, it is now recognized that autonomic dysregulation is central to the evolution of heart failure and arrhythmias. Autonomic regulation therapy is an emerging modality in the management of acute and chronic cardiac pathologies. Neuromodulation-based approaches that target select nexus points of this hierarchy for cardiac control offer unique opportunities to positively affect therapeutic outcomes via improved efficacy of cardiovascular reflex control. As such, understanding the anatomical and physiological basis for such control is necessary to implement effectively novel neuromodulation therapies. © 2016 American Physiological Society. Compr Physiol 6:1635-1653, 2016.

Parasympathetic Vagal Control of Cardiac Function

This brief review focuses on four new topics, with novel and clinically significant consequences, concerning the powerful influence of parasympathetic activity on cardiac function. In this short summary, we will highlight very recent and important work, published in the last 3-4 years, that (1) challenges the paradigm that parasympathetic activity to the heart is involved in the control of heart rate but plays little role in other cardiac functions, (2) characterizes important long-range synaptic pathways to parasympathetic cardiac vagal neurons that are involved in "higher" brain functions (such as arousal and emotional challenges), (3) asks whether implantable chronic vagal nerve stimulation is a promising clinical tool for treating cardiovascular diseases, and (4) describes newly identified neuropeptides and other modulators that can influence the generation and maintenance of parasympathetic activity to the heart.

The heart's 'little brain' controlling cardiac function in the rabbit

NEW FINDINGS

What is the topic of this review? The topic of the review is the intrinsic cardiac nervous system in the rabbit. What advances does it highlight? The anatomy of rabbit intrinsic ganglia is similar to that of other species, including humans. Immunohistochemistry confirms the presence of cholinergic and adrenergic neurones, with a striking arrangement of neuronal nitric oxide synthase-positive cell bodies. Activation of atrial ganglia produces effects on local and remote regions. Heart disease is a primary cause of mortality in the developed world, and it is well recognized that neural mechanisms play an important role in many cardiac pathologies. The role of extrinsic autonomic nerves has traditionally attracted the most attention. However, there is a rich intrinsic innervation of the heart, including numerous cardiac ganglia (ganglionic plexuses), that has the potential to affect cardiac function independently as well as to influence the actions of the extrinsic nerves. To investigate this, an isolated, perfused, innervated rabbit Langendorff heart preparation was considered the best option. Although ganglionic plexuses have been well described for several species, there was no full description of the anatomy and histochemistry of rabbit hearts. To this end, rabbit intrinsic ganglia were located using acetylcholinesterase histology (n = 33 hearts). This revealed six generalized ganglionic regions, defined as a left neuronal complex above the left pulmonary vein, a right neuronal complex around the base of right cranial vein, three scattered in the dorsal right atrium and a region containing numerous ventricular ganglia located on the conus arteriosus. Using immunohistochemistry, neurons were found to contain choline acetyltransferase or tyrosine hydroxylase and/or neuronal nitric oxide synthase in differing amounts (choline acetyltransferase > tyrosine hydroxylase > neuronal nitric oxide synthase). The function of rabbit intrinsic ganglia was investigated using a bolus application of nicotine or electrical stimulation at each of the above sites whilst measuring heart rate and atrioventricular conduction. Nicotine applied to different ganglionic plexuses caused a bradycardia, a tachycardia or a mixture of the two, with the right atrial plexus producing the largest chronotropic responses. Electrical stimulation at these sites induced only a bradycardia. Atrioventricular conduction was modestly changed by nicotine, the main response being a prolongation. Electrical stimulation produced significant prolongation of atrioventricular conduction, particularly when the right neuronal complex was stimulated. These studies show that the intrinsic plexuses of the heart are important and could be crucial for understanding impairments of cardiac function. Additionally, they provide a strong basis from which to progress using the isolated, innervated rabbit heart preparation.

