The heart as an electrical pump

Question 1

What are the 6 main components of the cardiac conduction system?

Answer 1

Sinoatrial node (SAN)
Internodal tracts
Atrioventricular node (AVN)
Bundle of His
Bundle branches (left & right)
Purkinje fibres

Question 2

Describe the location of the SAN.

Answer 2

Posterior and upper wall of the right atrium, close to the opening of the SVC

Question 3

The SAN has intrinsic automaticity – what does this mean?

Answer 3

It can spontaneously generate action potentials

Question 4

What is the frequency of spontaneous action potentials generated by the SAN?

Answer 4

Once every second

Question 5

Which structure prevents the electrical signal from propagating from the atria directly to the ventricles?

Answer 5

Fibrous cardiac skeleton – it electrically isolates the atria from the ventricles

Question 6

Why is it important that electrical signal is transmitted via the AVN as opposed to directly from the atria to the ventricles?

Answer 6

The AV node introduces a physiological delay of approximately 100ms in conduction, allowing time for the atria to contract and complete ventricular filling before the ventricles contract

Question 7

List the 3 latent pacemakers of the heart and their discharge rates.

Answer 7

AV node – 40-50 bpm

Bundle of His – 40-50 bpm

Purkinje fibres – 20 bpm

Question 8

Describe the pattern of contraction in the ventricles.

Answer 8

Contraction happens from the apex to the base

Question 9

What is the resting membrane potential of myocytes?

Answer 9

-80 mV

Question 10

What are the 2 types of cardiac action potentials?

Answer 10

Slow response potentials
Fast response potentials

Question 11

Which 2 places are slow response potentials found?

Answer 11

SAN
AVN

Question 12

Describe the speed of depolarization in slow response potentials and what this is due to.

Answer 12

Slowly depolarizing cells due to unstable resting membrane potential (‘pacemaker’ potential)

Question 13

Describe the speed of depolarization in fast response potentials and what this is due to.

Answer 13

Rapidly depolarizing cells due to stable resting membrane potential

Question 14

What is Phase 0 of a cardiac action potential?

Answer 14

Depolarisation

Question 15

What is Phase 1 of a cardiac action potential?

Answer 15

Rapid repolarization

Question 16

What is Phase 2 of a cardiac action potential?

Answer 16

Plateau

Question 17

What is Phase 3 of a cardiac action potential?

Answer 17

Repolarization

Question 18

What is Phase 4 of a cardiac action potential?

Answer 18

Resting

Question 19

What phases do slow response cardiac action potentials have?

Answer 19

Phases 0, 3, 4

Question 20

What phases do fast response cardiac action potentials have?

Answer 20

Phases 0, 1, 2, 3, 4

Question 21

Describe what happens during each phase of slow response action potentials.

Answer 21

Phase 0 – Ca2+ influx causes slow depolarization

Phase 3 – K+ efflux causes repolarization

Phase 4 – membrane is permeable to Na+ and K+. Net inward current causes slow depolarization

Question 22

Which current gives the pacemaker cells their intrinsic automaticity?

Answer 22

Funny current

Question 23

In terms of the parasympathetic nervous system – which nerve innervates the heart, which specific areas does it innervate, which neurotransmitter, which receptors, and what effect does it have?

Answer 23

Vagus nerve – mostly innervates SAN and AVN

Acetylcholine binds to muscarinic M2-cholinergic receptors, slowing the heart rate

Question 24

In terms of the sympathetic nervous system – which specific area of the heart does it innervate, which mediator, which receptors, and what effect does it have?

Answer 24

Innervates conduction system and myocardium

Noradrenaline binds to B1-adrenoreceptors, increasing the heart rate

Question 25

Describe the 3 ways in which the parasympathetic nervous system slows heart rate.

Answer 25

ACh reduces the influx of calcium ions, which leads to a less steep Phase 4, meaning more time is required to reach the threshold ACh causes K+ channels to open, increasing K+ efflux which leads to hyperpolarization, so more time is required to reach the threshold Calcium channels are modulated – the threshold for activation is increased.

Question 26

The parasympathetic nervous system causes what specific effect on the SAN?

