Regulation of Cardiac Function 2

Question 1

Describe a ventricular action potential

Answer 1

Rapid depolarisation

Long plateau phase

Question 2

What is the resting membrane potential in the ventricles?

Answer 2

-90mV

Question 3

What is the resting membrane potential in the atria?

Answer 3

-90mV

Question 4

What is the resting membrane potential in the Purkinje fibres?

Answer 4

-90mV

Question 5

Describe the ionic currents during a ventricular action potential (3)

Answer 5

Opening of Na+ channels = rapid depolarisation (voltage-gated Na+ channels and positive feedback) due to Na+ influx

Ca2+ channels open at peak depolarisation and Na+ channels close = plateau due to Ca2+ influx

K+ channels open after plateau and Ca2+ channels close = repolarisation due to K+ efflux

Question 6

What occurs in skeletal muscle when there is insufficient time to remove Ca2+?

Answer 6

Tetany

Question 7

Why is tetany not desirable in cardiac muscle?

Answer 7

Discrete contractions

Maximise volume of blood pumped

Question 8

How are discrete cardiac muscle contractions achieved?

Answer 8

Length of action potential and length of contraction and relaxation is synonymous

Question 9

How does lowering the Ca2+ concentration affect the ventricular action potential/contraction?

Answer 9

Shorter plateau

Less tension generated

Question 10

What is the decaying pacemaker potential?

Answer 10

Describes the SAN membrane potential

Gradual spontaneous depolarisation to allow spontaneous action potential to be generated

Question 11

What cells have a decaying pacemaker potential?

Answer 11

SAN

AV node and Purkinje fibres have some in case SAN is damaged

Question 12

Is the upstroke/depolarisation faster in the SAN or the ventricles?

Answer 12

Ventricles

Question 13

Describe the currents underlying the SAN action potential (4)

Answer 13

Long-acting Ca2+ channels generate slow-rising action potential by Ca2+ influx

At peak, K+ channels open and Ca2+ channels close = repolarisation by K+ efflux

Closure of K+ channels and opening of ‘funny current’ Na+ channels = start of pacemaker potential by Na+ influx

Towards end of pacemaker potential, transient and long-acting Ca2+ channels open = depolarisation

Question 14

What kind of ion channel is mostly absent in nodal cells?

Answer 14

Voltage-gated Na+ channels

Question 15

Why is it called the ‘funny current’ Na+ channel?

Answer 15

Opens on hyperpolarisation rather than depolarisation

Question 16

What is the difference between transient and long-acting Ca2+ channels?

Answer 16

Transient = open for less time but let in more Ca2+ at one time

Long-acting = open for longer but let in less Ca2+ at one time

Question 17

What drugs inhibit long-acting Ca2+ channels in the SAN?

Answer 17

Verapamil

Nifedipine

Question 18

What inhibits K+ channels in the SAN?

Answer 18

Barium

Question 19

What drug inhibits the ‘funny current’ Na+ channels in the SAN?

Answer 19

Ivabradine

Question 20

What type of agents modulate the pacemaker potential decay rate?

Answer 20

Chronotropic agents

Question 21

What do chronotropic agents modulate and how?

Answer 21

Decay of pacemaker potential

Affect ion conductance of channels

Question 22

What are two examples of chronotropic agents and what do they do?

Answer 22

Noradrenaline (sympathetic) = faster decay = faster heart rate

Acetylcholine (parasympathetic) = slower decay = slower heart rate

Question 23

What are the units that make up a gap junction?

Answer 23

Connexons

Question 24

What are the subunits of a connexon?

Answer 24

6 connexins

Question 25

Where are the gap junctions in the heart?

Answer 25

Intercalated discs

Question 26

Why are there gap junctions in the heart?

Answer 26

Allow ions to move between cells/electrically coupled So act as an electrical syncitium Smooth contraction = maximum blood ejection

Question 27

What two factors affect the rate of action potential conduction?

Answer 27

Resistance of gap junctions Membrane capacitance

Question 28

How can you change a gap junction to give faster action potential conduction?

Answer 28

More connexons (less resistance)

Question 29

What is the conduction pathway in the heart?

Answer 29

SAN AVN Bundle of His Left and right bundles Purkinje fibres

Question 30

How fast is action potential conduction in the atria?

Answer 30

1m/s

Question 31

How fast is action potential conduction via the AVN?