Myths and realities of the cardiac vagus

There is continuing belief that cardiac parasympathetic postganglionic fibres are sparse or absent from the ventricles. This review of the literature shows that the supposition is a myth. Early studies considered that fine silver-stained fibres coursing amongst ventricle myocardial cells were most likely cardiac parasympathetic postganglionic fibres. The conclusions were later supported by acetyl cholinesterase staining using a method that appeared not to be associated with noradrenaline nerve fibres. The conclusion is critically examined in the light of several recent histological studies using the acetyl cholinesterase method and also a more definitive technique (CHAT), that suggest a widespread location of parasympathetic ganglia and a relatively dense parasympathetic innervation of ventricular muscle in a range of mammals including man. The many studies demonstrating acetylcholine release in the ventricle on vagal nerve stimulation and a high density of acetylcholine M2 receptors is in accord with this as are tests of ventricular performance from many physiological studies. Selective control of cardiac functions by anatomically segregated parasympathetic ganglia is discussed. It is argued that the influence of vagal stimulation on ventricular myocardial action potential refractory period, duration, force and rhythm is evidence that vagal fibres have close apposition to myocardial fibres. This is supported by clear evidence of accentuated antagonism between sympathetic activity and vagal activity in the ventricle and also by direct effects of vagal activity independent of sympathetic activity. The idea of differential control of atrial and ventricular physiology by vagal C and vagal B preganglionic fibres is examined as well as differences in chemical phenotypes and their function. The latter is reflected in medullary and supramedullary control. Reference is made to the importance of this knowledge to understanding the normal physiology of cardiac autonomic control and significance to pathology.

Cardiac autonomic nerve stimulation in the treatment of heart failure

Research on the therapeutic modulation of cardiac autonomic tone by electrical stimulation has yielded encouraging early clinical results. Vagus nerve stimulation has reduced the rates of morbidity and sudden death from heart failure, but therapeutic vagus nerve stimulation is limited by side effects of hypotension and bradycardia. Sympathetic nerve stimulation that has been implemented in the experiment may exacerbate the sympathetic-dominated autonomic imbalance. In contrast, concurrent stimulation of both sympathetic and parasympathetic cardiac nerves increases myocardial contractility without increasing heart rate. This review assesses the current state of electrical stimulation of the cardiac autonomic nervous system to treat heart failure.

Evidence for impaired vagus nerve activity in heart failure

Parasympathetic control of the heart via the vagus nerve is the primary mechanism that regulates beat-to-beat control of heart rate. Additionally, the vagus nerve exerts significant effects at the AV node, as well as effects on both atrial and ventricular myocardium. Vagal control is abnormal in heart failure, occurring at early stages of left ventricular dysfunction, and this reduced vagal function is associated with worse outcomes in patients following myocardial infarction and with heart failure. While central control mechanisms are abnormal, one of the primary sites of attenuated vagal control is at the level of the parasympathetic ganglion. It remains to be seen whether or not preventing or treating abnormal vagal control of the heart improves prognosis.

Epicardial neural ganglionated plexus of ovine heart: anatomic basis for experimental cardiac electrophysiology and nerve protective cardiac surgery

BACKGROUND

Sheep are routinely used in experimental cardiac electrophysiology and surgery.

OBJECTIVE

The purpose of this study was to (1) ascertain the topography and architecture of the ovine epicardial neural plexus (ENP), (2) determine the relationships of ENP with vagal and sympathetic cardiac nerves and ganglia, and (3) evaluate gross anatomic differences and similarities of ENP in humans, sheep, and other species.

METHODS

Ovine ENP and extrinsic sympathetic and vagal nerves were stained histochemically for acetylcholinesterase in whole heart and/or thorax-dissected preparations from 23 newborn lambs, with subsequent examination by stereomicroscope.

RESULTS

Intrinsic cardiac nerves extend from the venous part of the ovine heart hilum along the roots of the cranial (superior) caval and left azygos veins to both atria and ventricles via five epicardial routes: dorsal right atrial, middle dorsal, left dorsal, right ventral, and ventral left atrial nerve subplexuses. Intrinsic nerves proceeding from the arterial part of the heart hilum along the roots of the aorta and pulmonary trunk extend exclusively into the ventricles as the right and left coronary subplexuses. The dorsal right atrial, right ventral, and middle dorsal subplexuses receive the main extrinsic neural input from the right cervicothoracic and right thoracic sympathetic T(2) and T(3) ganglia as well as from the right vagal nerve. The left dorsal is supplied by sizeable extrinsic nerves from the left thoracic T(4)-T(6) sympathetic ganglia and the left vagal nerve. Sheep hearts contained an average of 769 +/- 52 epicardial ganglia. Cumulative areas of epicardial ganglia on the root of the cranial vena cava and on the wall of the coronary sinus were the largest of all regions (P <.05).

