Chapter-10 Rhythmical Excitation Of The Heart

Question 1

How many times the heart is rhythmically excited each day approxiy?

Answer 1

100,000

Question 2

The cardiac conduction system is vulnerable to what type of insult?

Answer 2

Ischemia due to reduced coronary blood flow

Question 3

Anatomy of sinus node?

Answer 3

It is an ellipsoid structure located inferoateral to the opening of the superior vena cava. It is 1 mm thick, 15mm long and 3 mm wide. The diameter of the SA nodal fibers are only 3 to 5 micrometers.

Question 4

What is the resting membrane potential of the SA node?

Answer 4

-55 to -60 mV.

Question 5

What causes less negative resting membrane potential in SA node as compared to the cardiac muscles?

Answer 5

The SA nodal cells are inherently leaky to positively charged calcium and sodium ions, this incurs a lower negative resting membrane potential.

Question 6

Why is the onset and offset of SA nodal AP slower than the ventricular AP?

Answer 6

At -55 mV the fast sodium channels remain close. The AP generation and repolarization are mediated by the slow L type calcium channels and slow potassium channels respectively. This causes the depolarization dynamics slower in SA nodal cells.

Question 7

What is the mechanism of SA nodal self excitation?

Answer 7

The SA nodal self excitation is mediated by the opening of the L-type calcium channels, when the resting membrane potential reaches -40 mV due to the inward funny current flow occurs through the leaky sodium and calcium channels.

Question 8

What is the velocity of conduction in atrial muscles ?

Answer 8

0.3 m/sec.

Question 9

What is the velocity of conduction in anterior interatrial bundle( Bachmann’s bundle)?

Answer 9

1 m/sec

Question 10

What are the components of internodal pathway?

Answer 10

Anterior internodal pathway
Thorel’s bundle or posterior internodal pathway
Wenckebach bundle or middle internodal pathway

Question 11

What is the anatomical location of the AV node?

Answer 11

Posterior wall of the right atrium, immediately behind the tricuspid valve

Question 12

What is the cardiac impulse delay in AV node?

Answer 12

0.09 seconds

Question 13

How much time dose it take the SA nodal impulse to reach AV bundle

Answer 13

Atrial conduction time+ AV nodal delay= 0.12 seconds

Question 14

What is the cardiac impulse delay in penetrating branches of AV bundle?

Answer 14

0.04 seconds

Question 15

The duration of SA nodal impulse to reach ventricular muscles ?

Answer 15

0.16 sec

Question 16

The largest fibers in the cardiac conduction system are ?

Answer 16

Purkinje system fibers

Question 17

Purkinje fibers have the highest conduction velocity: how much ?

Answer 17

1.5 to 4.0 M/ sec. It is six times higher than the ventricular fibers velocity and 150 times higher than the AV bundle fibers velocity

Question 18

What is the cause of super fast conduction through the purkinje fibers?

Answer 18

It has plenty of Gap junctions at the intercalated discs+ they have little to no myofibrils

Question 19

What is the time required for the cardiac impulse to travel from the purkinje fibers to the epicardial surface?

Answer 19

60 Ms.

Question 20

What is the maximum time required for the cardiac impulse to spread in the right atrium and the left atrium ?

Answer 20

70 Ms. And 90 Ms respectively.

Question 21

What is the total time required for the cardiac impulse to reach the epicardial surface area from the beginning of it in the SA node?

Answer 21

220 Ms.

Question 22

What is the intrinsic rhythmical discharge rate of AV node ?

Answer 22

40 to 60 BPM

Question 23

What is the intrinsic rhythmical discharge rate of purkinje fibers?

Answer 23

20-40 beats per minute

Question 24

Why is sinus node the pacemaker of the normal heart?

Answer 24

Although AV node and purkinje system has intrensic rhythmical self excitation capacity, the SA node has the fastest self excitation rate thus it controls the peacemaking function of the heart

Question 25

What is Stoke Adams syndrome?

Answer 25

When there is complete block of cardiac conduction through the AV node and bundle of Hiss. The purkinje system will become the natural pacemaker of the heart l. However, it takes 5 to 20 seconds for the purkinje system to take over the pacing at a rate of 15-40 beats per minute as its intrensic rhythmical self excitation function was suppressed by the preceding SA nodal impulse. In the first 4 to 5 seconds of ventricular pumping failure the patient will faint due to lack of cerebral perfusion. This phenomenon is called stokes- Adams syndrome.

Question 26

What is the mechanism of the cardiac vagal effect?

Answer 26

It is incurred by acetylcholine released by the vagal nerv endings which increases SA nodal conductive fibers membrane permeability for potassium ions leading to more exocytosis of potassium resulting in hyperpolarization of the conductive cells the resting membrane potential of SA nodal cells will drop from the range of -55 to -60 mV to -65 to -75 mV.

Question 27

What is the effect of SA nodal hyperpolarization on rhythmical self excitation?

Answer 27

In a membrane potential of -65 to -75 the leaky sodium and calcium channels can’t mediate funny current induced drift of resting membrane potential to threshold potential, consequently the potential of the SA node to Rhythmically self excite decreases significantly with very strong stimulation arrest it completely

Question 28

Mechanism of sympathetic stimulation on the conductive system?

Answer 28

It is mediated by the activation of beta 1 adrenergic receptors by the norepinephrine released by the sympathetic nerve endings.