| || |
Overview the Cardiac Conduction
The sinoatrial node is situated in the upper part of the right atrium in the healthy heart, and also serves together the natural pacemaker (Figure 1). This nodal cell manifest voluntarily depolarizations and are for this reason responsible because that generating the regular cardiac rhythm; together a heart rate can likewise be described as intrinsic or automatic. Importantly, the frequency that this faster cardiac depolarization is fine modulated through both sympathetic and also parasympathetic efferent innervation. In addition, the nodal rate can likewise be modulated by local alters within perfusion and/or the chemical environment (i.e., neurohormonal, nutritional, oxygenation, etc.). Although the atrial rhythms usually emanate native the sinoatrial node, sports in the initiation site of atrial depolarization have actually been recorded outside that the histological nodal tissues, particularly when high atrial rates are elicited, and may encompass paranodal organization <10-14>.
One of the most conspicuous functions of sinoatrial nodal cells is the they own poorly emerged contractile apparati (a common feature to all myocytes committed for conduction), making up only about 50% the the intracellular volume <1,10,15>. In general, return it typically cannot be seen grossly, the ar of the sinoatrial node is on the roof of the right atrium at the almost right junction of the superior vena cava, the appropriate atrial appendage, and also the sulcus terminalis. In the adult human, the node is around 1 mm below the epicardium, 10-20 mm long, and up to 5 mm special <1,16>.
Figure 1. The conduction system of the heart. Typical excitation originates in the sinoatrial (SA) node, then propagates through both atria (internodal tracts shown as dashed lines). The atrial depolarization spreads to the atrioventricular (AV) node, passes through the bundle that His (not labeled), and also then come the Purkinje yarn which make up the left and right bundle branches; subsequently all ventricular muscle i do not care activated.
After early sinoatrial nodal excitation, depolarization spreads throughout the atria. The exact mechanisms associated in the spread out of impulses (excitation) indigenous the sinoatrial node across the atria space still today, somewhat controversial <1,17>. However, that is generally accepted that: 1) the spread of depolarizations native nodal cells deserve to go straight to nearby myocardial cells; and also 2) preferentially bespeak myofibril pathways allow this excitation to quickly transverse the right atrium come both the left atrium and also the atrioventricular node (Figure 1). It is thought by countless that there space three preferential anatomic conduction pathways indigenous the sinoatrial node come the atrioventricular node <1,18>. In general, these have the right to be considered as the shortest electrical routes in between the nodes. Keep in mind that there room microscopically i can identify structures, appearing to be preferentially oriented fibers, that provide a direct node-to-node pathway. In some hearts, pale staining Purkinje-like fibers have likewise been reported in this regions. An ext specifically, the anterior street is explained as expanding from the anterior component of the sinoatrial node, bifurcating right into the so-called Bachmann's bundle which important delivers impulses to the left atrium and also with a 2nd tract that descends follow me the interatrial septum that connects come the anterior component of the atrioventricular node. The middle (or Wenckebach's pathway) extends from the superior part of the sinoatrial node, operation posteriorly to the superior vena cava, then descends in ~ the atrial septum, and may sign up with the anterior bundle together it enters the atrioventricular node. The third pathway is described as gift posterior (Thorel's) which, in general, is thought about to expand from the inferior component of the sinoatrial node, passing through the crista terminalis and also the Eustachian valve previous the coronary sinus to go into the posterior part of the atrioventricular node. In enhancement to excitation along these preferential conduction pathways, general excitation diffusion from cabinet to cell throughout the whole atrial myocardium via the specialized connections in between cells, the space junctions, that typically exist in between all myocardial cell types (see below).
It then complies with that in the direction of the end of atrial depolarization, the excitation will the atrioventricular node via the aforementioned atrial routes, through the final result being excitation the the atrioventricular node. Further, these paths are well-known as the slow or fast pathways, which are thought about to be functionally and anatomically distinct. The slow-moving pathway typically crosses the isthmus in between the coronary sinus and also the tricuspid annulus; it has actually a longer conduction time, but a much shorter effective refractory period. The rapid pathway is generally a premium route, emanating from the interatrial septum, and has a much faster conduction price but, in turn, a longer effective refractory period. Common conduction during sinus rate occurs follow me the rapid pathway, but higher heart prices and/or premature beats room often carried out through the sluggish pathway, due to the fact that the rapid pathway may be refractory at these rates.
Though the primary duty of the atrioventricular node may seem simple, that is to relay conduction between the atria and also ventricles, its framework is very facility <1>. Together a way to define these complexities, math arrays and finite element analysis models have actually been created to elucidate the underlying structure-function partnership of the node <19>.
