Conducting System Of Heart

CONDUCTING SYSTEM OF HEART

Q. 1 Maximum velocity of conduction is seen in ?
 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje fibers

Q. 1 Maximum velocity of conduction is seen in ?
 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje fibers

Ans. D

Explanation:

Purkinje fibers REF: Guyton’s physiology 22nd edition page 105 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


Q. 2

Choose the TRUE/FALSE statements regarding cardicac conduction functions:

a) The SA node is composed of a cluster of small fusiform cells in the sulcus terminalis at the right atrial–superior vena caval junction
b) The SA nodal artery arises from the right coronary artery in 20 % and the left circumflex artery in 80 % of persons
c) SA node dysfunction may be completely asymptomatic 
d) Almost all  patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter
e) Failure to increase the heart rate with exercise is referred to as chronotropic incompetence
 
 A

a,b,c,d,e-True & None-False

 B

a,e-True & b,c,d-False

 C

a,c,e-True & b,d-False

 D

b,c,d-True & a,e-False

Q. 2

Choose the TRUE/FALSE statements regarding cardicac conduction functions:

a) The SA node is composed of a cluster of small fusiform cells in the sulcus terminalis at the right atrial–superior vena caval junction
b) The SA nodal artery arises from the right coronary artery in 20 % and the left circumflex artery in 80 % of persons
c) SA node dysfunction may be completely asymptomatic 
d) Almost all  patients with SA node dysfunction develop supraventricular tachycardia, usually atrial fibrillation or atrial flutter
e) Failure to increase the heart rate with exercise is referred to as chronotropic incompetence
 
 A

a,b,c,d,e-True & None-False

 B

a,e-True & b,c,d-False

 C

a,c,e-True & b,d-False

 D

b,c,d-True & a,e-False

Ans. C

Explanation:

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.

 
Ref: Harrisons Principles of Internal Medicine, 18th Edition, Page 1869

Q. 3

SA node acts as a pacemaker of the heart because of the fact that it:

 A

Is capable in generating impulses spontaneously

 B

Has rich sympathetic innervations

 C

Has poor cholinergic innervations

 D

Generates impulses at the highest rate

Q. 3

SA node acts as a pacemaker of the heart because of the fact that it:

 A

Is capable in generating impulses spontaneously

 B

Has rich sympathetic innervations

 C

Has poor cholinergic innervations

 D

Generates impulses at the highest rate

Ans. D

Explanation:

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. 

There is a hierarchy of intrinsic rates of subsidiary pacemakers that have normal automaticity: atrial pacemakers have faster intrinsic rates than AV junctional pacemakers, and AV junctional pacemakers have faster rates than ventricular pacemakers.
 
Ref: Chen P., Antzelevitch C. (2011). Chapter 38. Mechanisms of Cardiac Arrhythmias and Conduction Disturbances. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst’s The Heart, 13e.

Q. 4

Slowest conduction is seen in:

 A

SA node

 B

AV node

 C

Bundle of his

 D

Pukinje fibres

Q. 4

Slowest conduction is seen in:

 A

SA node

 B

AV node

 C

Bundle of his

 D

Pukinje fibres

Ans. B

Explanation:

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

Cardiac tissue conduction velocity (fastest –> slowest):
Purkinje system –> Atrial muscle –> Ventricular muscle –> AV node
The slowest conduction velocity is in the AV node 
  • 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
Cardiac tissue Conduction: The heartbeat originates in a specialized cardiac conduction system and spreads via this system to all parts of the myocardium. The structures that make up the conduction system are the sinoatrial node (SA node), the internodal atrial pathways, the atrioventricular node (AV node), the bundle of His and its branches, and the Purkinje system.
 
The factors related to the speed of conduction are the
  • 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.
AV nodal delay: There is delay of 0.1 s before the excitation spreads to ventricles. This delay is shortened by stimulation of the sympathetic nerves to the heart and lengthened by stimulation of the vagi. 
 

