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Overview of Cardiac Conduction
The sinoatrial node is situated in the upper part of the appropriate atrium in the healthy and balanced heart, and serves together the organic pacemaker (Figure 1). These nodal cell manifest spontaneous depolarizations and are for this reason responsible because that generating the normal cardiac rhythm; together a heart rate can also be defined as intrinsic or automatic. Importantly, the frequency the this earliest cardiac depolarization is fine modulated through both sympathetic and parasympathetic efferent innervation. In addition, the nodal price can also be modulated through local transforms within perfusion and/or the chemical environment (i.e., neurohormonal, nutritional, oxygenation, etc.). Back the atrial rhythms usually emanate native the sinoatrial node, variations in the initiation site of atrial depolarization have been documented outside the the histological nodal tissues, an especially when high atrial rates are elicited, and may include paranodal tissue <10-14>.
One that the many conspicuous attributes of sinoatrial nodal cells is that they own poorly emerged contractile apparati (a typical feature to all myocytes devoted for conduction), making up only around 50% the the intracellular volume <1,10,15>. In general, although it typically cannot be checked out grossly, the ar of the sinoatrial node is ~ above the roof of the right atrium in ~ the approximate junction of the premium vena cava, the appropriate atrial appendage, and also the sulcus terminalis. In the adult human, the node is about 1 mm listed below the epicardium, 10-20 mm long, and also up to 5 mm thick <1,16>.
Figure 1. The conduction system of the heart. Common excitation originates in the sinoatrial (SA) node, climate propagates through both atria (internodal tracts displayed as dashed lines). The atrial depolarization spreads to the atrioventricular (AV) node, passes through the bundle that His (not labeled), and also then to the Purkinje yarn which comprise the left and right bundle branches; ultimately all ventricular muscle i do not care activated.
After early sinoatrial nodal excitation, depolarization spreads throughout the atria. The precise mechanisms associated in the spread of impulses (excitation) native the sinoatrial node throughout the atria room still today, somewhat controversial <1,17>. However, it is generally embraced that: 1) the spread of depolarizations from nodal cells have the right to go straight to nearby myocardial cells; and also 2) preferentially notified myofibril pathways permit this excitation to rapidly transverse the ideal atrium come both the left atrium and also the atrioventricular node (Figure 1). The is believed by many that there space three preferential anatomic conduction pathways indigenous the sinoatrial node to the atrioventricular node <1,18>. In general, these have the right to be thought about as the shortest electrical routes in between the nodes. Keep in mind that there are microscopically i can identify structures, showing up to be preferentially oriented fibers, that administer a direct node-to-node pathway. In part hearts, pale staining Purkinje-like fibers have likewise been reported in these regions. Much more specifically, the anterior tract is explained as extending 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 with a 2nd tract that descends along the interatrial septum that connects to the anterior component of the atrioventricular node. The middle (or Wenckebach's pathway) extends from the superior part of the sinoatrial node, operation posteriorly come the exceptional vena cava, then descends within the atrial septum, and may sign up with the anterior bundle as it start the atrioventricular node. The 3rd pathway is described as gift posterior (Thorel's) which, in general, is taken into consideration to extend from the inferior component of the sinoatrial node, passing v the crista terminalis and the Eustachian valve previous the coronary sinus to enter the posterior part of the atrioventricular node. In enhancement to excitation follow me these preferential conduction pathways, basic excitation spreads from cell to cell throughout the entire atrial myocardium via the devoted connections in between cells, the gap junctions, that generally exist in between all myocardial cell types (see below).
It then follows that towards the end of atrial depolarization, the excitation reaches the atrioventricular node via the previously mentioned atrial routes, v the final result being excitation of the atrioventricular node. Further, these courses are recognized as the sluggish or rapid pathways, i beg your pardon are thought about to be functionally and anatomically distinct. The slow pathway commonly crosses the isthmus between the coronary sinus and the tricuspid annulus; it has actually a longer conduction time, but a much shorter effective refractory period. The fast pathway is frequently a superior route, emanating from the interatrial septum, and has a much faster conduction rate but, in turn, a longer efficient refractory period. Common conduction during sinus valuation occurs along the fast pathway, but greater heart prices and/or premature beats are often conducted through the slow pathway, due to the fact that the rapid pathway might be refractory at these rates.
Though the primary function of the atrioventricular node might seem simple, that is to relay conduction between the atria and ventricles, its structure is very complex <1>. As a means to define these complexities, math arrays and also finite element analysis models have been created to elucidate the basic structure-function relationship of the node <19>.
