cardiac muscle mechanics

Question 1

do T tubules exist in cardiac muscle?

Answer 1

yes, they invaginate at the z-line

Question 2

what is cardiac muscle called a “functional syncytium”?

Answer 2

because all the cardiac muscle cells are mechanically and electrically connected to one other- the entire tissue resembles 1 giant cell

Question 3

why does cardiac muscle sarcoplasm have large numbers of mitrochondria and abundant reserves of myoglobin?

Answer 3

because the cells are dependent on aerobic metabolism to obtain energy needed to continue contracting

Question 4

does this statement describe skeletal or cardiac muscle

“physical attachment between L type calcium channel and ryanodine receptors (the endfeet)”

Answer 4

skeletal muscle

Question 5

how are the ryanodine receptors opened in cardiac muscle cells?

Answer 5

calcium induced calcium release

Question 6

how long does a skeletal AP last? a cardiac AP?

Answer 6

skeletal- less than 5 msec

cardiac- around 250msec

Question 7

can there be summation in cardiac muscle action potentials?

Answer 7

NO! the refractory period prevents it- heart contracts ONLY by twitch

Question 8

what triggers phase 1 in a ventricle action potential?

Answer 8

closing of the Na+ channels

Question 9

what 2 stages in ventricle action potential correspond to the absolute refractory period?

Answer 9

phase 1 & 2

Question 10

how does phase 1 look on the graph?

Answer 10

slight dip (brief repolarization)

Question 11

how does phase 2 look on the graph?

Answer 11

plateau phase

Question 12

how do action potentials move between adjacent cells?

Answer 12

by means of charge displacement through gap junctions

Question 13

what percentage of the necessary calcium for a twitch in a cardiac cell comes from outside the cell?

Answer 13

20%

Question 14

what percentage of the necessary calcium for a twitch in a cardiac cell comes from the SR in the cell?

Answer 14

80%

Question 15

how much external calcium contributes to skeletal muscle transients and twitch?

Answer 15

ZERO percent

Question 16

what method of calcium extraction from the sarcoplasm contributes the greatest to calcium removal?

Answer 16

sarcoplasmic and endoplasmic retiuculm ATPase (SERCA)

about 80%

Question 17

what percentage of sarcoplasmic calcium is removed via the sodium/calcium exchanger (NCX)?

Answer 17

15%

Question 18

besides NCX and SERCA what is the last “major” way to get ride of calcium in the sarcoplasm?

Answer 18

PCMA-pump that removes calcium from the cell

5%

Question 19

in cardiac muscle, where does calcium bind to initiate the crossbridge cycle?

Answer 19

troponin C

Question 20

when is the twitch in a cardiac muscle terminated?

Answer 20

when there is calcium clearance from the sarcoplasm

Question 21

in cardiac muscle is there recruitment?

Answer 21

NO

Question 22

how long is the refractory period in an cardiac muscle cell action potential

Answer 22

300 msec

Question 23

what are two ways tension can be affected in cardiac muscle cells?

Answer 23

• by altering the sarcoplasmic [ca]-by an inotropic agent

- altering calcium sensitivity of the myofilament

Question 24

withOUT vagal stimulation, what is the heart rate triggered spontaneously by the SA node?

Answer 24

100 bpm

Question 25

with vagal stimulation, what is the heart rate triggered spontaneously by the SA node?

Answer 25

60 bpm

Question 26

at what sarcomere length is the maximal force generated in the human heart?

Answer 26

2.2 micrometers

Question 27

an increase in initial length in cardiac muscle will do what to PO (maximum load)?

Answer 27

increase it

Question 28

an increase in initial length in cardiac muscle will do what to velocity of shortening? amount of shortening?

Answer 28

increase both

Question 29

an increase in initial length in cardiac muscle will do what to work the heart does? power delivered by the heart?

Answer 29

increase both

Question 30

what is unique to cardiac muscle compared to skeletal muscle regarding muscle tension?

Answer 30

in cardiac muscle there is an additional mechanism that plays a role in tension besides just myosin/actin overlap

Question 31

what results in greater sarcomere stretching?

Answer 31

greater ventricle filling during diastole

Question 32

what is the result of greater sarcomere stretching?

Answer 32

increased troponin C affinity for calcium-->more crossbridge formation per AP--> greater force of contraction during systole

Question 33

how do inotropic agents increase force of contraction?

Answer 33

by increasing the amount of calcium release from the SR

Question 34

in the heart what type of receptor does norepi bind to?

Answer 34

beta 1 adrenergic receptor

Question 35

when NE binds to a beta 1 adrenergic receptor what 4 things increase?

Answer 35

cAMP PKA calcium tension

Question 36

after the administration of norepi how many beats does it usually take for the heart to reach its new steady state?

Answer 36

8 beats

Question 37

how does norepi shorten twitch duration?

Answer 37

by accelerating the SR calcium uptake pump and calcium removal from the sarcoplasm

Question 38

what does an increase in contractility do to twitch duration?

Answer 38

it decreases it

Question 39

what is the result of PKA phosphorylating phospholamban?

Answer 39

increases SR Ca++ pump sensitivity which increases the rate of Ca+ uptake into the SR and therefore decreasing duration of muscle contraction

Question 40

what are the two results of positive inotropic effects of beta adrenergic stimulation?

Answer 40

increased magnitude of contraction | decreased twitch duration

Question 41

what is the positive chronotropic effect of beta adrenergic stimulation?

Answer 41

increased HR

Question 42

what are the two ways to increase sensitivity of contractile apparatus to calcium?

Answer 42

1. use of certain drugs | 2. increase initial fiber length (starling mechanism)

Question 43

how would the curve be shifted in a tension vs calcium graph when NE is added?

Answer 43

the actual curve would not be shifted up but instead the muscle response would be shifted along the calcium tension curve graph shown on page 369 of notes

Question 44

would the curve be shifted in a tension vs length graph when NE is added? if so, which way?

Answer 44

YES the whole curve would be shifted up graph shown on page 397 of notes

Question 45

what is the effect on isotonic contraction of NE in the muscle?

Answer 45

isotonic contraction shortens twice the distance with the same load. -the same contraction performed twice the work with NE