Force Generation by the Heart

Question 1

What is the general structure of cardiac muscle?

Answer 1

Striated

Question 2

What does striated mean?

Answer 2

Regular arrangement of contractile proteins

Question 3

What kind of junction is present in cardiac muscle?

Answer 3

Gap junctions

Question 4

What is the purpose of desmosomes?

Answer 4

Provide mechanical adhesion

Question 5

What is the contractile unit of cardiac muscle?

Answer 5

Myofibril

Question 6

Describe the structure of myofibrils

Answer 6

Alternate between thick and thin protein filaments (myosin and actin)

Question 7

What is the functional unit of cardiac muscle?

Answer 7

Sarcomere

Question 8

How is muscle tension produced?

Answer 8

Sliding of actin filaments on myosin

Question 9

What chemical is responsible for formation of cross-bridges?

Answer 9

ATP

Question 10

What will inhibit contraction?

Answer 10

Absence of Ca

Question 11

What is the importance of calcium?

Answer 11

When it binds to troponin the myosin binding site on actin is exposed and the bridge can form in the presence of ATP

Question 12

What needs to be present for calcium to be released from the SR?

Answer 12

Extra-cellular calcium

Question 13

Describe the roll of calcium in the plateau phase of ventricular muscle action potential?

Answer 13

Calcium will flow through L-type channels and enter myocytes, stimulating the release of more calcium from within the SR within myocytes

Question 14

What happens to CICR after contraction

Answer 14

Calcium influx will cease and so will CICR

Question 15

When is it not possible to generate another action potential?

Answer 15

During the refractory period

Question 16

What state are the Na channels in during the plateau phase?

Answer 16

Closed depolarised state

Question 17

What state are the K channels in during the descending phase?

Answer 17

Open

Question 18

What is stroke volume?

Answer 18

Volume ejected per ventricle per minute

Question 19

What can change the stroke volume?

Answer 19

Changes in diastolic length of myocardial fibres (volume of blood within the ventricles at the end of the relaxation period)

Question 20

What is EDV dependant on?

Answer 20

Venous return to the heart

Question 21

What does the frank starling curve tell us about the relationship between EDV and SV?

Answer 21

The more the EDV the higher the SV, unless we strain the heart too much which is unlikely in a normal heart

Question 22

What does stretch do to Ca affinity for troponin?

Answer 22

Decreases it

Question 23

How is optimum length of cardiac muscle achieved?

Answer 23

By stretching it within the FS curve

Question 24

How does the FS curve compensate for decreased SV caused by increased after load?

Answer 24

Increasing the force of contraction until eventually ventricular hypertrophy occurs

Question 25

What is after load?

Answer 25

resistance into which the heart is pumping

imposed AFTER contraction

Question 26

What can extrinsically control SV?

Answer 26

Nerves and hormones

Question 27

How is ventricular muscle innervated?

Answer 27

sympathetic nerve fibres

Question 28

What does sympathetic stimulation cause?

Answer 28

Increased HR and force of contraction

Question 29

how does sympathetic stimulation cause a positive inotropic effect?

Answer 29

by activating Ca channels, mediated by cAMP, raising peak ventricular pressure and dP/Pt during systole increases, increasing the duration of systole and reduced diastole duration

Question 30

What shifts the FS curve left?

Answer 30

Sympathetic stimulation

Question 31

What shifts the FS curve right?

Answer 31

Heart failure

Question 32

What affect do adrenaline and noradrenaline have?

Answer 32

Positive inotropic and positive chronotropic effect