The Cardiac Cycle

Question 1

What are the 3 cations that are heavily involved in the cardiac cycle?

Answer 1

• Na+
• K+
• Ca2+

Question 2

There are 2 types of K+ channels that are involved in the cardiac cycle action potential. What is the inward rectifier K+ channel?

Answer 2

• opens at rest to maintain resting membrane potential (Vm)
• INWARD RECTIFIER is open leaking K+
• K+ leaking out keep Vm below -70mV

Question 3

There are 2 types of K+ channels that are involved in the cardiac cycle action potential. What is the delayed rectifier K+ channel?

Answer 3

• K+ channel that opens to facilitate depolarisation
• all gating occurs with a delay

Question 4

During a normal cardiac cycle, which cation begins to increase the Vm towards 0mV?

Answer 4

• Na+

Question 5

Once Na+ has increased sufficiently during a normal cardiac action potential the cell is to commited to an all or nothing action potential. What is the name given that the current has to pass to initiate depolarisation, and roughly what voltage is this?

Answer 5

• threshold (around -50mV)
• depolarisation reaches around +30mV

Question 6

Once the action potential of a cardiomyocyte has completed, what is the term given to the cell that occurs directly following depolarisation?

Answer 6

• repolarisation

Question 7

What happens to Na+ and K+ channels during repolarisation?

Answer 7

• Na+ channels close
• delayed rectifier K+ channels open
• Vm becomes negative again

Question 8

Once the cardiomyocyte has undergone repolarisation, there is a point where is goes below the resting Vm, what is this called?

Answer 8

• refractory period

Question 9

Once the cardiomyocyte has undergone repolarisation and the membrane has dropped below the resting Vm, called the refractory period, which channel opens to restore Vm?

Answer 9

• inward rectifier K+ channels
• K+ slowly released

Question 10

During the refractory period are cardiomyocytes able to recieve another acton potential?

Answer 10

• no
• refactory means no electrical impulse
• allows the heart to refil

Question 11

During the refractory period cardiomyocytes are not able to recieve another acton potential. Multiple action potentials, as those found in the muscle is called tatanus, which is what is found in skeletal muscle. Why is it important that cardiac tissue does not undergo tetanus?

Answer 11

• to ensure adequate filling time for the heart between contractions

Question 12

How many phases are there in normal ventricular myocyte action potentials?

Answer 12

• 5
• from 0 - 4

Question 13

What happens during phase 0 of ventricular myocyte action potential?

Answer 13

• depolarisation due to Na+ entering myocyte

Question 14

What is the positive feedback loop that occurs during phase 0 of the ventricular myocyte action potential?

Answer 14

• Na+ channels open intially during depolarisation
• once Vm reaches threshold more Na+ channels open
• so voltage is the positive feedback

Question 15

What happens during phase 1 of ventricular myocyte action potential?

Answer 15

• delayed rectified K+ channels open
• K+ leaves cell causing small drop in charge

Question 16

What happens during phase 2 of ventricular myocyte action potential?

Answer 16

• Ca2+ channels open
• Ca2+ entering and K+ leaving causes a plateau

Question 17

What happens during phase 3 of ventricular myocyte action potential?

Answer 17

• rapid repolarisation
• Ca2+ channels close
• K+ continue to leaves myocyte

Question 18

What happens during phase 4 of ventricular myocyte action potential?

Answer 18

• no hyperpolarisation
• resting Vm returns
• inward rectifier K+ opens and K+ leaks out to maintain Vm

Question 19

Do action potential from the sinus and atrioventricualr nodes have inward rectified K+ channels?

Answer 19

• no

Question 20

What is the name given to channels that facilitates action potentials from SN and AV nodes?

Answer 20

• the if current or funny current
• mainly Na+ into the cell, but a little K+ out of the cell

Question 21

Instead of Na+ initiating depolarisation like in cardio myocytes in phase 0, what cations initiates phase 0 in cardiac action potentials in sinus and AV nodes?

Answer 21

• opening of Ca2+ channels

Question 22

In cardiac action potentials in sinus and AV nodes there is no phase 1 or 2. What cation channel causes repolarisation in phase 3?

Answer 22

• delayed rectifier K+ channel

Question 23

What are hyperpolarisation activated cyclic nucleotide (HCN) gated channels?

Answer 23

• cells capable of generating their own rhythm
• SA and AV nodes are examples

Question 24

What are intercalted discs?

Answer 24

• connections between myocytes

Question 25

What 3 things make up an intercalated disc?

Answer 25

- desmosomes (structure holding cells together) - gap junctions (ion movement) - adherens junctions (anchor thin filaments)

Question 26

Why are gap junctions important in cardiac tissue?

Answer 26

- allow rapid conduction of action potentials

Question 27

What is a syncytium?

Answer 27

- a multinucleated cell - like syncytiotrophoblast in embryology

Question 28

There are 2 synctium in cardiac tissue, the atria and ventricles. Why is it important that these are seperate?