CONCLUSION

Despite substantial interindividual variability in the morphology of ovine ENP, right-sided epicardial neural subplexuses supplying the sinoatrial and atrioventricular nodes are mostly concentrated at a fat pad between the right pulmonary veins and the cranial vena cava. This finding is in sharp contrast with a solely left lateral neural input to the human atrioventricular node, which extends mainly from the left dorsal and middle dorsal subplexuses. The abundance of epicardial ganglia distributed widely along the ovine ventricular nerves over respectable distances below the coronary groove implies a distinctive neural control of the ventricles in human and sheep hearts.

Spatially divergent cardiac responses to nicotinic stimulation of ganglionated plexus neurons in the canine heart

Ganglionated plexuses (GPs) are major constituents of the intrinsic cardiac nervous system, the final common integrator of regional cardiac control. We hypothesized that nicotinic stimulation of individual GPs exerts divergent regional influences, affecting atrial as well as ventricular functions. In 22 anesthetized canines, unipolar electrograms were recorded from 127 atrial and 127 ventricular epicardial loci during nicotine injection (100 mcg in 0.1 ml) into either the 1) right atrial (RA), 2) dorsal atrial, 3) left atrial, 4) inferior vena cava-inferior left atrial, 5) right ventricular, 6) ventral septal ventricular or 7) cranial medial ventricular (CMV) GP. In addition to sinus and AV nodal function, neural effects on atrial and ventricular repolarization were identified as changes in the area subtended by unipolar recordings under basal conditions and at maximum neurally-induced effects. Animals were studied with intact AV node or following ablation to achieve ventricular rate control. Atrial rate was affected in response to stimulation of all 7 GPs with an incidence of 50-95% of the animals among the different GPs. AV conduction was affected following stimulation of 6/7 GP with an incidence of 22-75% among GPs. Atrial and ventricular repolarization properties were affected by atrial as well as ventricular GP stimulation. Distinct regional patterns of repolarization changes were identified in response to stimulation of individual GPs. RAGP predominantly affected the RA and posterior right ventricular walls whereas CMVGP elicited biatrial and biventricular repolarization changes. Spatially divergent and overlapping cardiac regions are affected in response to nicotinic stimulation of neurons in individual GPs.

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.

Differential control of cardiac functions by the brain

1. The idea is introduced that cardiac rate, contractility or atrioventricular (A-V) conduction spread may be controlled independently by the brain. Limited data from reflex studies are cited to support this view. 2. Evidence is presented that individual autonomic post- and preganglionic neurons have quite specific actions on the heart. Premotor and other central neurons can have preferential actions on heart rate, contractility or A-V conduction. 3. The functional implications of selective cardiac control are discussed.

Central control of atrio-ventricular conduction and left ventricular contractility in the cat heart: Synaptic interactions of vagal preganglionic neurons in the nucleus ambiguus with neuropeptide Y-immunoreactive nerve terminals

In the cat, vagal postganglionic controls of heart rate, atrio-ventricular (AV) conduction and left ventricular contractility are mediated by three separate intrinsic cardiac ganglia, the sinoatrial (SA), AV and cranioventricular (CV) ganglia, respectively. The vagal preganglionic neurons (VPNs) that project to these ganglia are located in the ventrolateral nucleus ambiguus (NA-VL). We have previously shown that the VPNs projecting to the SA, AV and CV ganglia are distinct from one another. We have also demonstrated that neuropeptide Y-immunoreactive (NPY-IR) axon terminals synapse upon VPNs projecting to the SA ganglion. In the present study, we test the hypothesis that those VPNs projecting to the AV ganglion (negative dromotropic VPNs) and those projecting to the CV ganglion (negative inotropic VPNs) are innervated by NPY-IR terminals in NA-VL. A retrograde tracer was injected into the AV or CV ganglion of the cat, and the brains subsequently processed for visualization of tracer and the immunocytochemical visualization of NPY by dual labeling electron-microscopic methods. We observed that 11+/-5% of all axodendritic synapses and 8+/-6% of all axosomatic synapses upon negative inotropic VPNs were NPY-IR. Furthermore, 19+/-14% of all axodendritic synapses upon negative dromotropic VPNs were NPY-IR. A few NPY-IR axosomatic synapses upon negative dromotropic neurons were also observed. NPY-IR terminals in NA-VL occasionally formed axosomatic synapses with NPY-IR neurons and axoaxonic synapses with unlabeled terminals. These results suggest that central NPY afferents to the NA-VL modulate the vagal preganglionic control of AV conduction and left ventricular contractility.

