Cardiac Conduction

Question 1

What is the intercalated disk?

Answer 1

The intercalated disk is a region of intracellular connections between cardiac cells. Gap junctions allow the propagation of APs between cardiac cells through connexons which determine the internal resistance.

Question 2

What is the function of the SA node?

Answer 2

Pacemaker - slow conduction

Question 3

What is the function of the AV node?

Answer 3

Pacemaker - slow conduction

Question 4

What is the function of atrial and ventricular muscle?

Answer 4

Contraction - rapid conduction

Question 5

What is the function of the His-Purkinje Bundle?

Answer 5

Very rapid conduction to coordinate the simultaneous contraction of the left and right ventricles

Question 6

How is the space constant related to the electrical conduction between cardiac cells?

Answer 6

The space constant is related to the membrane and internal resistances which is also related to the K+ permeability, the gap junction connexons and cell diameter.

Question 7

How does the upstroke of the action potential related to the rate of conduction of the cell?

Answer 7

Upstroke of the action potential is related to the Na channels available which is related to the rate of conduction.

Question 8

How is the resting membrane potential related to the availability of Na channels?

Answer 8

The higher the RMP is, and thus, the more depolarized it is, the less Na channels that will be available and the slower conduction will be.

Question 9

What is the P-R interval?

Answer 9

It is the conduction time from the atria to the ventricles

Question 10

What is the QRS complex?

Answer 10

It is the conduction through the ventricle sand it should be <100 ms

Question 11

What NT does the parasympathetic nervous system use to regulate the cardiac conduction?

Answer 11

Acetylcholine

Question 12

What NT does the sympathetic nervous system use to regulate the cardiac conduction?

Answer 12

Norepinephrine

Question 13

How does ACh affect the conduction of the AV node?

Answer 13

ACh increases K+ membrane permeability which leads to hyper polarization of the RMP, making it harder for it to reach thresholds and activate action potentials -> inhibition of the SA/AV nodes

Question 14

How does NE affect the conduction of the AV node?

Answer 14

NE binds β-adrenergic receptors, which increase cAMP leading to an increase in the slow Ca current -> affects all areas of the heart: increasing atrial and ventricular contraction and increases the SA node rate and AV node conduction and the P-R interval is decreased as a result.

Question 15

How will an ACh affected cardiac cycle show up on the EKG?

Answer 15

The P-R interval will be lengthened

Question 16

How will an NE affected cardiac cycle show up on the EKG?

Answer 16

The P-R interval will be shortened

Question 17

What is 1st degree AV block?

Answer 17

Abnormal elongation of the P-R interval.

Question 18

What is 2nd degree AV block?

Answer 18

Not all P-waves are followed by a QRS, indicating that the atria are failing to activate the ventricles.

Question 19

What is 3rd degree AV block?

Answer 19

Complete AV node block so that there is no consistent P-R interval. The ventricles and the atria are completely disconnected.

Question 20

What is supra ventricular tachycardia?

Answer 20

It is a rapid rhythm originating at or above the AV node and the conduction through the ventricles is normal with normal QRS duration. The HR is fast but ventricles are normal and so stroke volume is normal.

Question 21

What is ventricular tachycardia?

Answer 21

It is a rapid rhythm that originates in the ventricles because conduction is slow and it does not travel through the His-Purkinje system. The QRS duration is prolonged and ventricle motion is abnormal, compromising stroke volume.

Question 22

What is atrial fibrillation?

Answer 22

Due to reentry of excitation and the disorganized electrical activity causes asynchronous contraction which ceases pumping. Not life threatening due to most of the atrial blood not requiring contraction to pump into the ventricles.

Question 23

What is ventricular fibrillation?

Answer 23

Due to reentry of excitation and the disorganized electrical activity causes asynchronous contraction which ceases pumping. This is rapidly fatal as the ventricular contraction is crucial in order to perfuse the body and the heart itself.

Question 24

What is the injury current?

Answer 24

It is a current that occurs in the heart as a result of infarcted tissue. The gap junctions running through the cells are damaged and causes the abnormality.

Question 25

What is healing over?

Answer 25

An injury current can disappear from detection if the gap junctions connecting infarcted tissue are shut down. The current will disappear but that does not mean that the infarcted tissue has healed.

Question 26

What can influence the upstroke of the action potential?

Answer 26

Hyperkalemia
Premature Excitation during the Relative Refractory Period
Ischemia or Infarction