What structure is not part of the hearts conduction system?

2.5 Emergence of the Cardiac Conduction System

The cardiac conduction system (CCS) cooperates with the valves to ensure that blood flow is unidirectional by controlling electrical impulses from the apex of the ventricle to its base, allowing efficient blood ejection into the outflow tract. Development of the CCS has been described in four phases that correlate to morphological changes occurring throughout early cardiac development (Chi et al., 2008). The first phase corresponds to the linear conduction across the linear heart tube. During looping phases, the second phase leads to a significant delay across the forming AVC. The third phase occurs during the formation of trabeculation whereby an immature fast conduction network emerges in the ventricle. The final phase is completed by 2–3 weeks postfertilization, when the fully mature fast conduction network shows an apex-to-base activation pattern (Chi et al., 2008).

Cardiac Muscle in DM1

The cardiac conduction system is selectively vulnerable to the effects of DM1. Surface electrocardiograms (ECGs) show an increase of the PR interval or prolongation of QRS duration in 65% of affected individuals (54). Slowing can occur at any point along the conduction pathway but is most commonly localized to the His–Purkinje system (55). The effects on the conduction system are progressive over time, with an average increase of 5 msec/yr for the PR interval and 2 msec/yr for QRS duration (56). Eventually this can lead to atrioventricular block, resulting in severe bradycardia or asystole, complications that are largely preventable by insertion of a pacemaker (57). DM1 also predisposes to atrial and, less commonly, ventricular tachycardia (57). Cardiac dysrhythmia ranks second after respiratory failure among causes of death in DM1 (3). In a large prospective study the risk of sudden death was 1.1% per year in adults (56). Cardiac histology shows focal areas of fibrosis and fatty infiltration in the conduction system (58). However, it is not known whether these changes can account for the physiologic defect, or whether functional changes, perhaps resembling the chloride channelopathy in skeletal muscle, may be superimposed.

The effects of DM1 on cardiac contractility are much less profound. Conventional echocardiography has shown normal ventricular size and systolic function in most individuals (59,60). In a study of 406 affected individuals, 10% had clinically evident heart failure or echocardiographic changes suggesting left ventricular systolic dysfunction (LVSD) (61). The frequency of LVSD increased after age 40, reaching levels as high as 30% by the eighth decade.

Overview of the Cardiac Conduction System

The cardiac conduction system is a network of specialized cells responsible for the initiation and co-ordination of the heartbeat. Relative to the myocytes responsible for regulating cardiac contraction in a normal heart (~1 × 109), the cells that make up the cardiac conduction system are relatively few in number, but are essential for cardiac electrical signaling and normal physiology. The three main components of the system are the sinoatrial node (SAN), the atrioventricular node (AVN) and the His–Purkinje system (HPS). The SAN, located in the right atrium, is the primary pacemaker of the heart and thus is responsible for the normal initiation of the cardiac action potential (sinus rhythm; Figure 7.1). Cells in the conduction system (SAN, AVN, and Purkinje cells) are unique in their ability to generate an electrical impulse or action potential without an external stimulus. This property of automaticity requires a distinct ion channel profile and a finely controlled intracellular electrical coupling. With respect to SAN function, pacemaking requires a synchronized effort because of the principle of entrainment (whereby faster discharging cells are slowed by cells firing more slowly) in a highly heterogeneous complex.1 The innervation of the sinus node consists of post-ganglionic adrenergic and cholinergic terminals.2 Most of the efferent vagal fibers converge at the superior vena cava–aortic root fat pad in the right atria, which is also the site of the highest concentrations of norepinephrine.3 Subsequently the SAN is triggered to discharge after catecholamines bind to sympathetic nerve terminals, causing a positive heart-rate-dependent response via β-adrenergic receptors and the cyclic adenosine monophosphate signaling pathway. A negative chronotropic response is caused by vagal stimulation via acetylcholine binding to muscarinic receptors.4 Internodal tracts then lead from the SAN to the AVN to continue conduction.