In general, the atrioventricular node is situated in the so-called floor that the appropriate atrium, over the muscular part of the interventricular septum, inferior to the membranous septum: i.e., within the triangle of Koch, i m sorry is bordered by the coronary sinus, the tricuspid valve annulus follow me the septal leaflet, and also the tendon that Todaro (Figure 2). Following atrioventricular nodal excitation, the sluggish pathway conducts impulses come the His bundle, indicated by a much longer interval between atrial and His activation. Currently, there is interest in the capability to place pacing leader to preferentially activate the bundle of His; in together approaches, ultrasound or various other imaging modalities are supplied to map the electrical characteristic His potentials to position the pacing leader <20>.
After leaving the bundle of His, the typical wave the cardiac depolarization spreads an initial to both the left and also right bundle branches; this pathways rapidly and simultaneously carry depolarization to the apical areas of both the left and right ventricles (see figure 1). Finally, the signal extensively travels v the remainder that the Purkinje fibers and also ventricular myocardial depolarization spreads.
In specific pathological conditions, direct accessory relations from the atrioventricular node and also the penetrating section of the bundle of His come the ventricular myocardium have been explained <21>. Yet, the function and pervasiveness of these connections, termed Mahaim fibers, is poorly understood. A rare bundle that Kent, second aberrant pathway as soon as present, exists between the atria and also ventricles and also is connected with the clinical manifestation that ventricular tachycardias (also recognized as Wolff-Parkinson-White syndrome). Therapeutically, this accessory pathway is electrically identified and then commonly ablated as a curative procedure.
The left bundle branch splits right into fascicles together it travels under the left next of the ventricular septum just listed below the endocardium. That is fascicles extend for a street of 5 come 15 mm, fanning out over the left ventricle. Importantly, typically around midway to the apex that the left ventricle, the left bundle separates right into two significant divisions, the anterior and also posterior branches (or fascicles). These divisions extend come the basic of each papillary muscle and the adjacent myocardium. In contrast, the ideal bundle branch continues inferiorly, together if it were a extension of the bundle the His, traveling along the ideal side of the muscular interventricular septum. This bundle branch runs proximally, just beneath the endocardium, and its course operation slightly inferior to the septal papillary muscle the the tricuspid valve before dividing into fibers that spread throughout the appropriate ventricle. The complicated network of conducting fibers the extends from either the ideal or left bundle branches is composed of the quick conduction cells known as Purkinje fibers. Purkinje yarn in both the right and left ventricles act as preferential conduction pathways to carry out rapid activation, therefore to name: coordinates the excitation pattern in ~ the assorted regions the the ventricular myocardium. Most of these fibers travel within the trabeculations that the right and left ventricles, as well as within the myocardium itself. As result of tremendous variability in the degree and also morphology of the trabeculations currently both within and between species, the is likely that variations in the left ventricular conduction patterns likewise exist. It need to be detailed that among the many common and easily well-known conduction pathways uncovered in mammalian hearts is the moderator band, which includes Purkinje fibers from the ideal bundle branch (see: http://www.snucongo.org/atlas/right-ventricle/moderator-band/index.shtml). Furthermore, in plenty of human hearts, in ~ both the right and left ventricles, one deserve to identify conduction bands that room white in figure (e.g., view apex videos in ~ the right and left ventricles).
In 1910, Aschoff and also Monckeberg listed three criteria for considering a myocardial cell together a specialized conduction cell, including: 1) the capability to histologically recognize discrete features; 2) the ability to track cells from section to section; and also 3) insulation of the cell by fibrous sheaths from the nonspecialized contractile myocardium <22,23>. The is significant that just the cell within the bundle the His, left and also right bundle branches, and also Purkinje fibers accomplish all three criteria. No structure within the atria meets all three criteria, consisting of the Bachmann's bundle, sinoatrial node, and also atrioventricular node (which space all uninsulated tissues). Yet, with major advances in histo-molecular techniques, the is likely that new criterion will follow that much better define the uniqueness of committed conduction structures.
Figure 2. The conduction device of the heart. Left: common excitation originates in the sinoatrial (SA) node then propagates with both atria. The atrial depolarization diffusion to the atrioventricular (AV) node, and passes with the bundle the His come the bundle branches/Purkinje fibers. Right: The table mirrors conduction velocities and also intrinsic pacemaker prices of various structures within the cardiac conduction pathway. The structures are noted in the stimulate of activation throughout a common cardiac contraction, beginning with the sinoatrial node. Keep in mind that the intrinsic pacemaker price is slow in structures additional along the activation pathway. For example, the atrioventricular nodal price is slower than the sinoatrial nodal rate. This avoids the atrioventricular node native generating a spontaneous rhythm under regular conditions, because it stays refractory at prices