Location and variability of epicardiac ganglia in human fetuses

The aim of the study was to determine the morphology of epicardiac ganglia in human fetuses at different stages of their development as these ganglia are considered to be of a pivotal clinical importance. Twenty-one fetal hearts were investigated applying a technique of histochemistry for acetylcholinesterase to visualize the epicardiac neural ganglionated plexus with its subsequent stereoscopic examination on total organs. In all of the examined fetuses, epicardiac neural plexus with numerous ganglia was well recognizable and could be clearly differentiated into seven ganglionated subplexuses, topography and structural organization of which were typical for hearts of adult human. The largest ganglion number comprising 77% of all counted ganglia was identified on the dorsal atrial surface. Fetal epicardiac plexus in gestation period of 15-40 weeks contained 929 +/- 62 ganglia, but ganglion amount did vary substantially from heart to heart. In conclusion, this study implies that the human fetal epicardiac ganglia occupy their definitive location already at gestation period from 15 weeks and their number as well as distribution on heart surface presumably is not age dependent.

Stochastic behavior of atrial and ventricular intrinsic cardiac neurons

To quantify the concurrent transduction capabilities of spatially distributed intrinsic cardiac neurons, the activities generated by atrial vs. ventricular intrinsic cardiac neurons were recorded simultaneously in 12 anesthetized dogs at baseline and during alterations in the cardiac milieu. Few (3%) identified atrial and ventricular neurons (2 of 72 characterized neurons) responded solely to regional mechanical deformation, doing so in a tightly coupled fashion (cross-correlation coefficient r = 0.63). The remaining (97%) atrial and ventricular neurons transduced multimodal stimuli to display stochastic behavior. Specifically, ventricular chemosensory inputs modified these populations such that they generated no short-term coherence among their activities (cross-correlation coefficient r = 0.21 +/- 0.07). Regional ventricular ischemia activated most atrial and ventricular neurons in a noncoupled fashion. Nicotinic activation of atrial neurons enhanced ventricular neuronal activity. Acute decentralization of the intrinsic cardiac nervous system obtunded its neuron responsiveness to cardiac sensory stimuli. Most atrial and ventricular intrinsic cardiac neurons generate concurrent stochastic activity that is predicated primarily upon their cardiac chemotransduction. As a consequence, they display relative independent short-term (beat-to-beat) control over regional cardiac indexes. Over longer time scales, their functional interdependence is manifest as the result of interganglionic interconnections and descending inputs.

Frank-Starling mechanism contributes modestly to ventricular performance during atrial fibrillation

OBJECTIVES

The aim of this study was to assess whether Frank-Starling mechanism has an independent effect on left ventricular (LV) performance in atrial fibrillation (AF).

BACKGROUND

Ventricular performance in AF depends on variable contractility through the interval-force mechanism based on the ratio of preceding and pre-preceding RR intervals (RR(p)/RR(pp)). The impact of end-diastolic volume (EDV) variability, through the Frank-Starling mechanism, is not well understood.

METHODS

We induced AF in 16 open chest dogs. RR intervals, LV pressure, LV volume, and aortic flow were collected for >400 beats during rapid AF (ventricular cycle length 292 +/- 66 ms). In six of the dogs, additional data were collected while average ventricular cycle length was prolonged from 258 +/- 34 ms to 445 +/- 80 ms by selective vagal nerve stimulation of the AV node.