Following initiation in the sinoatrial node, the cardiac action potential propagates to the atrioventricular node (AVN; Figure 7.1), located at the apex of a triangle formed by the tricuspid annulus and the tendon of Todaro.5,6 The atria and the ventricles are separated by a ring of fibrous tissue, and the only conduction pathway between the two sets of chambers is the AVN, which is located at the base of the right atrium. Conduction of the action potential through the atrioventricular node is relatively slow, consistent with its role as a functional delay between atrial and ventricular systole to allow the atria to pump blood into the ventricles before they in turn contract. Due to automaticity of AVN cells, the AVN may serve as a secondary pacemaker in case of SAN failure (e.g. due to aging or disease). In the event of atrial tachyarrhythmia (e.g. atrial fibrillation or atrial flutter), the AVN plays an important role in limiting the number of action potentials conducted to the ventricles. The main function of the AVN is transmission of the atrial impulse to the His–Purkinje system (HPS) and the ventricles to stimulate chamber contractions.

The HPS in the ventricles consists of a common bundle (the bundle of His), the left and right bundle branches (which arise from the bundle of His), and a network of terminal Purkinje fibers (which arise from the bundle branches; Figure 7.1). The function of the HPS is to conduct the action potential rapidly (at velocities up to four meters/second; compared to 0.3–1 meter/second in ventricle7) to the ventricles to ensure that the ventricular muscle contracts simultaneously.8 The bundle of His or the penetrating portion of the AVN sends out extensions to the actual bundle branches.9 These fibers connect with the terminal Purkinje fibers on the endocardial surface of the ventricles. Here they form multicellular bundles in longitudinal strands that transmit the atrial action potential to the ventricle to stimulate myocyte contraction.10

Conduction system

The cardiac conduction system consists of the sinus node, internodal pathways, and atrioventricular conduction tissues. Its function is influenced by the innervation of the heart (described below). The sinus and atrioventricular nodes are both right atrial structures. It is important to note that all structures of the conduction system are specialized cardiac myocytes (not nerves) whose primary function is impulse propagation rather than contraction and relaxation.

The sinus node is located subepicardially, along the superior aspect of the sulcus terminalis, near the confluence of the superior vena cava, sinus venosus, and muscular right atrial free wall. These latter two structures are the reason the sinus node is sometimes referred to as the sinoatrial node. It is elliptical in shape with the sinoatrial nodal artery, which is often grossly identifiable, running centrally through the nodal tissue.

The internodal pathways serve as preferential tracts by which impulse propagation is transmitted from the sinus node to the atrioventricular node. They have been described in the atrial septum, right atrial free wall, and the crista terminalis. While electrophysiologically identifiable, these structures have not been definitively teased out morphologically, except for the Bachman bundle between the atria [72].

The atrioventricular node is located subendocardially (rather than subepicardially, like the sinus node) within the triangle of Koch. The triangle of Koch is the anatomic region bordered by the tricuspid annulus (of the septal tricuspid leaflet), the ostium of the coronary sinus, and the tendon of Todaro (described above). The apex of the triangle is adjacent to the central fibrous body. The atrioventricular nodal artery does not necessarily travel intranodally, like that of the sinus nodal artery.

The atrioventricular bundle (bundle of His) extends from the node, traveling through the central fibrous body, to the basal ventricular septum, adjacent to the membranous septum. It then splits into both its right and left bundle branches. The right bundle branch is a cord-like structure that travels subendocardially to the moderator band and then out to the free wall of the right ventricle. The broaderleft bundle branch also travels subendocardially toward the left ventricular apex, while fanning out into fascicles of Purkinje cells.

Molecular Markers of the Cardiac Conduction System

Visualization of the developing CCS has been greatly enhanced by the development of conduction system reporter mice (Table 29.121–45). Each reporter mouse delineates different components of the CCS at various developmental time points using LacZ, green fluorescent protein (GFP), or Cre transgenes. The CCS-LacZ,46 Contactin-2-EGFP (Cntn2-EGFP),47 and Cx40-eGFP48 mouse lines are representative examples of well-established markers of the specialized conduction system. (Fig. 29.3A–C). Using the CCS-LacZ and Cntn2-EGFP reporter lines, novel regulators of Purkinje cell specification and function have been identified, such as the transcription factor ETV1.49 The Etv1 nuclear-LacZ (Etv1nlz)49a reporter gene is expressed throughout the CCS but has the highest levels of expression in regions of rapid conduction, namely the pectinated atrial myocardium and the trabeculated ventricular myocardium, which matures into the VCS49 (Fig. 29.3D). Etv1-nlz reporter demonstrates overlapping expression with Cntn2-EGFP in Purkinje cells throughout the VCS49 (Fig. 29.3E). While some CCS reporters delineate the entire conduction system, others are more restricted in their expression pattern. All CCS reporter lines have some degree of expression outside of the specialized conduction system, such as in the atria, coronary arteries, cardiac nerves, valves, or extracardiac sites.

