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630: Medical Physiology > Cardiac Muscle Contraction > Flashcards

Cardiac Muscle Contraction Flashcards

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USMLE® Step 1 flashcards
1
Q

Is the basic contractile machinery in cardiac and skeletal muscle similar?

A

Yes, their cell structure is basically the same

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2
Q

Describe the steps involved in excitation-coupling in cardiac muscle

A

1) The cardiac action potential is initiated in the myocardial cell membrane, and the depolarization spreads to the interior of the cell via the T tubules which results in an inward Ca2+ current
2) Entry of Ca2+ into the myocardial cell produces an increase in intracellular Ca2+ concentration which triggers the release of more Ca2+ from the endoplasmic reticulum
3) Ca2+ binds to troponin C, moving tropomyosin out of the way allowing actin and myosin to interact
4) Cross-bridges form and then break, resulting in the thin and thick filaments to move past each other to produce tension

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3
Q

When does relaxation occur?

A

When Ca2+ is reaccummulated in the sarcoplasmic reticulum by the action of Ca2+ ATPase

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4
Q

What is contractility, or iontropism?

A

The intrinsic ability of myocardial cells to develop force at a given muscle cell length

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5
Q

Agents that produce an increase in contractility are said to have _____ inotropic effects

A

positive

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6
Q

What do positive inotropic effects do?

A

They increase both the rate of tension development and the peak tension

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7
Q

Agents that produce an decrease in contractility are said to have _____ inotropic effects

A

negative

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8
Q

What do negative inotropic effects do?

A

They decrease both the rate of tension development and the peak tension

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9
Q

What is contractility directly related to?

A

intracellular calcium concentration

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10
Q

What are the 2 factors in which the amount of Ca2+ released from the sarcoplasmic reticulum depend on?

A
  • size of the inward Ca2+ current during the plateau of the myocardial action potential
  • the amount of Ca2+ previously stored in the sarcoplasmic reticulum for release
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11
Q

Stimulation of the sympathetic nervous system has a ____ inotropic effect

A

positive

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12
Q

What are the 3 important features of the positive inotropic effect of the sympathetic nervous system?

A
  • increased peak tension
  • increased rate of tension development
  • faster rate of relaxation
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13
Q

Faster relaxation means that the contraction is _____.

A

shorter

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14
Q

What are the 2 proteins that are phosphorylated to produce an increase in contractility?

A
  • sarcolemmal Ca2+ channels

- phospholamban

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15
Q

How does phosphorylation of the sarcolemmal Ca2+ channels produce an increase in contractility?

A

There is increased inward Ca2+ current during the plateau and increased trigger Ca2+, which increases the amount of Ca2+ released from the sarcoplasmic reticulum

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16
Q

What is phospholamban?

A

A protein that regulates Ca2+ ATPase in the sarcoplasmic reticulum

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17
Q

How does phosphorylation of phospholamban increase contractility?

A

It stimulates the Ca2+ ATPase, resulting in greater uptake and storage of Ca2+ by the sarcoplasmic reticulum

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18
Q

What are the 2 effects of increased Ca2+ uptake by the sarcoplasmic reticulum?

A
  • it causes faster relaxation

- it increases the amount of stored Ca2+ for release on subsequent beats

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19
Q

Stimulation of the parasympathetic nervous system has a negative inotropic effect on what?

A

the atria

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20
Q

The G protein that is coupled to the muscarinic receptors of the parasympathetic nervous system causes a _____ in contractility. Explain why…

A

decrease

Because it is inhibitory

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21
Q

What are the 2 factors that are responsible for the decrease in atrial contractility caused by the parasympathetic nervous system?

A
  • ACh decreases inward Ca2+ current during the plateau of the action potential
  • ACh shortens the duration of action potential and, indirectly, decreases the inward Ca2+ current (by shortening the plateau phase)
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22
Q

What are cardiac glycosides?

A

a class of drugs that act as positive inotropic agents

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23
Q

What is the action of cardiac glycosides?

A

inhibition of Na+ - K+ ATPase

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24
Q

Inhibition of Na+ - K+ ATPase results in ____ inotropic effects

A

positive

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25
Q

Describe the mechanism of the positive inotropic effect of cardiac glycosides

A

1) cardiac glycosides inhibit Na+ - K+ ATPase at the extracellular K+ binding site
2) less Na+ is pumped out of the cell, increasing the intracellular Na+ concentration
3) a Ca2+ - Na+ exchanger pumps Ca2+ out of the cell against an electrochemical gradient in exchange for Na+ moving into the cell
4) less Ca2+ is pumped out of the cell and intracellular Ca2+ concentration increases
5) tension increases due to an increase in intracellular Ca2+ concentration

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26
Q

What are cardiac glycosides typically used for therapeutically?

A

in the treatment of congestive heart failure

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27
Q

What does the maximal tension that can be developed by a myocardial cell depend on?

A

its resting length

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28
Q

At what length do cardiac cells develop maximal tension?

A

2.2 micrometers

At this point there is maximal overlap thick and thin filaments

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29
Q

What are the 2 other length-dependent mechanisms in cardiac muscle that alter tension developed?

A
  • increasing muscle length increases the Ca2+ sensitivity to troponin C
  • increasing muscle length increases Ca2+ release form the sarcoplasmic reticulum
30
Q

The length-tension relationship for single myocardial cells can be extended to a length-tension relationship for the _____.

A

ventricles

31
Q

The length of a single ventricular muscle fiber just prior to contraction corresponds to left ventricular _____ volume

A

end-diastolic

32
Q

Why do cardiac muscle fibers not operate over their entire length-tension curve?

