Physiology: Force Generation by the Heart

Question 1

Describe the organisation of cardiac muscle

Answer 1

• Striated, due to regular protein arrangement

- No neuromuscular junctions

Question 2

List and describe the connections between cardiac myocytes

Answer 2

Gap junctions:

• Protein channels
• Allow low resistance electrical communication between neighbouring cells

Desmosomes:

• Provide mechanical adhesion between cells
• Ensure that tension produced by one cell is transmitted to the next

Question 3

Describe the All-or-none Law of the heart

Answer 3

If the stimulus exceeds the threshold potential, heart will give a complete response, otherwise there is no response

Question 4

Describe a myfibril

Answer 4

• Contractile units of muscle

- Alternating segments of actin and myosin

Question 5

Describe the appearance of actin filaments in a myofibril

Answer 5

• Thin

- Lighter colour

Question 6

Describe the appearance of myosin filaments in a myofibril

Answer 6

• Thick

- Darker colour

Question 7

Describe a sarcomere

Answer 7

• A subunit of a myofibril
• Smallest contractile subunit
• From one Z line to the next

Question 8

What is the basic principle of how muscle tension is produced?

Answer 8

By actin filaments sliding on myosin filaments

e.g. Sarcomeres shorten

This uses ATP

Question 9

Describe the process by which myosin binds to actin

Answer 9

• Troponin-tropomyosin blocks the myosin cross bridge binding sites
• Ca2+ binds to troponin, causing a conformational change
• Myosin binding sites exposed
• Myosin cross bridge binds to actin via binding sites

Question 10

Give another term for a muscle fibre

Answer 10

Muscle cell (myocyte)

Question 11

Describe the sarcoplasmic reticulum (SR)

Answer 11

• Stores Ca2+ for release during contraction mechanism
• Forms a mesh enclosing the myofibrils
• Has transverse tubules (T-tubule) connected by lateral sacks

Question 12

Define the sarcolemma

Answer 12

The membrane that envelops myocytes

Question 13

Define transverse tubules (T-tubules)

Answer 13

Extensions of the sarcolemma that penetrate into the centre of the myocyte

Question 14

Describe the purpose of the transverse tubules (T-tubules)

Answer 14

To bring the membrane potential closer to the myofibrils (and thus sarcomeres)

Question 15

Describe the mechanism by which adequate Ca2+ are acquired to free up the myosin cross bridge binding sites

Answer 15

Ca2+ induced Ca2+ release (CICR):

• Ca2+ influx during phase 2 of action potential
• Ca2+ influx induces Ca2+ stores in the Sarcoplasmic reticulum to be released
• Enough Ca2+ present to full expose the myosin cross bridge binding sites

Question 16

Is there enough Ca2+ influx during phase 2 of the action potential to allow the myosin cross bridge binding sites to move?

Answer 16

No, it requires Ca2+ release from the sarcoplasmic reticulum

This occurs via Ca2+ - induced Ca2+ release (CICR)

Question 17

Define the refractory period of a cardiac myocyte

Answer 17

The period after an action potential in which it is impossible to produce another action potential

Question 18

Describe the importance of the long refractory period of cardiac myocytes

Answer 18

Prevents tetanic contractions

Question 19

Define tetanic contractions

Answer 19

Sustained muscle contraction due to very rapidly occurring action potentials

Question 20

What factors contribute to the existence of the refractory period?

Answer 20

• The Na+ channels are closed after phase 0, preventing further depolarisation
• Phase 2 lasts a long time due to the slowly inactivating L-type Ca2+ channels
• In phase 3 the outflux of K+ prevents another depolarisation

Question 21

Define the stroke volume (SV) of the heart

Answer 21

The volume of blood ejected a ventricle during systole

Question 22

Give an equation for determining stroke volume from the end volumes of systole and diastole

Answer 22

SV = EDV - ESV

Question 23

Define the end diastolic volume (EDV)

Answer 23

The volume of blood within the ventricle at the end of diastole

i.e. the max volume of blood in the ventricle

Question 24

What determines the end diastolic volume?