Answer 28

- if it was one then the heart would contract as one like skeletal muscle

Question 29

There is a delay of 120-200ms when the AV node receives the action potential from the sinus node. Why is thius required?

Answer 29

- allows ventricles to fill before contraction

Question 30

Although the sinus and AV node are able to maintain their own rhythm, what neurological system can influence this, and what are the effects of this?

Answer 30

- autonomic - sympathetic = increased action potential speed - parasympathetic = decreased action potential speed

Question 31

What is starlings law?

Answer 31

- law stating the relationship between stroke volume and venous return - venous return generally measured in end diastolic volume

Question 32

In sterlings law, venous return (preload) stretches the ventricles. What is the relationship between the number of cross bridges between actin (thin filaments) and myosin (thick filaments) and the lenght of the muscle?

Answer 32

- increased lenght = ⬆️ stretch of myocytes - ⬆️ stretch of myocytes = ⬆️ number of cross bridges - increased muscle lenght and crossbridge = ⬆️ force generation - increased muscle lenght and crossbridge = ⬆️ duration of contraction

Question 33

What is an example of intrinsic and extrinsic regulation of contractile force in cardiac tissue?

Answer 33

- intrinsic = starling mechanism - extrinsic = sysmpathetic stimulation

Question 34

Intrinsic regulation of contractile force of cardiac tissue is dependent on starlings law. As the number of cross bridges increase as the muscle is stretched what happens to the force and duration of the muscle contraction?

Answer 34

- increased force generation - increased contraction duration

Question 35

Extrinsic regulation of contractile force of cardiac tissue is dependent on sympathetic stimulation. As the number of cross bridges does not change, what happens to the force and duration of the muscle contraction?

Answer 35

- increased force generation - similiar duration of force generation as HR increases so duration cannot increase as well - faster and stronger contraction

Question 36

When looking at the cardiac cycle below it shows a single beat of the heart. What does the peak and the plateau of the cardiac cycle relate to?

Answer 36

- plateau along the bottom = diastolic - peak = systolic

Question 37

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle at what point in the image would the mitral valve open and close?

Answer 37

- mitral valve opens at end of systolic = bottom right - mitral closes at end of diastolic = bottom left

Question 38

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle at what point in the image would the aortic valve open and close?

Answer 38

- aortic valve opens at start of systolic. Valve opens as pressure in left ventricle exceeds aortic pressure - aortic valve closes at end of systolic. Valve closes as pressure in left ventricle is below aortic pressure

Question 39

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle, where would isovolumetric contraction occur and what is this?

Answer 39

- occurs between mitral valve closing - before aortic valve opens - blood cannot leave but pressure increases

Question 40

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle, where would isovolumetric relaxation occur and what is this?

Answer 40

- after aortic valve closes - before mitral valve opens - blood remaining cannot go anywhere so volume remains

Question 41

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle, at what point would blood be pumped out of the heart on the figure?

Answer 41

- in systolly - between aortic valve opening and closing

Question 42

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle, what are the 3 stages that occur once the mitral valve has opened and before the atria contract again?

Answer 42

1 - passive filling 2 - diastasis 3 - active filling

Question 43

At what point in the figure below would you hear the 2 sounds of the heart, referred to as S1 and S2 and what happens at each one?

Answer 43

- S1 = mitral valve (and tricuspid valve) closing - S2 = aortic valve (and pulmonary valve) closing

Question 44

When looking at the cardiac cycle below it shows a single beat of the heart. If we consider this to be from the left ventricle, what happens to the LV volume in systolic and diastolic?

Answer 44

- LV progressively decreases in systolic as blood is pumped out - LV progressively fills during diastolic

Question 45

What is the name given to the plots where all the cardiac cycle data is put onto the same plot?

Answer 45

- Wiggers plots

Question 46

What is a left ventricle pressure volume loop?

Answer 46

- a plot that represents one heart beat - plot includes volume and pressure of LV - includes systolic and diastolic

Question 47

In the left ventricle volume pressure loop where would you expect to find the following: a = isovolumic contraction b = ejection c = isovolumic relaxation d = diastolic filling

Answer 47

a = isovolumic contraction is right verticle b = ejection is curved line at top c = isovolumic relaxation is left verticle line d = diastolic filling is line on bottome

Question 48

In the left ventricle volume pressure loop where would you expect to find the following: a - aortic valve opens b - aortic valve closes c - mitral valve opens d - mitral valve closes

Answer 48

- a - aortic valve opens - top right corner - b - aortic valve closes -top left corner - c - mitral valve opens - bottom left corner - d - mitral valve closes - bottom right corner

Question 49

What is the most accurate marker for a myocardial infarction that is released into the blood when cardiac muscle is damaged?

Answer 49

- cardiac troponin T or I - specific to cardiac muscle

Question 50

Although not specific as troponin T or I, there is an enzyme that can be useful for identifying if a patient has has a myocardiac infarction. What is this called?

Answer 50

- creatine kinase-MB (CK-MB) - creatine kinase is found throughout body - CK-MB is specific to cardiac tissue