Answer 31

0.05m/s

Question 32

How fast is action potential conduction through the Bundle of His, left and right bundles and Purkinje fibres to the ventricular mass?

Answer 32

4m/s

Question 33

How fast is action potential conduction from the endocardium to the epicardium?

Answer 33

0.3m/s

Question 34

Why is conduction speed so slow in the AVN?

Answer 34

Allow last of atrial blood to enter ventricles

Question 35

What type of cells make up the conduction pathway in the heart?

Answer 35

Muscle

Question 36

What part of the nervous system modulates heart rate?

Answer 36

Autonomic NS

Question 37

What is Ca2+-induced Ca2+ release?

Answer 37

Action potential causes long-acting Ca2+ channels to open in T-tubules = Ca2+ influx Ca2+ binds to ryanodine receptors on sarcoplasmic reticulum to allow Ca2+ release from internal store

Question 38

What receptor do Ca2+ ions bind to on the sarcoplasmic reticulum?

Answer 38

Ryanodine

Question 39

What protein does Ca2+ bind to in muscle contraction?

Answer 39

Troponin

Question 40

Why must Ca2+ be removed after heart contraction?

Answer 40

Allow relaxation before the next contraction

Question 41

How is Ca2+ removed after heart contraction?

Answer 41

Active Ca2+ uptake into sarcoplasmic reticulum via SERCA pump Ca2+ removal by Na-Ca exchanger

Question 42

What pump is used in the uptake of Ca2+ into the sarcoplasmic reticulum?

Answer 42

SERCA

Question 43

How many ions are transported by the Na-Ca exchanger at one time?

Answer 43

1 Ca2+ out 3Na+ in

Question 44

How does noradrenaline act as an positive inotropic agent?

Answer 44

Faster decay of pacemaker potential = faster heart rate (positive chronotropic agent) Less time to remove Ca2+ so builds up Ca2+ required for contraction so increased contractile force (staircase/Treppe effect)

Question 45

What is the Treppe effect?

Answer 45

Increased heart rate increases contractile force

Question 46

What is the difference between chronotropic agents and inotropic agents?

Answer 46

Chronotropic agents affect rate Inotropic agents affect contractility

Question 47

Give an example of a positive chronotropic agent

Answer 47

Noradrenaline

Question 48

Give an example of a negative chronotropic agent

Answer 48

ACh Propanolol (b-blocker) Atenolol (b1-blocker) Adenosine Digoxin (glycosides)

Question 49

How does adenosine decrease heart rate?

Answer 49

Activates nodal K+ channels = hyperpolarisation

Question 50

How do glycosides decrease heart rate?

Answer 50

Increase vagal tone

Question 51

Give an example of a positive inotropic agent

Answer 51

Noradrenaline Isoprenaline, adrenaline (b-agonists) Digoxin (glycosides)

Question 52

How does digoxin increase contractility?

Answer 52

Inhibits Na-K ATPase Na+ not removed = Ca2+ not removed

Question 53

What can act as a negative inotropic agent?

Answer 53

Ischaemia Hypoxia Acidity

Question 54

How does an electrocardiogram work?

Answer 54

Measures alterations in currents at skin's surface to detect de/repolarisation of heart muscle

Question 55

What is Einthoven's triangle?

Answer 55

Placement of electrodes in a 3-lead ECG

Question 56

Which type of 3-lead ECG is used most often and why?

Answer 56

Lead II Depolarisations measured are parallel to the interventricular septum (best view of PQRST waves)

Question 57

What is the positioning of electrodes in the Lead II 3-lead ECG?

Answer 57

Cathode on right clavicle Anode on last left rib Earth on left clavicle

Question 58

What is the positioning of electrodes in the Lead III 3-lead ECG?

Answer 58

Cathode on left clavicle Anode on last left rib Earth on right clavicle

Question 59

What is the positioning of electrodes in the Lead I 3-lead ECG?

Answer 59

Cathode on right clavicle Anode on left clavicle Earth on last left rib

Question 60

Why do we take 6 or 12 lead ECGs?

Answer 60

Allows us to see different planes of heart to determine where a blockage in conduction is

Question 61

In a Lead II 3-lead ECG, why is there positive and negative deflection?

Answer 61

Positive = depolarisation in same direction of anode Negative = depolarisation in opposite direction to anode