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Medical Physiology > Cardiac Muscle Contraction > Flashcards

Cardiac Muscle Contraction Flashcards

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USMLE® Step 1 flashcards
1
Q

what is preload

A

amount of stretch before contraction

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2
Q

what is afterload

A

load during active shortening

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3
Q

is recruitment a factor in cardiac muscle?

A

no, every cardiac muscle cell is recruited during contraction

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4
Q

what is contractility

A

increased work in the heart for the same preload

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5
Q

does skeletal muscle have calcium current during AP?

A

no

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6
Q

describe excitation contraction coupling in cardiac muscle

A

actin and myosin are regulated by troponin C but cardiac cells rely on extracellular Ca for contraction through Calcium influx during AP

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7
Q

which is the most reliable measure of left ventricular preload

A

muscle fiber length, but it is not practical, so left ventricular volume during ECHO

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8
Q

order of reliability of measure of left ventricular preload

A
  1. muscle fiber length
  2. left ventricular volume
  3. left ventricular diameter
  4. left ventricular end diastolic pressure
  5. left atrial pressure
  6. pulmonary capillary wedge pressure
  7. pulmonary artery pressure
  8. central venous pressure
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9
Q

how to determine left ventricular diameter

A

xray

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10
Q

problem with using leeft ventricular end diastolic pressure as a measure

A

if the heart or vessels around it are stiff , there would be high pressure in the ventricle itself, but the ventricle could actually not be stretched

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11
Q

how is force-velocity relationship altered in cardiac muscle?

A

extrinsic regulatory mechanisms like sympathetic nerves – would increase curve, parapsympathetic would decrease curve

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12
Q

what occurs if preload increases

A

increased force of contraction and ejection

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13
Q

what occurs if preload decreases

A

decreased force of contraction and ejection

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14
Q

what is ionotropic

A

effects on force of contraction by external influences (physiological or pathological)

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15
Q

what is contractility in cardiac muscle

A

extrinsic control of heart by autonomic nerves and hormones that regulate response to preload and afterload

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16
Q

what is the primary pacemaker

A

SA node

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17
Q

secondary pacemaker

A

AV node

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18
Q

how does cardiac relaxation occur?

A
  1. SERCA pumps Ca back to SR

2. Ca efflux by active transport (Na/Ca exhanger)

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19
Q

what regulates cardiac force

A

calcium influx during AP

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20
Q

what is DHPR

A

dihydropyridine receptors which are L type voltage gated channels that respond to Ca influx. they open and trigger release of Ca from SR by binding to RYR

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21
Q

what is RYR

A

calcium gated calcium channel, releases Ca from SR

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22
Q

describe calcium induced Ca release from SR

A

1, cardiac AP

  1. Ca enters cell during plateau
  2. Ca binds to DHPR
  3. DHPR opens RYR
  4. Ca leaves SR
  5. Ca binds to troponin C
  6. cross bridge
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23
Q

what is a calcium pulse

A

rise in Ca to trigger contraction via DHPR (L type voltage gated channels)

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24
Q

what does calcium pulse indicate in terms of cardiac relaxation?

A

Ca must be actually removed from cell, not just resequestered in the SR

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25
Q

how does cell remove Ca

A

Na/Ca exchanger, sends 3 Na into the cell to bring 1 Ca out

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26
Q

what is Na/Ca

A

secondary active transport, bc follows Na concentration gradient that has been established via Na/K ATPase

27
Q

what occurs if there is ischemia or mitochondrial damage?

A

lower ATP means transport of Ca is limited (Na/K ATPase not doing concentration), so Ca builds up in the cell and interferes with normal resting potential and AP

28
Q

what does increased Ca in cardiac muscle do

A

interferes with normal membrane potential and AP, so you see reduced force and inadequate relaxation

29
Q

why is duration of AP, calcium pulse and active force important

A

refractory periods extend well into force production to prevent tetany and limits ability of increased frequency to increase force

30
Q

what controls force production in cardiac muscle

A

calcium influx

31
Q

what would be seen in hypocalcemia?