Answer 26

Negative chronotropy - decreases pacemaker firing (decreases heart rate)

Question 27

The parasympathetic nervous system causes what specific effect on the AVN?

Answer 27

Negative dromotropy - decreases conduction velocity

Question 28

Describe the 2 ways in which the sympathetic nervous system increases heart rate.

Answer 28

Noradrenaline increases the influx of calcium, which leads to a steeper Phase 4, causing faster depolarization Calcium channels are modulated – the threshold for activation is decreased

Question 29

Which 2 places are fast response action potentials found?

Answer 29

Atrial + ventricular myocytes Purkinje fibres

Question 30

Describe what happens during each phase of fast response action potentials.

Answer 30

Phase 0 – Rapid influx of Na+ causes rapid depolarization Phase 1 – initial K+ efflux, causing rapid and transient repolarization Phase 2 – influx of Ca2+ balances K+ efflux, creating a sustained plateau Phase 3 – K+ efflux causes repolarization Phase 4 – stable resting membrane potential

Question 31

What are the 2 refractory periods in a fast response potential?

Answer 31

Absolute refractory period Relative refractory period

Question 32

What cannot happen during the absolute refractory period?

Answer 32

No new action potential can be generated

Question 33

When does the absolute refractory period occur?

Answer 33

From the beginning of Phase 0 through to Phase 3

Question 34

When does the relative refractory period occur?

Answer 34

From Phase 3 through to the early part of Phase 4

Question 35

Describe the generation of action potentials in the relative refractory period.

Answer 35

Action potentials can be generated but a stronger than normal stimulus is needed

Question 36

Give 2 reasons why the refractory period in fast response potentials is important.

Answer 36

Ensures unidirectional propagation of the action potential Ensures adequate time for ventricular filling prior to subsequent contraction

Question 37

What is the outer membrane of cardiomyocytes called?

Answer 37

Sarcolemma

Question 38

What are the thick filaments of sarcomeres made of?

Answer 38

Myosin

Question 39

What are the thin filaments of sarcomeres made of?

Answer 39

Actin, tropomyosin, and troponin

Question 40

In cardiomyocytes, what are transverse tubules and what is their function?

Answer 40

Invaginations of the sarcolemma that extend deep into the cardiomyocyte They allow the depolarization of the membrane to penetrate muscle fibre

Question 41

What is the function of the sarcoplasmic reticulum?

Answer 41

Intracellular store of calcium

Question 42

Cardiomyocytes connect to each other via what?

Answer 42

Intercalated discs

Question 43

Which 2 types of cell junctions are found in cardiomyocytes?

Answer 43

Gap junctions Desmosomes

Question 44

Which proteins are gap junctions composed of?

Answer 44

Connexins

Question 45

What is excitation-contraction coupling?

Answer 45

The mechanism that translates a cardiac action potential into muscle contraction

Question 46

Describe what happens during excitation-contraction coupling.

Answer 46

T tubule membrane depolarisation. This causes the opening of voltage-gated calcium channels. A small influx of calcium binds to and activates ryanodine receptors on the sarcoplasmic reticulum, causing a large release of calcium. Calcium binds to troponin C, causing contraction. Calcium removal from the cytoplasm leads to relaxation – it is actively pumped back into the SR by SR Ca2+-ATPase (SERCA) and removed from the cell via NCX

Question 47

What is the process called where calcium binding to ryanodine receptors on the SR releases calcium?

Answer 47

Calcium-induced calcium release

Question 48

Which part of the heart's conduction system has the fastest conduction velocity?

Answer 48

Purkinje fibres

Question 49

Which part of the heart's conduction system has the slowest conduction velocity?

Answer 49

AVN

Question 50

Which channels are found in the funny current?

Answer 50

HCN channels

Question 51

In calcium-induced calcium release, which receptor does calcium bind to?

Answer 51

Ryanodine

Question 52

Ryanodine receptors are found where?

Answer 52

Sarcoplasmic reticulum

Question 53

After calcium-induced calcium release, by which 2 receptors and mechanisms are intracellular calcium levels lowered?

Answer 53

Re-uptake by SERCA (Ca2+-ATPase) on the sarcoplasmic Removed from cell by NCX