A

Because it has a high resting tension, and small increases in length produce large increases in resting tension

33
Q

What is preload?

A

The resting length from which the muscle contracts

34
Q

Preload is equal to what?

A

left ventricular end-diastolic volume

35
Q

The relationship between preload and developed tension is based on what?

A

The degree of overlap of thick and thin filaments

36
Q

The afterload for the left ventricle is what?

A

aortic pressure

37
Q

When is the velocity of shortening of cardiac muscle maximal?

A

When afterload is zero

38
Q

Velocity of shortening decreases as afterload _____.

A

increases

39
Q

What are the 3 parameters in which the function of the ventricles are described?

A
  • stroke volume
  • ejection fraction
  • cardiac output
40
Q

What is stroke volume?

A

The volume of blood ejected on one ventricular contraction

41
Q

Stroke volume is the difference between what 2 values?

A

The volume of blood in the ventricle before ejection and the volume remaining in the ventricle after ejection

42
Q

Stroke volume is typically about __ mL

A

70 mL

43
Q

What is ejection fraction?

A

the fraction of the end-diastolic volume that is ejected in one stroke

44
Q

What is ejection fraction normally?

A

55%

45
Q

What is the ejection fraction an indicator for?

A

contractility

46
Q

An increase in contractility results in ____ in ejection fraction.
A decrease in contractility results in _____ in ejection fraction

A

increases

decreases

47
Q

What is cardiac output?

A

The total volume of blood ejected per unit time

48
Q

Cardiac output is depends on what 2 values?

A

stroke volume and heart rate

49
Q

What is normal cardiac output?

A

5000 mL/min

50
Q

What does the Frank-Starling relationship state?

A

That the volume of blood ejected by the ventricle depends on the volume present in the ventricle at the end of diastole, which correlates with venous return

51
Q

What does the Frank-Sterling law of the heart ensure?

A

that cardiac output equals venous return

52
Q

As venous return increases, what 2 other things increase?

A

end-diastolic volume and stroke volume

53
Q

Positive inotropic agents produce _____ in stroke volume and cardiac output for a given end-diastolic volume.

A

increases

54
Q

What is the end result of positive inotropic agents?

A

A larger fraction of the end-diastolic volume is ejected per beat and there is an increase in ejection fraction

55
Q

Negative inotropic agents produce _____ in stroke volume and cardiac output for a given end-diastolic volume.

A

decreases

56
Q

What is the end result of negative inotropic agents?

A

A smaller fraction of the end-diastolic volume is ejected per beat and there is a decrease in ejection fraction

57
Q

What does the ventricular pressure-volume loop describe?

A

One complete cycle of ventricular contraction, ejection, relaxation and filling

58
Q

The isovolumetric contraction occurs between what 2 points on the ventricular pressure-volume loop?

A

Begins at point 1, which marks the end of diastole. And ends at point 2, which is when the ventricle is activated, it contracts, and ventricular pressure increases dramatically

59
Q

Left ventricular pressure _____ and left ventricular volume _____ between points 1 and 2 on the ventricular pressure-volume loop

A

increases

remains the same

60
Q

The ventricular ejection occurs between what 2 points on the ventricular pressure-volume loop?

A

Begins at point 2, which is the point at which ventricular pressure surpasses aortic pressure, forcing the aortic valve to open. Ends at point 3. During the 2 points blood is rapidly ejected, driven by the pressure gradient between the left ventricle and the aorta

61
Q

Left ventricular pressure _____ and left ventricular volume _____ between points 2 and 3 on the ventricular pressure-volume loop

A

remains high

decreases dramatically

62
Q

The volume remaining in the ventricle at point 3 is the end-_____ volume

A

systolic

63
Q

End-systolic volume is approximately __ mL.

A

70 mL

64
Q

The width of the ventricular pressure-volume loop is equal to what?

A

the volume of blood ejected, or the stroke volume

65
Q

The isovolumetric relaxation occurs between what 2 points on the ventricular pressure-volume loop?

A

Begins at point 3, in which systole ends and the ventricle relaxes because ventricular pressure decreases below aortic pressure and the aortic valve closes. Ends at point 4, which is the point at which ventricular pressure is less than left atrial pressure

66
Q

Left ventricular pressure _____ and left ventricular volume _____ between points 3 and 4 on the ventricular pressure-volume loop

A

decreases rapidly

remains constant

67
Q

Ventricular filling occurs between what 2 points on the ventricular pressure-volume loop?

A

Begins at point 4, which is the point in which ventricular pressure is less than left atrial pressure, causing the mitral valve to open. 1, which is the point in which the left ventricle has been filled with blood (end-diastolic)

68
Q

Left ventricular pressure _____ and left ventricular volume _____ between points 4 and 1 on the ventricular pressure-volume loop

A

increases slightly (due to the compliance of the ventricles)

increases back to end-diastolic volume (=140 mL)

69
Q

What 3 changes can alter the ventricular pressure-volume loop?

A
  • increased preload
  • increased afterload
  • increased contractility
70
Q

How does an increased preload alter the ventricular pressure-volume loop?

A

Stroke volume, which is measured by the width of the pressure-volume loop, increases

71
Q

How does an increased afterload alter the ventricular pressure-volume loop?

A

Less blood is ejected from the ventricle during systole, which results in decreased stroke volume and increased end-systolic volume

72
Q

How does increased contractility alter the ventricular pressure-volume loop?

A

The ventricle can develop greater tension and pressure during systole and eject a larger volume of blood than normal. Therefore stroke volume increases and end-systolic volume decreases

630: Medical Physiology (24 decks)

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