RESULTS

The relations of maximal LV power (LVPower) and peak LV pressure derivative (dP/dt) versus RR(p)/RR(pp) were fitted to the equation y = A * (1 - EXP (RR(p)/RR(pp)min - RR(p)/RR(pp))/C) and the residuals (RES) of these relations were analyzed. LVPower and dP/dt strongly correlated with RR(p)/RR(pp) (r(2) = 0.67 +/- 0.12 and 0.66 +/- 0.12, P < .0001 for all correlations). Importantly, RES-LVPower and RES-dP/dt showed linear correlation with EDV (r(2) = 0.20 +/- 0.14 and r(2) = 0.24 +/- 0.17, P < .01 for all correlations). In the six dogs with slowed average ventricular rate, the slope of both residual relationships (RES-LVPower vs EDV and RES- dP/dt vs EDV) decreased (P < .03 for both).

CONCLUSIONS

The Frank-Starling mechanism contributes to ventricular performance in AF independently of the interval-force effects of the beat-to-beat variability in cardiac contractility. The Frank-Starling mechanism is sensitive to the average ventricular rate.

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.

Parasympathetic control of the heart. I. An interventriculo-septal ganglion is the major source of the vagal intracardiac innervation of the ventricles

The locations, projections, and functions of the intracardiac ganglia are incompletely understood. Immunocytochemical labeling with the general neuronal marker protein gene product 9.5 (PGP 9.5) was used to determine the distribution of intracardiac neurons throughout the cat atria and ventricles. Fluorescence microscopy was used to determine the number of neurons within these ganglia. There are eight regions of the cat heart that contain intracardiac ganglia. The numbers of neurons found within these intracardiac ganglia vary dramatically. The total number of neurons found in the heart (6,274 +/- 1,061) is almost evenly divided between the atria and the ventricles. The largest ganglion is found in the interventricular septum (IVS). Retrogradely labeled fluorescent tracer studies indicated that the vagal intracardiac innervation of the anterior surface of the right ventricle originates predominantly in the IVS ganglion. A cranioventricular (CV) ganglion was retrogradely labeled from the anterior surface of the left ventricle but not from the anterior surface of the right ventricle. These new neuroanatomic data support the prior physiological hypothesis that the CV ganglion in the cat exerts a negative inotropic effect on the left ventricle. A total of three separate intracardiac ganglia innervate the left ventricle, i.e., the CV, IVS, and a second left ventricular (LV2) ganglion. However, the IVS ganglion provides the major source of innervation to both the left and right ventricles. This dual innervation pattern may help to coordinate or segregate vagal effects on left and right ventricular performance.

Parasympathetic control of the heart. III. Neuropeptide Y-immunoreactive nerve terminals synapse on three populations of negative chronotropic vagal preganglionic neurons

The vagal postganglionic control of cardiac rate is mediated by two intracardiac ganglia, i.e., the sinoatrial (SA) and posterior atrial (PA) ganglia. Nothing is known about the vagal preganglionic neurons (VPNs) that innervate the PA ganglion or about the neurochemical anatomy of central afferents that innervate these VPNs. These issues were examined using light microscopic retrograde labeling methods and dual-labeling electron microscopic histochemical and immunocytochemical methods. VPNs projecting to the PA ganglion are found in a narrow column exclusively in the ventrolateral nucleus ambiguus (NA-VL). These neurons are relatively large (37.6 +/- 2.7 microm by 21.3 +/- 3.4 microm) with abundant cytoplasm and intracellular organelles, rare somatic and dendritic spines, round uninvaginated nuclei, and myelinated axons. Previous physiological data indicated that microinjections of neuropeptide Y (NPY) into the NA-VL cause negative chronotropic effects. The present morphological data demonstrate that NPY-immunoreactive nerve terminals formed 18 +/- 4% of the axodendritic or axosomatic synapses and close appositions on VPNs projecting to the PA ganglion. Three approximately equal populations of VPNs in the NA-VL were retrogradely labeled from the SA and PA ganglia. One population each projects to the SA ganglion, the PA ganglion, or to both the SA and PA ganglia. Therefore, there are both shared and independent pathways involved in the vagal preganglionic controls of cardiac rate. These data are consistent with the hypothesis that the central and peripheral parasympathetic controls of cardiac rate are coordinated by multiple potentially redundant and/or interacting pathways and mechanisms.

