Force Generation by the Heart

Question 1

Describe cardiac muscle?

Answer 1

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

Question 2

What causes striations?

Answer 2

Regular arrangement of contractile proteins

Question 3

Why are there no neuromuscular junctions in the heart?

Answer 3

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

Question 4

What do protein channels in cardiac muscle ensure?

Answer 4

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

Question 5

What do the desmosomes between intercalated provide?

Answer 5

Provide mechanical tension between adjacent cardiac cells

they ensure the tension developed by one cell is transferred to the next cell

Question 6

What does each muscle contain?

Answer 6

Myofibrils - contractile units of muscle

Question 7

What do myofibrils have?

Answer 7

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

Question 8

How are actin and myosin arranged in a myofibril?

Answer 8

Into sarcomeres (smallest functional unit of a muscle)

Question 9

How is muscle tension produced?

Answer 9

Sliding of actin filaments on myocin filaments

Question 10

What is the sliding filament theory?

Answer 10

Explanation of how muscle shortens and produces force

Question 11

What does force generation depend on?

Answer 11

ATP dependant interaction between actin and myocin filaments

Question 12

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

Answer 12

True

Question 13

What is Ca2+ required for?

Answer 13

Switching on cross bridge formation

Question 14

Where is Ca2+ released from?

Answer 14

Sarcoplasmic reticulum

Question 15

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

Answer 15

Presence of extracellular Ca2+

Question 16

Systole- what happens with calcium?

Answer 16

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

Question 17

What happens when an AP has passed after systole?

Answer 17

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

Question 18

Describe the muscle fibre when relaxed?

Answer 18

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

Question 19

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

Answer 19

Power stroke that pulls actin in during contraction

Question 20

Describe the muscle fibre when excited?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

What triggers contraction?

Answer 22

The ventricular muscle AP

Question 23

What is a refractory period?

Answer 23

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

Question 24

Theory of a refractory period?

Answer 24

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

Question 25

Define Stroke volume?

Answer 25

Volume of blood ejected from each ventricle per heart beat

Question 26

Stroke volume equation?

Answer 26

SV= End diastolic volume - end systolic volume

Question 27

What is SV regulated by?

Answer 27

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

Question 28

What are changes in stroke volume brought about by?

Answer 28

Changes in the diastolic length/diastolic stretch of myocardial fibers

Question 29

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

Answer 29

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

Question 30

What is the end diastolic volume determined by?

Answer 30

Venous return to the heart

Question 31

Another name for the Frank-starling mechanism ?

Answer 31

Starling's law of the heart

Question 32

What does the Frank starling mechanism state?

Answer 32

The more the ventricle is filled with blood during diastole (END DIASTOLIC VOLUME), the greater the volume of ejected blood will be during the resulting systolic contraction (STROKE VOLUME)

Question 33

What relationships does Starling's law of the heart outline?

Answer 33

Relationship between venous return, end diastolic volume and stroke volume

Question 34

What does diastolic stretch increase?

Answer 34

Affinity of troponin for Ca++

Question 35

How is optimal length achieved in cardiac muscle?

Answer 35

Stretching the muscle. In skeletal muscle it is achieved at resting muscle length

Question 36

What happens if venous return to right atrium increases?

Answer 36

EDV of right ventricle increases which leads to increased SV into pulmonary artery

Question 37

Venous return to left atrium from pulmonary vein increases?

Answer 37

EDV of left ventricle increases= larger SV into aorta

Question 38

Define afterload?

Answer 38

The resistance into which the heart is pumping

Question 39

When is the afterload imposed?

Answer 39

After the heart has contracted

Question 40

What happens if the afterload increases?

Answer 40

-At first the heart is unable to eject full SV so the end diastolic volume increases If it continues-Eventually the ventricular muscle mass increases to overcome resistance

Question 41

What is an increase in ventricular muscle mass called?

Answer 41

Ventricular hypertrophy

Question 42

What do you call increased afterload existing continuously?

Answer 42

Hypertension

Question 43

What type of nerves is the ventricular muscle supplied by

Answer 43

Sympathetic nerve fibers

Question 44

What is extrinsic neurotransmitter?

Answer 44

Noradrenaline

Question 45

What does stimulation of sympathetic nerves do to the heart?

Answer 45

Increases the force of contraction - positive inotropic effect Positive chronotropic effect- - increases the HR

Question 46

What does extrinsic control involve?

Answer 46

Nerves and hormones

Question 47

What does heart failure do to Frank-Starling curve?

Answer 47

Shifts it to the right

Question 48

What does sympathetic nerve stimulation do to Frank-Starling curve?

Answer 48

Shifts it to the left

Question 49

Effect of parasympathetic nerves on ventricular contraction ?

Answer 49

Very little innervation of ventricles by vagus in man- little/no effect on SV Vagal stimulation has influence on rate not force of contraction

Question 50

Hormones that control stroke volume?

Answer 50

Adrenaline and noradrenaline These are released from adrenal medulla and have inotropic and chronotropic effects The effects are normally minor compared to effects of noradrenaline from sympathetic nerves

Question 51

Define cardiac output?

Answer 51

The volume of blood pumped by each ventricle per minute is known as the Cardiac Output (CO)

Question 52

CO equation?

Answer 52

CO= SV x HR

Question 53

What is the resting CO in a healthy adult?

Answer 53

5 litres per minute