Force generation by the heart

Question 1

What causes the striation of cardiac muscle?

Answer 1

regular arrangement of contractile protein within the cardiac muscle cells

Question 2

What ensures that the electrical excitation reaches all of the cardiac myocytes?

Answer 2

Gap junctions

Question 3

What are desmosomes?

Answer 3

structures within the intercalated discs provide mechanical adhesion between adjacent cardiac cells.
-They ensure that the tension developed by one cell is transmitted to the next

Question 4

what are myofibrils?

Answer 4

Intracellular organelles in the muscle fibres. They are the contractile units of the muscle

Question 5

What is a sarcomere?

Answer 5

it is the functional unit of the muscle. Actin and myosin are arranged into sarcomeres within each myofibril

Question 6

What is the functional unit of a cell?

Answer 6

The smallest unit that can do all the functions of the tissue

Question 7

How do muscles contract?

Answer 7

Sliding of the actin filaments on the myosin filaments.

Question 8

What has to be present for cross bridges to generate?

Answer 8

ATP

Question 9

What two molecules must be present for contraction and relaxation of muscle?

Answer 9

ATP and Ca++

Question 10

How does calcium switch on cross bridge formation?

Answer 10

• myosin binding sites on actin are covered by regulatory proteins (troponin and tropomyosin)
• when calcium binds to these regulatory proteins, it causes a conformational change causing the proteins to release from the myosin binding site on actin.
• Thereby allowing myosin to form a cross bridge

Question 11

Where is calcium stored in cardiac muscle cells?

Answer 11

in the lateral sacs of the sarcoplasmic reticulum

Question 12

What is required for SR to release calcium?

Answer 12

The presence of extra cellular Ca++

Question 13

What happens when there is not enough intracellular calcium?

Answer 13

Muscles relax

Question 14

What happens during the plateau phase of action potential?

Answer 14

Calcium will flow through the L type Ca++ channels into the cardiac myocytes. This will then cause SR to release calcium

Question 15

What happens when the action potential has passed?

Answer 15

, Ca++ influx ceases, Ca++ re-sequestered in SR by Ca++-ATPase, heart muscle relaxes

Question 16

True/ False. During the plateau phase of ventricular action potential, the Na+ channels are in the hyperpolarised, open state

Answer 16

False,
During the plateau phase of ventricular action potential the Na+ channels are in the depolarised closed state i.e. they are not available for opening

Question 17

What is the benefit of the long refractory period?

Answer 17

To protect the heart muscle via preventing generation of tetanic contractions.

Question 18

What is the long refractory period?

Answer 18

is a period following an action potential in which it is not possible to produce another action potential

Question 19

True/False. During the descending phase of action potential the K+ channels are open and the membrane can not be depolarised

Answer 19

True

Question 20

What is stroke volume?

Answer 20

This is defined as “the volume of blood ejected by each ventricle per heart beat”

Question 21

What is the equation for Stroke volume?

Answer 21

SV = End Diastolic Volume (EDV) – End Systolic Volume (ESV

Question 22

What is the stroke volume regulated by>

Answer 22

Intrinsic and extrinsic mechanisms.

• intrinsic- within the heart muscle itself
• extrinsic- nervous, hormonal control

Question 23

What causes changes in stroke volume?

Answer 23

changes in the DIASTOLIC LENGTH of MYOCARDIAL FIBERS

Question 24

What is the end diastolic volume?

Answer 24

the volume of blood within each ventricle at the end of diastole.
-It is determined by the venous return to the heart

Question 25

What is the cardiac preload?

Answer 25

How much the heart is loaded with blood before it contracts

Question 26

What is the Starling’s law of the heart?

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)

Question 27

What does Afterload mean?

Answer 27

Resistance into which the heart is pumping

Question 28

How does the body respond to increased afterload?

Answer 28

• increased afterload leads to decreased stroke volume and therefore EDV increases to compensate
• hypertrophy is afterload increases over time.

Question 29

What is a positive inotropic effect?

Answer 29

Increase in the force of contraction

Question 30

What are the effects of noradrenaline on ventricular contraction?

Answer 30

Leads to a stronger and faster contraction.

• force of contraction increases by activation of Ca channels
• peak ventricular pressure rises
• rate of pressure change during systole increases
• reducing the duration of systole
• rate of ventricular relaxation increases
• this reduces the duration of diastole

Question 31

Any end diastolic value will lead to an increased stroke volume if sympathetic stimulation occurs. T/F?

Answer 31

True

Question 32

What is the extrinsic control of stroke volume?

Answer 32

Hormones adrenaline and noradrenaline are released from adrenal medulla and have inotropic and chronotropic effects

Question 33

What is cardiac output?

Answer 33

The volume of blood pumped by each ventricle per minute

Question 34

What is the equation for cardiac output?

Answer 34

CO=SV x HR