In general, the atrioventricular node is situated in the so-called floor the the ideal atrium, end the muscular component of the interventricular septum, inferior to the membranous septum: i.e., in ~ the triangle that Koch, i m sorry is bordered by the coronary sinus, the tricuspid valve annulus follow me the septal leaflet, and the tendon of Todaro (Figure 2). Following atrioventricular nodal excitation, the slow pathway conducts impulses to the His bundle, indicated by a much longer interval in between atrial and His activation. Currently, there is interest in the capacity to place pacing leader to preferentially activate the bundle that His; in such approaches, ultrasound or other imaging modalities are offered to map the electric characteristic His potentials to position the pacing leader <20>.
After leaving the bundle that His, the regular wave of cardiac depolarization spreads very first to both the left and also right bundle branches; these pathways rapidly and simultaneously carry depolarization to the apical areas of both the left and also right ventricles (see number 1). Finally, the signal broadly travels with the remainder the the Purkinje fibers and ventricular myocardial depolarization spreads.
In certain pathological conditions, direct accessory connections from the atrioventricular node and also the penetrating part of the bundle of His come the ventricular myocardium have actually been explained <21>. Yet, the function and prevalence of this connections, termed Mahaim fibers, is poorly understood. A rare bundle the Kent, an additional aberrant pathway as soon as present, exists in between the atria and also ventricles and also is linked with the clinical manifestation that ventricular tachycardias (also known as Wolff-Parkinson-White syndrome). Therapeutically, this accessory pathway is electrically identified and then typically ablated as a curative procedure.
The left bundle branch splits right into fascicles as it travels under the left next of the ventricular septum just below the endocardium. The fascicles prolong for a distance of 5 to 15 mm, fanning the end over the left ventricle. Importantly, typically around midway come the apex that the left ventricle, the left bundle separates right into two significant divisions, the anterior and posterior branches (or fascicles). These departments extend come the basic of every papillary muscle and the nearby myocardium. In contrast, the best bundle branch proceeds inferiorly, as if it to be a continuation of the bundle that His, traveling follow me the appropriate side the the muscular interventricular septum. This bundle branch operation proximally, simply beneath the endocardium, and also its course runs slightly inferior to the septal papillary muscle the the tricuspid valve prior to dividing right into fibers that spread throughout the right ventricle. The complicated network that conducting fibers the extends from one of two people the ideal or left bundle branches is written of the quick conduction cells known as Purkinje fibers. Purkinje fibers in both the right and left ventricles act together preferential conduction pathways to administer rapid activation, for this reason to name: coordinates the excitation pattern in ~ the various regions of the ventricular myocardium. Most of this fibers take trip within the trabeculations the the right and also left ventricles, and also within the myocardium itself. Because of tremendous variability in the degree and also morphology that the trabeculations existing both within and between species, the is most likely that variations in the left ventricular conduction patterns also exist. It should be detailed that among the most common and easily recognized conduction pathways found in mammalian mind is the moderator band, which includes Purkinje fibers from the appropriate bundle branch (see: http://www.civicpride-kusatsu.net/atlas/right-ventricle/moderator-band/index.shtml). Furthermore, in numerous human hearts, within both the right and left ventricles, one have the right to identify conduction bands that room white in illustration (e.g., check out apex videos within the right and also left ventricles).
In 1910, Aschoff and Monckeberg listed three criteria for considering a myocardial cell together a dedicated conduction cell, including: 1) the capacity to histologically recognize discrete features; 2) the capacity to monitor cells from ar to section; and also 3) insulation the the cell by fibrous sheaths from the nonspecialized contractile myocardium <22,23>. It is noteworthy that only the cells within the bundle that His, left and also right bundle branches, and also Purkinje fibers accomplish all 3 criteria. No structure within the atria meets all 3 criteria, including 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 most likely that new criterion will certainly follow that much better define the uniqueness of committed conduction structures.
Figure 2. The conduction mechanism of the heart. Left: regular excitation originates in the sinoatrial (SA) node then propagates with both atria. The atrial depolarization spreads to the atrioventricular (AV) node, and passes through the bundle that His come the bundle branches/Purkinje fibers. Right: The table shows conduction velocities and intrinsic pacemaker rates of assorted structures in ~ the cardiac conduction pathway. The structures are noted in the order of activation during a normal cardiac contraction, start with the sinoatrial node. Keep in mind that the intrinsic pacemaker rate is slow in structures additional along the activation pathway. Because that example, the atrioventricular nodal rate is slower than the sinoatrial nodal rate. This avoids the atrioventricular node indigenous generating a spontaneous rhythm under common conditions, due to the fact that it continues to be refractory at prices