Tissue

Conduction speed (m/s)

Sino atrial node

0.05

Atrial pathways

1

Atrio ventricular node

0.05

Bundle of His

1

Purkinje system

4

Ventricular muscle

1

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


Q. 5

The following statements are true regarding the SA node except:

 A

Is located at the right border of the ascending aorta

 B

It contains specialized nodal cardiac muscle

 C

It is supplied by the artial branches of the right coronary artery

 D

It initiates cardiac conduction

Q. 5

The following statements are true regarding the SA node except:

 A

Is located at the right border of the ascending aorta

 B

It contains specialized nodal cardiac muscle

 C

It is supplied by the artial branches of the right coronary artery

 D

It initiates cardiac conduction

Ans. A

Explanation:

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.


Q. 6

Which of the following calcium channel blocker would be useful in the treatment of supra-ventricular tachycardia by suppressing AV node conduction ‑

 A

Amlodipine

 B

Nimodipine

 C

Verapamil

 D

Nifedipine

Q. 6

Which of the following calcium channel blocker would be useful in the treatment of supra-ventricular tachycardia by suppressing AV node conduction ‑

 A

Amlodipine

 B

Nimodipine

 C

Verapamil

 D

Nifedipine

Ans. C

Explanation:

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.


Q. 7

Highest rate of impulse generation is given in:

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje system

Q. 7

Highest rate of impulse generation is given in:

 A

SA node

 B

AV node

 C

Bundle of HIS

 D

Purkinje system

Ans. A

Explanation:

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


Q. 8

Arterial supply to SA node is by:       

 A

Left anterior descending coronary artery

 B

Posterior interventricular (descending) artery

 C

Left coronary artery

 D

Right coronary artery

Q. 8

Arterial supply to SA node is by:       

 A

Left anterior descending coronary artery

 B

Posterior interventricular (descending) artery

 C

Left coronary artery

 D

Right coronary artery

Ans. D

Explanation:

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


Q. 9

True statement regarding purkinje fibres:

September 2009, March 2013

 A

Are myelinated fibres

 B

Have action potential about a tenth as long as those in the heart muscle

 C

Have conduction velocity of four times than that of the heart muscle

 D

All of the above

Q. 9

True statement regarding purkinje fibres:

September 2009, March 2013

 A

Are myelinated fibres

 B

Have action potential about a tenth as long as those in the heart muscle

 C

Have conduction velocity of four times than that of the heart muscle

 D

All of the above

Ans. C

Explanation:

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


Q. 10

True about SA node are all except ‑

 A

Supplied by nodal artery

 B

Primary pacemaker

 C

Supplied by left vagus nerve

 D

Made up of nodal cells and connective tissue

Q. 10

True about SA node are all except ‑

 A

Supplied by nodal artery

 B

Primary pacemaker

 C

Supplied by left vagus nerve

 D

Made up of nodal cells and connective tissue

Ans. C

Explanation:

 

  • 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 suplied 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.
  • 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).

Q. 11

Which of the following is situated in the upper part of Crista terminalis ‑

 A

AV node

 B

Bundle of his

 C

Right Bundle branch

 D

SA node

Q. 11

Which of the following is situated in the upper part of Crista terminalis ‑

 A

AV node

 B

Bundle of his

 C

Right Bundle branch

 D

SA node

Ans. D

Explanation:

 SA node


Q. 12

SA node is located in ‑

 A

Triangle of Koch’s

 B

In crista terminalis

 C

In membranous part of interventricular septum

 D

Upper part of interatrial septum

Q. 12

SA node is located in ‑

 A

Triangle of Koch’s

 B

In crista terminalis

 C

In membranous part of interventricular septum

 D

Upper part of interatrial septum

Ans. B

Explanation:

In crista terminalis


Q. 13

Purkinje fibres are ‑

 A

Modified nerve fibres

 B

Modified smooth muscle

 C

Modified cardiac muscle

 D

Fibrous tissue

Q. 13

Purkinje fibres are ‑

 A

Modified nerve fibres

 B

Modified smooth muscle

 C

Modified cardiac muscle

 D

Fibrous tissue

Ans. C

Explanation:

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 : ‑

1) 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.

2) 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.

3) 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.

4) 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.


Q. 14

AV node is situated in ‑

 A

Opening of SVC

 B

Interventricular septum

 C

Koch’s triangle

 D

None

Q. 14

AV node is situated in ‑

 A

Opening of SVC

 B

Interventricular septum

 C

Koch’s triangle

 D

None

Ans. C

Explanation:

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



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