Physiology 2 - Force Generation by the Heart

Question 1

Is cardiac muscle striated or non-striated?Why?

Answer 1

StriatedDue to the regular arrangement of contractile protein

Question 2

What part of the intercalated disc provides mechanical adhesion between adjacent cardiac cells?

Answer 2

The desmosomes

Question 3

What are myocytes made up of?

Answer 3

Myofibrils which contain actin and myosin

Question 4

Out of actin and myosin, what is the thicker filament and what is the thinner filament?

Answer 4

Thicker = myosin (darker appearance)Thinner = actin (lighter appearance)

Question 5

What is the actin and myosin in myofibrils arranged into?

Answer 5

Sacromeres

Question 6

What are sarcomeres?

Answer 6

The functional unit of muscle

Question 7

Does actin slide over myosin or does myosin slide over actin?

Answer 7

Actin slides over myosin

Question 8

Is ATP required for the contraction or relaxation of cardiac muscle fibres?

Answer 8

Contraction and relaxation

Question 9

What happens to the myosin head when ATP is present but there is no Ca2+ present?

Answer 9

It is energised but since there is no Ca2+, it does not bind to actin

Question 10

What happens to the myosin head when both ATP and Ca2+ are present?

Answer 10

The myosin head is energised and since there is Ca2+ present, it can bind to actin and creates a bending movement called a power stroke causing the release of energy and ADP + Pi

Question 11

What must be available for myosin to detach from actin?

Answer 11

ATP

Question 12

What 2 molecules are found attached to actin?

Answer 12

TroponinTropomyosin

Question 13

How does the presence of Ca2+ lead to myosin being able to bind to actin?

Answer 13

Ca2+ binds to topping which causes a conformational change which slides the troponin-tropomyosin complex away from the binding site on actin (if we don’t have Ca2+ this does not happen and therefore the troponin-tropomyosin complex stays covering the binding site)

Question 14

What is the intracellular store of calcium in a cell?

Answer 14

In the sarcoplasmic reticulum

Question 15

how is calcium released from the sarcoplasmic reticulum?

Answer 15

Due to the presence of extracellular calcium = calcium induced calcium release

Question 16

How does the length of the action potential and muscle contraction compare between cardiac smooth muscle and skeletal muscle?

Answer 16

much longer in cardiac smooth muscle (refractory period is also longer)

Question 17

What is the refractory period?

Answer 17

The period following stimulation in which it is not possible to produce another action potential

Question 18

Why is the long refractory period good for the heart?

Answer 18

It is protective, preventing generation of titanic (sustained) contraction

Question 19

What is the stroke volume?

Answer 19

The volume of blood ejected from each ventricle per heart beat

Question 20

SV = ?

Answer 20

SV = end diastolic volume - end systolic volume

Question 21

What are the 2 categories of mechanisms by which the stroke volume can be regulated?

Answer 21

Intrinsic (within the heart itself)Extrinsic (nervous and hormonal control)

Question 22

What are the intrinsic factors affecting stroke volume?

Answer 22

Changes in diastolic length of myocardial fibresThis is determined by the volume of blood in each ventricle at the end of diastole (end diastolic volume)

Question 23

What is the cardiac preload?

Answer 23

Length of sarcomeres/ tension in cardiac muscle prior to contraction

Question 24

What determines the cardiac preload?

Answer 24

The end diastolic volume

Question 25

What determines the end diastolic volume?

Answer 25

Venous return to the heart

Question 26

Starling’s law/ Frank-Starling curve?

Answer 26

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) - as we increase the end diastolic volume, we increase the stroke volume unless we stretch the heart too much

Question 27

How is the optimal length of cardiac muscle achieved?

Answer 27

By stretching the muscle

Question 28

What is the afterload?

Answer 28

Resistance into which the heart is pumping

Question 29

What can happen if an increased afterload persists e.g. untreated hypertension?

Answer 29

the ventricular muscle will increase to overcome the resistance = ventricular hypertrophy

Question 30

What type of nerve supplies the ventricular muscle?

Answer 30

Sympathetic nerves

Question 31

What happens when the sympathetic nerves suppying the ventricular muscle is stimulated?

Answer 31

It increases the force of contraction - positive ionotropic effect

Question 32

How does the force of ventricular contraction increase due to sympathetic stimulation?What mediates this effect?

Answer 32

Due to increased activation of Ca2+ channels - peak ventricular pressure rises, rate of pressure change during systole also increases, rate of ventricular relaxation also increases due to increased rate of Ca2+ pumpingcAMP

Question 33

What are the 2 axis on the frank starling curve?

Answer 33

EDVStroke volume

Question 34

What effect does sympathetic stimulation have on the Frank-stirling curve?

Answer 34

Shifts it to the left and peak is increased

Question 35

What effect does heart failure have on the frank-starling curve?

Answer 35

Shifts it to the right - negative ionotropic effect - less stroke volume for same EDV due to lower force of contraction

Question 36

Effect of parasympathetic nerve on ventricular contraction?

Answer 36

Very little innervation of ventricles by ages in man therefore little, if any, direct effect on SV (large impact on rate but not force or contraction)

Question 37

what hormones have a chronotropic and ionotropic effect?

Answer 37

Adrenaline and noradrenaline released from the adrenal medulla (effects normally minor)

Question 38

What is the cardiac output?

Answer 38

The volume of blood pumped by each ventricles per minute

Question 39

What is the equation for cardiac output?

Answer 39

CO = SV X HR