CONDUCTING SYSTEM OF HEART

Ans. is. D. Purkinje fibers 

Velocity of Signal Conduction in Cardiac Muscle:

• The velocity of conduction of the excitatory action potential signal along both atrial and ventricular muscle fibers is about 0.3 to 0.5 m/sec, or about 1/250 the velocity in very large nerve fibers and about 1/10 the velocity in skeletal muscle fibers. 
• The velocity of conduction in the specialized heart conductive system in the Purkinje fibers is as great as 4 m/sec in most parts of the system, which allows reasonably rapid conduction of the excitatory signal to the different parts of the heart

The SA nodal artery arises from the right coronary artery in 55–60% and the left circumflex artery in 40–45% of persons. One-third to one-half of patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter.

The sinoatrial (SA) node normally displays the highest intrinsic rate. All other pacemakers are referred to as subsidiary or latent pacemakers because they take over the function of initiating excitation of the heart only when the SA node is unable to generate impulses or when these impulses fail to propagate. 

The Atrio Ventricular (AV) node is the slowest conducting structure with the longest refractory period in the heart.

• Allows time to the atria to contract
• Allows complete ventricular filling
• The fastest Conduction velocity is in the Purkinje fibers 
• Allows the ventricles to contract at the same time simultaneously

• Magnitude and rate of rise of the action potential 
• Diameter of the fibres (AV node radius of the fibres 7 µm and purkinje fibres is close to 50 µm).
• The conduction also depends on the tight junctions: The AV node conduction is slow due to diminished gap junctions between cells, taking ~ 0.09 sec and 0.04 sec, respectively; there is an abundance of gap junctions in intercalated discs between Purkinje fiber cells, therefore traversing entire expanse of ventricle only takes 0.03 sec after depolarization front exits the septum.

Ref: Cardiovascular physiology, By William R Milnor, Page 88.

A i.e. Is located at the right border of ascending aorta

S.A node is located at the junction of superior venacave with right atriumQ just deep to the epicardium, near the superior end of sulcus terminalis and AV node is located in the right postero-inferior region of interatrial septum near the opening of coronary sinus.

Ans. is ‘c’ i.e., Verapamil

o Amongst calcium channel blockers, verapamil has the most prominent cardiac electrophysiological action.
o CCBs primarily act on SA node and AV node (slow channel – Ca⁺² channel action potential) →↓ automaticity in SA node and decreased conduction in AV node.

Answer is A (SA Node)

The rate of impulse generation/rate of discharge is highest in the SA node.

Note:

Highest Rate of impulse generation in seen in     : SA NodeQ

Highest Rate of conduction is seen in                  : Purkinje SystemQ

The right coronary artery arises from the anterior aortic sinus of the ascending aorta and runs forward between the pulmonary trunk and the right auricle.

The artery of the sinoatrial node (branch of the right coronary artery) supplies the SA node and the right and left atria; in 40% of individuals, it arises from the left coronary artery.

Ans. C: Have conduction velocity of four times than that of the heart muscle

• Conduction rate is 4 m/s in Purkinje fibers whereas it is 1 m/s in ventricular muscles, atrial pathways, and Bundle of His. 
• SA Node and AV Node has a conduction speed of 0.05 m/s

 Ans: C. Supplied by the left vagus nerve

• SA node is located in the upper part of the crista terminalis at the junction of SVC and the right atrium.
• It is the pacemaker of the heart and generates impulse at a rate of 70-100/min.
• SA node is supplied by a nodal artery, a branch of RCA in 65% cases and a branch of the circumflex branch of LCA in 35% cases.
• SA node is supplied by the right vagus/parasympathetic (inhibitory) and right sympathetic (excitatory) system as it develops from structures on the right side of the embryo.
• The SA node consists of connective tissue stroma containing an irregular whorled network of cardiac nodal cells, the SA nodal artery and numerous nerve endings (postganglionic parasympathetic and postganglionic sympathetic).

 SA node

In crista terminalis

Ans. is ‘c’ i.e., Modified cardiac muscle

Conducting system of heart

The conducting system is made of specialized myocardium (cardiac muscle), that is capable for initation and conduction of cardiac impulse. It has following parts : ‑

• SA node :- SA node is located in the upper part of crista terminalis at the junction of SVC and the right atrium. It is the pacemaker of the heart and generates impulse at a rate of 70-100/min. SA node is sniffled by nodal artery, a branch of RCA in 65% cases and a branch of circumflex branch of LCA in 35% cases. SA node is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatory) system as it develops from structures on the right side of embryo.
• AV node :- It lies in the right atrial floor near the interatrial septum in the ‘triangle of koch’. It is supplied by AV nodal artery, a branch of RCA. AV node develops from left side of heart, thus is supplied by left vagus and left sympathetic fibers.
• Atrioventricular bundle or bundle of His :- It arises from AV node and crosses the AV ring (annulus fibrosus).
• In the muscular septum it divides into right and left branches. It has a dual blood supply from AV nodal artery (branch of RCA) and anterior descending (interventricular) branch of LCA.
• Right bundle branch (RBB) and left bundle branch (LBB) :- These bundle branches consist of modifeid muscle fibers (Purkinje fibers). Both RBB and LBB are supplied by LCA, except a small part of the LBB which is supplied by RCA.

Ans. is ‘c’ i.e., Koch’s triangle 

• AV node is situated in triangular part (triangle of Koch) in the lower part of interatrial septum.
• Triangle of Koch is landmark for AV node with its transition zone. It is bounded by tendon of todaro (superiorly), tricuspid valve septal leaflet (inferiorly) and coronary sinus orifice (basally).

Conducting system           Location in heart

SA node                             Upper end of crista terminalis, near opening of SVC

AV node                             Triangle of Koch’s (near interatrial septum)

Bundle of his                      Membranous part of interventricular septum

RBB                                   Right surface of interventricular septum

LBB                                    Left surface of interventricular septum