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Flashcards in Force Generation by the Heart Deck (53)
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1

Describe cardiac muscle?

Striated
No neuromuscular junctions
Protein channels which form low resistance electrical communication pathways between neighboring myocytes
Intercalated discs - desmosomes
Contains gap junctions

2

What causes striations?

Regular arrangement of contractile proteins

3

Why are there no neuromuscular junctions in the heart?

Because cardiac muscle does not require nervous stimulation- it is myogenic

4

What do protein channels in cardiac muscle ensure?

Ensure that each electrical excitation reaches all of the cardiac myocytes (All or none law of the heart)

5

What do the desmosomes between intercalated provide?

Provide mechanical tension between adjacent cardiac cells
they ensure the tension developed by one cell is transferred to the next cell

6

What does each muscle contain?

Myofibrils - contractile units of muscle

7

What do myofibrils have?

Alternating segments of thick and thin protein filaments (actin and myosin) actin is lighter appearance, myosin is darker

8

How are actin and myosin arranged in a myofibril?

Into sarcomeres (smallest functional unit of a muscle)

9

How is muscle tension produced?

Sliding of actin filaments on myocin filaments

10

What is the sliding filament theory?

Explanation of how muscle shortens and produces force

11

What does force generation depend on?

ATP dependant interaction between actin and myocin filaments

12

ATP is required for both contraction and relaxation. True or false?

True

13

What is Ca2+ required for?

Switching on cross bridge formation

14

Where is Ca2+ released from?

Sarcoplasmic reticulum

15

What is the release of Ca2+ from SR dependent on in cardiac muscle?

Presence of extracellular Ca2+

16

Systole- what happens with calcium?

Ca2+ influx during plateau phase
Ca2+ induced Ca2+ release from sarcoplasmic
reticulum
Ca2+ influx activates contractile machinery

17

What happens when an AP has passed after systole?

Ca++ influx ceases
Ca++ is re-sequestered in sarcoplasmic reticulum by Ca++ ATPase
Heart muscle relaxes

18

Describe the muscle fibre when relaxed?

No cross-bridge binding site because the cross-bridge binding site on actin is physically covered by the troponin-tropomyosin complex

19

What does the bindning of actin and myosin cross-bridge trigger?

Power stroke that pulls actin in during contraction

20

Describe the muscle fibre when excited?

Ca++ binds with troponin
Pulling troponin-tropomyosin complex aside to expose cross-bridge binding site
Cross-bridge binding occurs

21

What is the importance of a refractory period in the cardiac cycle?

The long refractory period is protective for the heart preventing generation of tetanic contractions in the cardiac muscle

22

What triggers contraction?

The ventricular muscle AP

23

What is a refractory period?

Period following an AP in which it is not possible to produce another AP

24

Theory of a refractory period?

During plateau phase of a ventricular AP: The Na+ channels are in the depolarised closed state- not available for opening
During the descending phase of AP- The K+ channels are open and membrane cannot be depolarized

25

Define Stroke volume?

Volume of blood ejected from each ventricle per heart beat

26

Stroke volume equation?

SV= End diastolic volume - end systolic volume

27

What is SV regulated by?

Intrinsic and extrinsic mechanisms
Intrinsic= within the heart itself
Extrinsic= Nervous and hormonal control

28

What are changes in stroke volume brought about by?

Changes in the diastolic length/diastolic stretch of myocardial fibers

29

What is the diastolic length/stretch (CARDIAC PRELOAD) determined by?

The volume of blood within each ventricle at the end of diastole - The END DIASTOLIC VOLUME

30

What is the end diastolic volume determined by?

Venous return to the heart