Conducting System Of Heart

SINO ATRIAL NODE:
  • The sinus node is crescent like in shape with a mean length of 13.5 mm in the adult present in sulcus/cristaterminalis at the superior vena cava–atrial junction
  • In most cases the head is subepicardial, whereas the tail penetrates inferiorly into the myocardium of the terminal crest to lie closer to the subendocardium.
  • SA node is composed of a cluster of small fusiform cells 
  • Although the specialized myocytes of the nodal cells are set in a fibrous matrix, the node is not encased in a fibrous sheath, with frequent interdigitations between nodal and ordinary atrial myocytes
  • The node is richly supplied with nerves from both the sympathetic chains and the vagus nerve.
  • SA node acts as a pacemaker of the heart because of the fact that it Generates impulses at the highest rate
  • The artery supplying the sinus node branches from the RCA (55 to 60 percent) or the LCA (40 to 45 percent)
  • SA node is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatory) system
Internodal and Intraatrial Conduction :
  • The anterior internodal pathway

Bachmanns bundle
2. Middle internodal pathway
Wenkhebach bundle
3. Posterior internodal pathway
Tract Of Thorel
4.These groups of internodal tissue are best referred to as internodal atrial myocardium, not tracts, because they do not appear to be histologically discrete specialized tracts.
Bachmann bundle
  • Most prominent interatrial bridge
  • Broad muscular band that runs in the subepicardium connecting the anterior right atrial wall of the SVC RA junction with the anterior wall of the LA.
  • SAN artery and its branches are the principal vascular supply of BB
  • Less visible in patients with severe coronary artery disease, atrial fibrillation, and interatrial conduction block
  • Changes in the musculature of BB could block or prolong interatrial conduction resulting in abnormal atrial excitability, atrial dysfunction, AF, and other arrhythmias
ATRIOVENTRICULAR JUNCTION
  • AV junctional area can be divided into distinct regions:
  1. The transitional cell zone:connect the atrial myocardium with the compact portion of the AV node
  2. The compact portion, or the AV node :superficial structure lying just beneath the right atrial endocardium at the apex of triangle of Koch ,5 mm long and wide.Slowest in
  3. conduction 
  4. The penetrating part of the AV bundle (His bundle)compact portion of the AV node is divide & form penetrating portion of the his bundle at the point where it enters the central fibrous body
  • In triangle of Koch, the tendon of Todaro, which forms one side of the triangle of Koch, is absent in about two thirds of hearts.
  • Fibers in the lower part of the AV node may exhibit automatic impulse formation
BUNDLE OF HIS:
  • Connects with the distal part of the compact AV node, perforates the central fibrous body, and continues through the annulus fibrosis, where it is called the nonbranching portion as it penetrates the membranous septum
  • Proximal cells of the penetrating portion are heterogeneous and resemble those of the compact AV node; distal cells are similar to cells in the proximal bundle branches.
  • Branches from the anterior and posterior descending coronary arteries supply the upper muscular interventricular septum with blood, which makes the conduction system at this site more resistant to ischemic damage unless the ischemia is extensive.
Characteristics of the Right Bundle
  • Long, thin, discrete, and vulnerable
  • Consists of fast response Purkinje fbers.
  • Courses down the right side of interventricular septum near the endocardium in its upper third, deeper in the muscular portion of the septum in the middle third, and then again near the endocardium in its lower third. 
  • Do not divide throughout most of its course, and begins to ramify as it approaches the base of the right anterior papillary muscle, with fascicles going to the septal and free walls of the RV.
Characteristics of the Left Bundle and Its Fascicles 
  • Penetrates the membranous portion of the IVS under the aortic ring and then divides into several fairly discrete branches.
  • LAF crosses the LVOT and terminates in the Purkinje system of the anterolateral wall of the LV.
  • LPF appears as an extension of the main LB and is large in its initial course. It then fans out extensively posteriorly toward the papillary muscle and inferoposteriorly to the free wall of the LV.
  • An estimated 65% of individuals have a, the left median fascicle (LMF).The LMF runs to the interventricular septum, and it arises in most cases from the LPF, or LAF or from both, or independent origin from the central part of the main LB at the site of its bifurcation.
PURKINJE FIBERS:
  • Purkinje fibres are modified cardiac muscle
  • Connect ends of the BBs to ventricles muscle, which transmit the cardiac impulse almost simultaneously to the entire RV and LV endocardium.
  • Tend to be less concentrated at the base of the ventricle and at the papillary muscle tips.
  • Penetrate only the inner 1/3rd of the endocardium
  • Maximum velocity of conduction 
Exam Question
  • Maximum velocity of conduction is seen in purkinje fiber
  • Slowest conduction is in AV node
  • The SA node is composed of a cluster of small fusiform cells in the sulcus/crista terminalis at the right atrial–superior vena caval junction
  • SA node acts as a pacemaker of the heart because of the fact that it Generates impulses at the highest rate
  • Arterial supply to SA node is by RCA
  • SA node is Supplied by nodal artery
  • SA node is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatory) system
  • Purkinje fibres are modified cardiac muscle

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