A

decreased AP amplitude, decreased Ca influx and decreased force

32
Q

what occurs if Na/K pump is inhibited by digitalis?

A

Na would build up in the cell. therefore, concentration gradient of Na not established, and Na/Ca pump could not work. Ca would increase in the cell and cause increased force of contraction. (positive ionotropic effect)

33
Q

why is preload important for contractility

A
  1. actin and myosin have more overlap/drawn closer together when stretched aka more potential to interact
  2. stretch increases Ca sensitivity of troponin
34
Q

what is intrinsic regulatino

A

preload and afterload

35
Q

what is extrinsic regulation

A

amount of response to preload and afterload

36
Q

what is positive ionotropic effect

A

increased contractility due to increased intracellular Ca

37
Q

what is negative ionotropic effeect

A

decreased calcium or decreased ability to produce force

38
Q

what is chronotropy

A

factors that change HR

39
Q

what is lusitropy

A

rate of relaxation

40
Q

what would occur if preload decrease, as with hemorrhage

A

sympathetic system would provoke contractility to keep alive

41
Q

what is frank starling law of heart

A

input must equal output, that is cardiac output must match venous return

42
Q

how does one change response to preload to match frank starling?

A

change contractility

43
Q

what is compliance?

A

ability to be stretched without permanent damage of shape, but takes time to return to size

44
Q

what is the equation for compliance

A

change in volume/change in pressure

45
Q

what is elasticity

A

production of tension as a material is stretched (hard to stretch makes increased tension, but snaps back fast)

46
Q

equation for elasticity

A

change in pressure/change in volume

47
Q

what occurs in the case of a hypertrophic heart?

A

increased mass of heart means decreased compliance. which takes more pressure to fill

48
Q

what occurs in the case of a dilated heart

A

high compliance except for at very high volumes

49
Q

what is the la place relationship

A

Tension = hydrostatic pressure x radius

50
Q

calculate wall stress

A

hydrostatic pressure x radius/2xwall thickness

51
Q

describe dilated heart in terms of wall tension and la place

A

dilated heart would have increased radius and decreased thickness. therefore, tension is increased, and wall stress is also increased. this means it has to work harder to eject the same amount of blood

52
Q

what does cardiac muscle get its energy from

A

fatty acid oxidation, dependent on oxygen

53
Q

what is the impact of oxygen on cardiac muscle

A

CARDIAC FORCE DEPENDENT ON OXYGEN SUPPLY; there is a direct relationship between cardiac work, cardiac oxygen consumption and coronary blood flow

54
Q

what is the direct effect of ichemia on the heart? (post MI_)

A

decrease in oxygen leads to a decrease in active force and work within 30 seconds of occlusion

55
Q

what is the impact of loss of ATP on the heart

A

slight ATP depletion means huge depletion of ATP in heart.

  1. loss of allosteric control of L type Ca channels and a transient decrease of Ca influx.
  2. low ATP inhibits SR Ca release
  3. reduces energy of hydrolysis of ATP required for force production
  4. inability to resequester or pump Ca out of cell poisons mitochondrial energy production
  5. excess Ca increases risk of free radical damage
56
Q

what sympathetic receptors are on cardiac cells

A

adrenergic beta 1

57
Q

what does SNS do to alter ion conductance?

A

cAMP

58
Q

what does cAMP do in the heart?

A

increases Ca influx and release of Ca from SR leading to increased transport and resequesteration of Ca.
makes stronger, faster, shorter duration (more frequent)

59
Q

parasympathetic receptors on cardiac cells

A

muscarinic M2 receptors (Ach)

60
Q

what do muscarinic receptors do to cAMP?

A

decrease cAMP

61
Q

what does reduced cAMP do?

A

weaker, slower, longer duration

62
Q

what does hypertension do to the heart

A

has chronic high afterload, meaning concentric left ventricular hypertrophy to overcome afterload

63
Q

what are examples of heart remodeling

A

dilation and hypertrophy

64
Q

what is remodeling due to

A

altered gene expression

Medical Physiology (21 decks)

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