Molecular Markers of the Cardiac Conduction System

Visualization of the developing CCS has been greatly enhanced by the development of conduction system reporter mice (Figure 29-3). Each reporter mouse delineates different components of the CCS at various developmental time points using LacZ or green fluorescent protein (GFP) expression. The CCS-LacZ and minK-LacZ mouse lines are representative examples of well-established markers of the specialized conduction system.17-19 The CCS-LacZ mouse was created serendipitously through a complex genomic rearrangement involving the MC4/engrailed-2-LacZ cassette (see Figure 29-3, A).18 CCS-LacZ reporter expression can first be detected at E8.5 in the SAN primordium within the venous pole. At subsequent stages, β-gal expression is detected in the developing and mature AVN and His-Purkinje system. All CCS reporter lines have some degree of cardiac expression outside of the conduction system. In the adult CCS-LacZ heart, significant β-gal expression is seen within the right atrium.18

The minK-LacZ reporter mouse was created by replacing the minK gene with a nuclear-targeted LacZ cassette.17 Early in development, β-gal expression was noted in the SA ring, AV ring, interventricular ring, and the VA ring. Subsequently, β-gal expression was confined to the AVN and the proximal conduction system, as well as in the venous valves, AV ring, and VA valves.17,19

The Cx40-eGFP reporter mouse has become a widely used tool to characterize normal and abnormal patterning of the mature His-Purkinje system (see Figure 29-3, C).20 Developmentally, Cx40 expression is not restricted to the VCS, with significant expression in the trabecular myocardium. In addition, Cx40 is not expressed in the distal AVN or His bundle before E14.5. In the mature heart, Cx40 is enriched in atrial myocardium and in coronary endothelial cells.20

Contactin-2 (Cntn-2) was recently identified as a CCS-enriched factor using differential gene profiling of adult mouse Purkinje fibers versus working myocytes (see Figure 29-3, B).21 Cntn-2 is a cell adhesion molecule that has a role in neuronal patterning and ion channel clustering. Both Cntn2-LacZ knock-in mice and Cntn2-EGFP BAC transgenic reporter mice delineated the entire cardiac conduction system in postnatal hearts. Currently, a functional role for Cntn-2 in the CCS has not been identified.21

CARDIAC CELL ACTION POTENTIAL

CAs alter the cardiac conduction system in a myriad of ways. The most distinctive toxicity relates to the inhibition of the fast sodium channels in the His-Purkinje tissue, leading to a slowing of phase 0 depolarization.46,47 This “membrane stabilizing” or “quinidine-like” effect is analogous to that of Vaughn Williams (VW) class I antidysrhythmic drugs.48 Impaired depolarization of cells within the His-Purkinje system slows the propagation of ventricular depolarization. This appears on the electrocardiogram (ECG) as prolongation of the QRS interval, the hallmark of TCA toxicity. The degree of conduction delay is rate-dependent and worsens with tachycardia.49 The QRS morphology is generally that of nonspecific intraventricular conduction delay, with discrete bundle branch block being less common. However, the longer refractory period of the right bundle relative to the left leads to the characteristic rightward axis deviation of the terminal 40 msec of the QRS complex seen in many patients with TCA toxicity.50 On the ECG this appears as an increased R wave amplitude in lead aVR and a deep S wave in leads I and aVL (Fig. 27-2).51 A less specific finding is prolongation of the corrected QT interval (QTc). This delay in myocyte repolarization may result from a direct effect of CAs on potassium channels.52,53 A prolonged QTc may also be seen in therapeutic dosing.

Cardiac Manifestations

MMD commonly involves the cardiac conduction system at an early stage, with pathological studies showing diffuse interstitial fibrosis, fatty infiltration, and focal vacuolar degeneration.157,158 Conduction abnormalities are the most common cardiac manifestations in MMD patients and predict cardiac events and sudden death. Both conduction disease and cardiac events are correlated with the length of CTG repeats and the age of the patient.159,160 In fact, the QRS interval has been reported to increase by 0.54 ms/year.161 Patients with >1000 repeats seem to be at a higher risk for a rapid progression of conduction system disease.159 MMD most commonly affects the His–Purkinje system but may involve the sinoatrial and AV nodes. Sixty-five percent of MMD patients are reported to have abnormalities on ECG, including prolonged PR, QRS, and QTc; nonspecific ST-T changes; AV blocks; bundle branch block; and premature ventricular beats.162 Studies have shown that surface ECG changes are important markers of sudden death and all-cause mortality.163 Patients with conduction defects may be asymptomatic or may experience shortness of breath, dizziness, syncope, and, in more severe cases, sudden death.164

