Impulse conduction of the heart

Question 1

Contractile tissue of the heart

Answer 1

-myocardium which is composed of cardiomyocytes

Question 2

Conduction system position

Answer 2

-buried in sub endocardial surface of heart (part that joins myocardium and endocardium)

Question 3

Cells of the conduction system

Answer 3

Specialized cells= Autorhythmic cells
1.Pacemaker cells (SA and AV nodes)
2.Conduction fibers (spread throughout the heart)

Question 4

AP rate of pacemaker cells and conduction fibers

Answer 4

-Both generate APs spontaneously
- But Pacemaker cells are much faster than conduction fibers

Question 5

What sets the pace of AP firing?

Answer 5

-SA node has the fastest rate so will set the pace

Question 6

Autorhythmic cell abundance

Answer 6

Only ~1% of cells in heart

Question 7

Do both the SA node and AV node need to fire everytime for the heart to beat?

Answer 7

-No, if one misfires, than the other one can pick up that beat.

Question 8

If SA node does not fire, what part of contraction might be missed?

Answer 8

-Atrial contraction
-Not detrimental because 2/3rds of filling will actually occur passively. The contraction is only needed to fill the last 1/3rd of the atrium

Question 9

Syncytium

Answer 9

-the cardiomyocytes work together as a unit through intercalated discs (gap junctions and anchoring proteins)

Question 10

Conduction steps of heart

Answer 10

1.AP begins at SA node (under normal conditions) which is located near vena cava. **Pacemaker step of heart
2.Depolarization wave: AP travels to the AV node through internodal pathways in the right atrium and it contracts (excitation contraction coupling)
3. AP reaches AV node at atrial septum and the impulse conduction speed is slowed down
-Allows for complete atrial depolarization and contraction to occur while ventricles fill more
4. Impulse than travels down Atrioventricular bundle (bundle of His) and then the right and left bundle branches along the septum towards the apex of the heart.
-causes a left to right depolarization of the interventricular septum
5.Impulse then spreads through the walls of the heart to the base through purkinje fibres
-depolarization of ventricles (simultaneous)
6.Relaxation occurs as everything repolarizes

Question 11

Rate of travel of impulse/AP along internodal pathway

Answer 11

• 1.5m/s
• From SA node to AV node

Question 12

Does the rate of travel of impulse/AP change at AV node?

Answer 12

-Yes it slows down. Now going at 0.5m/s

Question 13

Speed of AP/impulse at bundle branches

Answer 13

2m/s

Question 14

Speed of AP/impulse at Purkinje fibers

Answer 14

4 m/s

Question 15

Refractory period

Answer 15

-period after contraction/stimulation of both SA and AV nodes

Question 16

Impulse conduction system modulation by Autonomic nervous system

Answer 16

Autonomic nerves innervate the SA and AV nodes and can increase or decrease heart rate

Sympathetic
-Increase rate of depolarization
-Increase conduction velocity
**nerves also spread out more and enter the myocardium which means sympathetic NS can also increase contractility

Parasympathetic
-Decrease rate of depolarization
- Decrease conduction velocity

Question 17

Sympathetic Branch/Nerves

Answer 17

-release norepinephrine
-bind to beta adrenergic receptors
-generally speeds up

Question 18

Parasympathetic (Vagus) Branch/Nerves

Answer 18

-release acetylcholine
-bind to muscarinic receptors
-mostly effects conduction components, not much on inotropy (generally slows down)

Question 19

Parasympathetic vs. sympathetic

Answer 19

-constantly active together under normal conditions. Usually one is more active than the other depending on the situation the individual is in

Question 20

ECG

Answer 20

-impulse spreads through the heart and then through the surface of the body. Picked up by electrodes which can pick up the impulse

White to right, smoke over fire

Question 21

ECG-Atrial depolarization

Answer 21

-initial part of wave; slight increase (right side) and than decrease (left side of heart)
-captured with P wave

Question 22

ECG- Atrial contraction

Answer 22

-flattened part of ECG
-wave spreads through AV node and bundle
-conduction velocity between SA and AV nodes
-captured in PR interval (time interval between the atrial and ventricular depolarization)

Question 23

ECG- Ventricular depolarization and contraction

Answer 23

-spike of ECG
-conduction begins just after Q through to T wave
- known as the QRS wave (electrical event!)
-Note: atrial repolarization would also occur here but the ventricular depolarization masks this otherwise small event on the ECG

Question 24

ECG-ventricular repolarization

Answer 24

-Captured as T wave
-ventricles repolarize and relax (and prepare to do it all over again)

Question 25

QT interval

Answer 25

-time required for ventricles to depolarize and repolarize

Question 26

PR interval

Answer 26

-time interval between atrial and ventricular depolarization

Question 27

Autonomic control of heart rate

Answer 27

-If you have less time between R-R intervals=increased heart rate
-if you have more time between R-R intervals=decreased heart rate

Question 28

Heart rate variation

Answer 28

-heart rate is going up and down (always varying slightly) due to the modulation effects from the presence of norepinephrine or acetylcholine (which always exists to some extent)

Question 29

High disease states and heart rate

Answer 29

-often linked with high sympathetic states and less variability of heart rate
- less variability often results in greater chances of death

Question 30

Why does parasympathetic branch modulate heart rate much quicker?

Answer 30

-due to the fact that acetylcholine is very quickly broken down by acetylcholinesterase. So need to constantly pump out acetylcholine to bring heart rate down. When this is not happening, means that heart rate is high, not variable and sympathetic branch is taking over.