Parasympathetic control of the heart. II. A novel interganglionic intrinsic cardiac circuit mediates neural control of heart rate

Intracardiac pathways mediating the parasympathetic control of various cardiac functions are incompletely understood. Several intracardiac ganglia have been demonstrated to potently influence cardiac rate [the sinoatrial (SA) ganglion], atrioventricular (AV) conduction (the AV ganglion), or left ventricular contractility (the cranioventricular ganglion). However, there are numerous ganglia found throughout the heart whose functions are poorly characterized. One such ganglion, the posterior atrial (PA) ganglion, is found in a fat pad on the rostral dorsal surface of the right atrium. We have investigated the potential impact of this ganglion on cardiac rate and AV conduction. We report that microinjections of a ganglionic blocker into the PA ganglion significantly attenuates the negative chronotropic effects of vagal stimulation without significantly influencing negative dromotropic effects. Because prior evidence indicates that the PA ganglion does not project to the SA node, we neuroanatomically tested the hypothesis that the PA ganglion mediates its effect on cardiac rate through an interganglionic projection to the SA ganglion. Subsequent to microinjections of the retrograde tracer fast blue into the SA ganglion, >70% of the retrogradely labeled neurons found within five intracardiac ganglia throughout the heart were observed in the PA ganglion. The neuroanatomic data further indicate that intraganglionic neuronal circuits are found within the SA ganglion. The present data support the hypothesis that two interacting cardiac centers, i.e., the SA and PA ganglia, mediate the peripheral parasympathetic control of cardiac rate. These data further support the emerging concept of an intrinsic cardiac nervous system.

Interactions within the intrinsic cardiac nervous system contribute to chronotropic regulation

The objective of this study was to determine how neurons within the right atrial ganglionated plexus (RAGP) and posterior atrial ganglionated plexus (PAGP) interact to modulate right atrial chronotropic, dromotropic, and inotropic function, particularly with respect to their extracardiac vagal and sympathetic efferent neuronal inputs. Surgical ablation of the PAGP (PAGPx) attenuated vagally mediated bradycardia by 26%; it reduced heart rate slowing evoked by vagal stimulation superimposed on sympathetically mediated tachycardia by 36%. RAGP ablation (RAGPx) eliminated vagally mediated bradycardia, while retaining the vagally induced suppression of sympathetic-mediated tachycardia (-83%). After combined RAGPx and PAGPx, vagal stimulation still reduced sympathetic-mediated tachycardia (-47%). After RAGPx alone and after PAGPx alone, stimulation of the vagi still produced negative dromotropic effects, although these changes were attenuated compared with the intact state. Negative dromotropic responses to vagal stimulation were further attenuated after combined ablation, but parasympathetic inhibition of atrioventricular nodal conduction was still demonstrable in most animals. Finally, neither RAGPx nor PAGPx altered autonomic regulation of right atrial inotropic function. These data indicate that multiple aggregates of neurons within the intrinsic cardiac nervous system are involved in sinoatrial nodal regulation. Whereas parasympathetic efferent neurons regulating the right atrium, including the sinoatrial node, are primarily located within the RAGP, prejunctional parasympathetic-sympathetic interactions regulating right atrial function also involve neurons within the PAGP.

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.

Selective AV nodal vagal stimulation improves hemodynamics during acute atrial fibrillation in dogs

Although the atrioventricular node (AVN) plays a vital role in blocking many of the atrial impulses from reaching the ventricles during atrial fibrillation (AF), a rapid irregular ventricular rate nevertheless persists. The goals of the present study were to explore the feasibility of novel epicardial selective vagal nerve stimulation for slowing of the ventricular rate during AF and to characterize the hemodynamic benefits in vivo. Electrophysiological-echocardiographic experiments were performed on 11 anesthetized open-chest dogs. Hemodynamic measurements were performed during three distinct periods: 1) sinus rate, 2) AF, and 3) AF with vagal nerve stimulation. AF was associated with significant deterioration of all measured parameters (P < 0.025). The vagal nerve stimulation produced slowing of the ventricular rate, significant reversal of the pressure and contractile indexes (P < 0.025), and a sharp reduction in one-half of the abortive ventricular contractions. The present study provides comprehensive evidence that slowing of the ventricular rate during AF by selective ganglionic stimulation of the vagal nerves that innervate the AVN successfully improved the hemodynamic responses.