The most commonly reported tachyarrhythmias are AF and AFL, likely promoted by atrial wall fibrosis and fatty infiltration.165 The possible mechanisms leading to VT are varied and include triggered activity, reentry around fibrofatty infiltrates, fascicular VT, and torsades de pointes induced by prolonged QT and bundle branch reentrant VT.166,167 Recognition of the latter is essential as its induction requires specific pacing protocols (long-short–coupled extra stimuli), infusion of drugs (isoproterenol or procainamide), or both and is readily amenable to ablation.168

LV dysfunction occurs in MMD patients to a lesser extent and at later disease stages than conduction disturbances; nonetheless, it increases all-cause as well as cardiovascular death rates (relative risk [RR] of 3.9 and 5.7, respectively).169,170 LVH and LV dilation as well as systolic dysfunction were reported to have a prevalence of 15%–20%, correlating with patient age and CTG repeats.170 Clinical HF occurs at a lower rate of approximately 2%–6%. Other structural abnormalities reported in MMD include ischemic heart disease, LV dyssynchrony, LV relaxation dysfunction (or myocardial myotonia), HF with preserved EF, left atrial dilation, mitral valve prolapse, and RV dysfunction.171–173

Morphology and Immunohistochemistry of the Sinoatrial Node Innervation in Humans and Other Mammals

All regions of the CCS possess a significantly higher density of nerve fibers than the adjacent working myocardium.3,20,38,39 The SAN is defined as the most densely innervated region of the human CCS.3 The highest density of nerve fibers immunoreactive for general neuronal marker protein gene product (PGP) 9.5 has also been observed in the guinea pig SAN compared with a significantly lower density of nerve fibers in the surrounding right atrium.8 More than a three-fold higher density of PGP 9.5–immunoreactive innervation relative to that of the surrounding atrial myocardium is confirmed in the pig heart.20

Fluorescence immunohistochemistry has shown that the mouse SAN CMs positive for HCN4 are accompanied by a dense fine meshwork of nerve fibers. Slender and narrow HCN4-immunoreactive myocytes from the main mass of the SAN extend toward the right auricle, the root of the right PV, and the root of the caudal vein (Fig. 37.3). Compared with the right atrial areas adjacent to the root of the right cranial vein (the right cranial vein prevailing in many mammalian species corresponds to the SVC in humans), the density of nerve fibers amid the cardiac pacemaker cells positive for HCN4 is three- to four-fold higher.9

The density of nerve fibers and their phenotypes vary between the zones of the SAN, and such variability is species dependent.8,20,21,38 Significantly more nerve fibers are distributed in the central zone of the human SAN surrounding the nodal artery compared with the nodal periphery, including the perivascular innervation of small arteries and arterioles in the atrial myocardium.3 However, no significant difference was found in the total percentage of the stained area of nerves immunoreactive for PGP 9.5 between the central and peripheral nodal regions of the pig heart.20

Electron microscopic data conclusively demonstrate that all nerve fibers identified in the mouse SAN are exclusively composed of unmyelinated nerve fibers and involve axons with both cholinergic and adrenergic neurotransmitters.9 The axons within unmyelinated nerve fibers have varicosities with abundant round, small, clear, and a few dense-cored vesicles. A number of unmyelinated nerve fibers have axons that are incompletely enveloped by Schwann cells with a fragment of their plasma membrane in direct contact with the basal lamina surrounding the whole unmyelinated nerve fiber (Fig. 37.4). These unmyelinated nerve fibers have varicosities and are distributed regularly in the vicinity of cardiac pacemaker cells.9 The density of nerve fibers within the distinct zones of the root of the right cranial vein (SVC, in humans) is significantly higher than that in the neighboring atrial zones, which correlates well between corresponding data of fluorescent and electron microscopy. Unmyelinated nerve fibers with numerous axonal varicosities are situated predominantly close to cardiac pacemaker cells. The closest unmyelinated nerve fibers are located 0.06 μm from cardiac pacemaker cells, but occasionally some were 2 μm or more away from such cells. In the mouse SAN, the average distance between cardiac pacemaker cells and unmyelinated nerve fibers is less than 0.5 μm,9 whereas it is only about 80 nm in the guinea pig SAN.40 SAN cells are closely associated with at least one unmyelinated nerve fiber or axon, but the majority of these cells usually are in close proximity to two to three unmyelinated nerve fibers.9

