Conduction System of the Heart

Question 1

What does the sinoatrial (SA) node do

Answer 1

Spontaneously generate electrical impulses

Question 2

What is the pacemaker of the cell

Answer 2

Sinoatrial (SA) node

Question 3

Where is the sinoatrial (SA) node located

Answer 3

Superior end of the crista terminalis

Question 4

At what rate does the SA node stimulate contractions

Answer 4

70 per minute

Question 5

What do impulses of the SA node stimulate

Answer 5

Contraction of the atria

Question 6

Where are impulses of the SA conducted to

Answer 6

Atrioventricular (AV) node

Question 7

Where are the AV node located

Answer 7

Located at the inferior end of the interatrial septum

Question 8

Where do the impulses travel after the AV node

Answer 8

Conducting fibres from the atrioventricular bundle

Question 9

What is the other name for the atrioventricular bundle

Answer 9

Bundle of His

Question 10

What does the atrioventricular bundle divide into

Answer 10

Two groups of fibres

Right and left bundle branches

Question 11

What do the left and right bundle branches give rise to

Answer 11

Purkinje fibres

Question 12

Where do the Purkinje fibres enter

Answer 12

Myocardium of the left and right ventricles

Stimulate contractions

Question 13

What is the order of conduction

Answer 13

SA node generates electrical impulses

Impulses from SA stimulate contraction of atria

Impulses are conducted to AV node

AV node to atrioventricular bundle to right and left bundle branches to Purkinje fibres

Purkinje fibres enter myocardium of right and left ventricles and stimulate contractions

Question 14

What is the SA node supplied by

Answer 14

Right coronary artery in 60% of people

Left coronary artery in 40% of people

Question 15

What is the AV node supplied by

Answer 15

Posterior interventricular artery

Question 16

What supplied the bundle of his in most people

Answer 16

Left coronary artery

Question 17

What does sympathetic fibres stimulation cause

Answer 17

Increase the heart rate and force of contraction

Question 18

What does parasympathetic fibres stimulation cause

Answer 18

Slows the heart rate and force of contraction

Question 19

What is the heart also innervated by

Answer 19

Visceral afferent

Question 20

What do visceral afferent fibres convey

Answer 20

Sensory information from the heart back to the CNS

Question 21

What is referred pain

Answer 21

Pain cannot be pinpointed

Generally felt in left side of neck and or left arm

Question 22

What does a myocyte membrane pump

Answer 22

K+ pumped in to the cell

Na+ and Ca2+ pumped OUT the cell

Against conc. gradient (ATP)

Question 23

What causes repolarisation back to the resting potential

Answer 23

Outward K+ current

Question 24

Describe a cardiac action potential

Answer 24

1. Resting Potential
   - Na/K ATPase - 2K+ in 3Na+ out
   - Na/K leak channels
2. Depolarisation
   - large number of Na+ enter the cell (voltage gated Na channel) causing the charge to increase from -90mv to +20mv
3. Initial repolarisation
   - transient outward current of K+ ions leaving the cell causing a small repolarisation
4. Plateau
   - Calcium channels open, causing calcium to enter the cell and maintain depolarised state
5. Repolarisation
   - Outward K+ current causes repolarisation back to resting potential

Question 25

What number stage is resting potential and what happens

Answer 25

4 - Na/K ATPase - 2K+ in 3Na+ out - Na/K leak channels

Question 26

What number stage is depolarisation and what happens

Answer 26

0 - large number of Na+ enter the cell (voltage gated Na channel) causing the charge to increase from -90mv to +20mv

Question 27

What stage is initial repolarisation and what happens

Answer 27

1 - transient outward current of K+ ions leaving the cell causing a small repolarisation

Question 28

What stage is plateau and what happens

Answer 28

2 - Calcium channels open, causing calcium to enter the cell and maintain depolarised state

Question 29

What stage is repolarisation and what happens

Answer 29

3 - Outward K+ current causes repolarisation back to resting potential

Question 30

Describe action potential propagation

Answer 30

1. Local depolarisation activates nearby Na+ channels 2. Influx of sodium ions 3. Further influx of sodium ions causes a wave of depolarisation Action potential spreads across membrane Gap junctions allow cell to cell conduction and propagation of action potential throughout the whole myocardium

Question 31

Describe excitation-contraction coupling

Answer 31

1. influx of calcium through surface ion channels 2. Amplification of Ca2+ with NaCa 3. Calcium induced calcium release

Question 32

Describe the troponin-tropomyosin-actin complex

Answer 32

Calcium binds to troponin Conformational change in tropomyosin reveals myosin binding sites Myosin heads cross-links with actin Myosin head pivots causing muscle contraction

Question 33

How much longer does cardiac contraction last compared to skeletal muscle

Answer 33

15x longer duration Due to slow calcium channels Decreased permeability of membrane to potassium after action potential

Question 34

What is the threshold for the SAN

Answer 34

35mV

Question 35

What is the phase slop of the SAN affected by

Answer 35

Autonomic tone Drugs Hypoxia Electrolytes Age

Question 36

Delaying of the impulse by the AV node allows for what

Answer 36

Atria to empty with blood into the ventricles Fewer gap junctions AV fibres are smaller than atrial fibres - Limits dangerous tachycardias

Question 37

What is the velocity of conduction by the atrial and ventricular muscle fibres

Answer 37

0.3-0.5 m/s

Question 38

What is the velocity of conduction by the purkinje fibres

Answer 38

4 m/s

Question 39

How long is the normal refractory period

Answer 39

0.25s

Question 40

Why is there a refractory period

Answer 40

Allow adequate time for the heart to fill Prevents excessively frequent contraction