Inherent Properties of Cardiac Muscle

Question 1

What is excitation contraction coupling?

Answer 1

the linking of the electrical excitability of a cell to its ability to generate force

Question 2

Why are the issues when disease occurs in the heart?

Answer 2

ventricular myocytes do not divide and are not replaced

Question 3

What are the features of cardiac muscle?

Answer 3

cannot fatigue

is enriched with large mitochondria

Question 4

What is the sarcomere?

Answer 4

the single basic unit that the muscle is made up of

Question 5

What is the distance between two Z disks?

Answer 5

2 microns

Question 6

What is attached to the Z disk?

Answer 6

actin

Question 7

What is the m-line?

Answer 7

the point where myosin is attached

Question 8

Where are t-tubules found?

Answer 8

along the surface membrane diving down around the z-disks

Question 9

What shape are ventricular myocytes?

Answer 9

brick shaped

Question 10

Why are cardiac muscle fibres branched?

Answer 10

allows them to make contact with numerous neighbours

Question 11

Why is the refractory period important?

Answer 11

prevents cardiac muscle from tetanising

protects against re-entrant arrhythmias

Question 12

What prevents the heart from filling with blood properly?

Answer 12

any excitation outside the regular beat prevents the heart from filling with blood

Question 13

What the three general states of most ion channels?

Answer 13

open
closed
inactivated

Question 14

transition to the inactivated state is dependent on what?

Answer 14

time - the ion channel will inactivate after a certain amount of time

Question 15

transition to the closed state is dependent on what?

Answer 15

voltage - it will happen when the membrane repolarises

Question 16

What is unique about cardiac muscle compared to skeletal muscle?

Answer 16

it requires calcium to initiate contraction

Question 17

What is the calcium transient?

Answer 17

the rise in IC Ca that gives rise to contraction after a delay

Question 18

What is the resting Ca level?

Answer 18

100nM

Question 19

What is the peak of Ca during a transient?

Answer 19

1uM

Question 20

What are some of the properties of each dyad?

Answer 20

10-25 L-Type Ca channels

100-200 RyRs

Question 21

What is a dyad?

Answer 21

the structure where the t-tubule meets the SR in the myocyte Z line

Question 22

How long does it take for Ca to rise in the cell?

Answer 22

30ms

Question 23

How is Ca removed from the cytoplasm?

Answer 23

SR CaATPase (SERCA) and the sarcolemmal Na/Ca exchanger - RELAXATION

Question 24

What aids the uniform and simultaneous release of Ca across the cell?

Answer 24

the t-tubule network

Question 25

What are the 3 accessory proteins of Actin?

Answer 25

tropomyosin I, TnT, TnC

Question 26

What is required to release the myosin head from actin?

Answer 26

ATP

Question 27

What happens in the absence of ATP?

Answer 27

myosin head remain attached and form rigor bonds

Question 28

What does Ca do to allow contraction?

Answer 28

binds TnC

Question 29

What does Ca binding TnC do?

Answer 29

changes the conformation pulling the tropomyosin out of the groove between actin filaments exposes myosin binding site

Question 30

What does the continued cycle with ATP, Ca, Myosin and Actin do?

Answer 30

pulls the sarcomere together

Question 31

What is the state of TnI, TnC and Actin at rest?

Answer 31

TnC weakly interacts with TnI TnI strongly interacts with Actin troponin/Tropomyosin complex blocks myosin/actin interaction

Question 32

What is the state of TnI, TnC and Actin in the presence of Ca?

Answer 32

TnC interacts with TnI and destabilised interaction with actin Removes steric hindrance to actin-myosin activates the ATPase activity of myosin

Question 33

How far to the filaments slide per cross-bridge cycle?

Answer 33

5-10nm

Question 34

What is the length tension relationship?

Answer 34

the basis of the Frank-Starling Law of the Heart - amount of force generated depends on the how stretched the muscle is - the more stretched the muscle, the greater the force

Question 35

What does increasing sarcomere length initially do?

Answer 35

facilitates the overlap between thick and thin filaments and increases the no. of cross-bridges that can form

Question 36

What is the maximal length where the overlap between thick and thin filaments is facilitated?

Answer 36

2.25uM

Question 37

what do changes in overlap account for in cardiac muscle?

Answer 37

20% of the increase of force over the physiological range of sarcomere lengths

Question 38

what does increasing sarcomere length do?

Answer 38

increase Ca sensitivity and maximally activated force

Question 39

What is the effect on Ca sensitivity mediated by?

Answer 39

an increase in sensitivity of TnC for Ca

Question 40

What does increasing sarcomere length do?

Answer 40

increase cross-bridge overlap | increased Ca sensitivity of myofilaments

Question 41

When increased cardiac frequency, what happens?

Answer 41

the SR content increases and the force of contraction increases

Question 42

Why does the SR content increase with increased bpm?

Answer 42

- L-type channels open more frequently and thus there is more Ca per second - some of the extra Ca is sequestered in the SR stores - bigger stores = bigger Ca release = increased force

Question 43

Why does the force-frequency relationship decrease in the failing heart?

Answer 43

down-regulation of SERCA | up-regulation of NcX

Question 44

What are the effects of down-regulating SERCA and upregulating NCX?

Answer 44

more calcium is extruded from the cell between beats and less Ca going into the SR

Question 45

What is isovolumic contraction?

Answer 45

where the volume is not changing but the pressure does not exceed that of the aorta so blood cannot move out the ventricles

Question 46

What are the stages of the cardiac cycle?

Answer 46

- diastole - chambers fill with blood - atrial systole - atria contract - blood enters ventricles - isovolumic contraction - bicuspid valve closes - 1st heart sound - ventricular systole - isovolumic relaxation - 65mls left in heart - can increase output if needed - aortic valve closes - 2nd heart sound

Question 47

When does the 1st heart sound occur on the ECG?

Answer 47

with the QRS complex

Question 48

When does the 2nd heart sound occur on the ECG?

Answer 48

just after T wave

Question 49

What is the average Stroke Volume?

Answer 49

70mls