Nerve fibers immunoreactive for tyrosine hydroxylase (TH) are observed within large nerves running adjacent to AChE-positive nerves located within the nodal tissue but close to the border with the atrial musculature and also at the nodal surfaces adjacent to both the SVC and the terminal crest.20 TH immunoreactivity is detectable in a significantly lower proportion of nerves and nerve fibers than that for AChE activity, representing approximately 30% of the total nodal innervations in pig hearts. Axonal profiles of TH immunoreactivity distribute in nerves that also possess AChE-positive fibers.20 TH-immunoreactive nerves represented 40%–45% of the total SAN innervation as displayed by PGP 9.5 immunoreactivity in the guinea pig heart; however, unlike AChE-positive nerves, a large number of TH-immunoreactive nerves are associated with perivascular plexuses both in and around the SAN.8 A meshwork rich in both cholinergic and adrenergic nerve fibers and possessing axons with a high amount of varicosities fills the mouse SAN region. Choline acetyltransferase (ChAT)-immunoreactive and TH-immunoreactive nerve fibers are equally abundant in the mouse SAN.9

Neuropeptide Y (NPY)–immunoreactive nerves are distributed similarly and occupy a similar area as TH-immunoreactive nerves. NPY-immunoreactive nerve fibers represent the predominant peptide-containing nerve subpopulation in the SAN and are significantly higher than NPY-containing nerves in the surrounding right atrium in the pig20 and the guinea pig heart.8 Examinations by Steele and Choate41 of the guinea pig heart determined that the entire SAN was densely innervated by sympathetic axons, the majority of which were NPY immunoreactive. However, only a few axons were revealed to be immunoreactive for TH. In the human heart, the relative density of sympathetic nerve fibers immunoreactive for NPY and TH is significantly greater in the central SAN region compared with the periphery SAN region.3

After NPY, the other predominant peptidergic nerve subpopulations are immunoreactive for the sensory peptide substance P (SP) and calcitonin gene–related peptide (CGRP).8,41 Somatostatin (SOM)-immunoreactive nerves are less abundant than SP- and CGRP-containing nerves and possess a distinct pattern of distribution compared with other nerve populations. Vasoactive intestinal peptide (VIP)-immunoreactive nerves are very sparse in both the SAN and the surrounding right atrium. Nerve fibers displaying immunoreactivity for VIP are rare in the pig SAN area. When found, these fibers are closely associated with small blood vessels, mainly arterioles, within the nodal tissue and also, to a lesser extent, in the atrial myocardium.20 Altogether, these nerves appear to represent a relatively minor component of the sinus nodal innervation exhibiting a percentage of stained area 10- to 40-fold less than that of NPY- and TH-immunoreactive nerves, respectively.3,8 Individual VIP- and SOM-immunoreactive nerve fibers scatter among SAN cells, whereas SP- and CGRP-immunoreactive nerve fibers occur mainly in epicardial nerves and are also found surrounding cell bodies in cardiac ganglia. No difference is found between the area of SP- and CGRP-stained nerves.3 The great majority of CGRP-immunoreactive neural tissue in the canine heart exists adjacent to the SAN where varicose nerve processes course in numerous large nerve bundles.42 Immunoreactivity for CGRP is localized to isolated nerve fibers within AChE-positive nerves. CGRP-positive nerve fibers occupy a significantly lower percentage stained area than the subpopulation of nerves immunoreactive for NPY and represent approximately 8% of the total neural population. SOM-immunoreactive nerves are relatively sparse compared with either NPY- or CGRP-immunoreactive nerves, representing less than 4% of the overall innervation.

In the study by Steele and Choate,41 intrinsic parasympathetic neurons from the guinea pig heart were extrinsically denervated by placing them in organotypic culture to allow degeneration of extrinsic axons. These experiments demonstrated several distinct populations of parasympathetic nerves innervating only a small, discrete part of the SAN. Such populations were immunoreactive for NPY, SOM, or VIP alone or for SOM combined with NPY, SOM with dynorphin B, and SOM with SP. These results highlighted a remarkable difference in the pattern of innervation of the SAN by the sympathetic and parasympathetic nervous systems.