Answer 24

The venous return of to the heart

Question 25

Define cardiac preload

Answer 25

The initial stretching of the cardiac myocytes prior to contraction It is determined by the end diastolic volume (how full the ventricle gets)

Question 26

Define the end systolic volume (ESV)

Answer 26

The volume of blood within the ventricle at the end of systole

Question 27

Describe the Frank-Starling Mechanism (Starling's Law)

Answer 27

- The relationship between end diastolic volume and stroke volume - The more blood in the ventricle during diastole (EDV), the greater the volume of blood ejected during systolic contraction (SV)

Question 28

What conditions are required to achieve optimal force generation from the heart?

Answer 28

- Optimal fibre length

Question 29

Does the the myocyte fibre length increase past the optimal fibre length for maximising stroke volume

Answer 29

Not in a healthy heart

Question 30

Give the effect that myocyte fibre stretch has on troponin affinity for Ca2+

Answer 30

Fibre stretch increases the affinity of troponin for Ca2+

Question 31

Compare the optimum muscle fibre lengths of skeletal and cardiac muscle

Answer 31

Skeletal: - Unstretched Cardiac: - After stretching

Question 32

Describe why the optimum muscle fibre length of cardiac muscle is achieved after stretch

Answer 32

- As stretch increases the affinity of troponin for Ca2+ - Leading to more cross bridge formation - More cross-bridges better contraction

Question 33

Define the venous return of to the heart

Answer 33

The blood that returns to the an atrium of the heart Can refer to either the right or left atria

Question 34

Describe the effect of increased venous return into the right atrium on: - The EDV of the right ventricle - Stroke volume into the pulmonary artery

Answer 34

EDV increases so Stroke volume increases

Question 35

Describe the effect of increased venous return into the left atrium: - The EDV of the left ventricle - Stroke volume into the aorta on the EDV of the left ventricle

Answer 35

EDV increases so Stroke volume increases

Question 36

Define cardiac afterload

Answer 36

The resistance into which the heart is pumping Is imposed after contraction

Question 37

Describe the heart's response to an increased cardiac afterload

Answer 37

- At first the heart cannot eject the full stroke volume So - EDV increases Then - The force on contraction increases via the Frank-Starling mechanism

Question 38

Describe the outcome of continued increases afterload e.g. hypertension

Answer 38

Ventricular hypertrophy To increase contractile force to overcome the increased afterload

Question 39

List the 2 types of extrinsic control of stroke volume

Answer 39

- Hormonal | - Nervous

Question 40

Describe the role of sympathetic nervous stimulation in controlling stroke volume

Answer 40

- Ventricular muscle is supplied by sympathetic nerves - Stimulation increases the contraction force (+ve inotropic effect) - Thus the peak ventricular pressure also increases - Causes increased rate of contraction and relaxation as well

Question 41

Describe the effect of sympathetic nervous stimulation on the rate of ventricular contraction and relaxation

Answer 41

- Increased rate of contraction | - Increased rate of relaxation

Question 42

Describe the impact of increased rate of contraction and increased rate of relaxation

Answer 42

Increased rate of contraction: - Reduces systole duration Increased rate of relaxation: - Reduces diastole duration Thus it has a +ve chronotropic effect

Question 43

Why does sympathetic stimulation increase ventricular contraction force?

Answer 43

- Activation of Ca2+ channels | - Greater influx of Ca2+

Question 44

Describe the effect sympathetic stimulation has on the the Frank-Starling Mechanism

Answer 44

- Shifts the graph to the left and upwards | - A greater stroke volume is achieved at the same EDV

Question 45

Describe the impact of a +ve and a -ve inotropic effect on the the Frank-Starling Mechanism

Answer 45

A +ve inotropic effect: - A greater stroke volume is achieved at the same EDV - Graph moves upwards A -ve inotropic effect: - A lower stroke volume is achieved at the same EDV - Graph moves downwards

Question 46

Describe the role of sympathetic nervous stimulation in controlling stroke volume

Answer 46

- Very little innervation of ventricles by vagus nerve | - Therefore little effect of stroke volume

Question 47

Describe the role of hormones in controlling stroke volume

Answer 47

- Adrenaline and noradrenaline have a +ve inotropic and chronotropic effect - They are released from the adrenal medulla - Have minor effects compared to that of sympathetic nerves

Question 48

Define cardiac output (CO)

Answer 48

The volume of blood pumped by each ventricle per minute

Question 49

Give an equation to calculate cardiac output from stroke volume and heart rate

Answer 49

CO = SV x HR

Question 50

What is an average healthy cardiac output?

Answer 50

~5 litres