Negative chronotropism of the heart is inhibited with lesions of the caudal medulla in the rat

Neurons in the ventrolateral medulla are essential for cardiorespiratory regulation. It has been suggested that neurons in the caudal ventrolateral medulla are responsible for the negative chronotropic effect of the heart, at least in carnivores, because injection of glutamate into this area decreases heart rate significantly. In the present study, we monitored heart rate both before and after injections of the excitotoxin ibotenic acid into the most caudal part of the ventrolateral medulla in rats. We found that resting heart rate increased significantly by more than 53% (P<0.0001) after the ibotenic acid injections. This result suggests that neurons located in the caudal ventrolateral medulla are responsible for the negative chronotropic effect of the heart in the rat, especially its most caudal part.

Effects of right and left vagal stimulation on left ventricular acetylcholine levels in the cat

To test the effectiveness of, and the interactions between, right and left vagal stimulation on left ventricular acetylcholine (ACh) levels, we applied the dialysis technique to the heart of anaesthetized cats. Dialysis probes were implanted in the left ventricular myocardium and perfused with Krebs-Henseleit buffer containing eserine. Dialysate ACh content was measured as an index of ACh release from post-ganglionic vagal nerve terminals in the left ventricular myocardium. We electrically stimulated the right and left cervical vagal nerves separately or together and investigated the dialysate ACh response. In two different regions of the left ventricle, substantial dialysate ACh responses were observed by the stimulation (20 Hz) of both right and left cervical vagal nerves. At stimulation frequencies of both 10 and 20 Hz, the dialysate ACh response to the bilateral vagal stimulation was almost algebraically the calculated sum of the individual dialysate ACh responses to unilateral vagal stimulation. In conclusion, ACh levels in the left ventricle are affected by both right and left vagal nerves and show little evidence of interactions between right and left vagal nerves at the level of the cardiac ganglia.

Control of negative inotropic vagal preganglionic neurons in the dog: synaptic interactions with substance P afferent terminals in the nucleus ambiguus?

Previous research from this laboratory has shown that substance P-immunoreactive (SP) terminals synapse upon negative chronotropic vagal preganglionic neurons (VPNs), but not upon negative dromotropic VPNs, of the ventrolateral nucleus ambiguus (NA-VL). Moreover, SP agonists injected into NA-VL cause bradycardia without decreasing AV conduction. In the current study, we have: (1) defined the electron microscopic characteristics of the SP neurons of NA-VL in dog; and (2) tested the hypothesis that SP nerve terminals synapse upon negative inotropic VPNs of NA-VL, retrogradely labeled from the cranial medial ventricular (CMV) ganglion. Numerous SP terminals and a few SP neurons were observed in the vicinity of retrogradely labeled neurons. SP terminals were observed forming synapses with unlabeled dendrites and with SP dendrites, but never with the retrogradely labeled neurons. Together, these results and earlier findings suggest that SP agonists may be able to induce bradycardia without decreasing AV conduction or ventricular contractility.

Negative inotropic vagal preganglionic neurons in the nucleus ambiguus of the cat: neuroanatomical comparison with negative chronotropic neurons utilizing dual retrograde tracers

The vagal postganglionic controls of cardiac rate and left ventricular contractility are mediated by separate intracardiac ganglia, the sino-atrial (SA) and cranio-ventricular (CV) ganglia, respectively. We injected a different retrograde tracer into each of these ganglia (in the same animal) and subsequently examined the brain for the presence of single labeled or double labeled vagal preganglionic neurons. Retrogradely labeled cells from either ganglion were found exclusively in the ventrolateral nucleus ambiguus (NA-VL). There was considerable overlap in the distribution of labeled cells from either ganglion, however fewer than 3% of labeled neurons were double labeled. The data are consistent with the hypothesis that the preganglionic controls of cardiac rate and left ventricular contractility are mediated by largely separate but overlapping groups of cardioinhibitory neurons originating from the NA-VL. These neurons have parallel but morphologically independent pathways projecting to the SA and CV ganglia. Physiological experiments are needed to support this hypothesis.