lecture 2: mechanical properties of the heart: episode 1

Question 1

what is required for contraction?

Answer 1

Ca2+ ions are needed for contraction

Question 2

what is needed for an action potential to happen?

Answer 2

• first there is a calcium transient (this is an influx of calcium in the sarcoplasm) - then there is a contractile event

Question 3

why is external calcium important?

Answer 3

• cardiac muscles CANNOT beat without external calcium where as skeletal muscles CAN contract without external calcium this is because in skeletal muscles there is a mechanical linkage

Question 4

• what is the structure of a cardiac myocyte?

Answer 4

• has a striated structure - 100 micrometers in length - width is 15 micrometers - the T tubule diameter is 200nm - has finger like invaginations of the cell surface - lots of myofibrils - large amount of mitochondria due to large amount of ATP required

Question 5

where is calcium stored in the cell?

Answer 5

in the sarcoplasmic reticulum

Question 6

what is the first step of excitation contraction coupling?

Answer 6

• the depolarisation is detected by L type calcium channel - external calcium then enters the cell a. some calcium causes contraction b. the rest binds to ryanodine receptors - this causes the release of calcium from the sarcoplasmic reticulum

Question 7

what is the second step of the excitation contraction coupling?

Answer 7

• after the effect has happened some of the calcium is taken back up into the sarcoplasmic reticulum by CaATPase channels

Question 8

what is the third step of excitation contraction coupling?

Answer 8

the same amount of calcium that came into the cell is then effluxes by a sodium calcium exchanger this does not require energy as the calcium co transports out with sodium down the concentration gradient

Question 9

what is the force production and intracellular calcium ion relationship?

Answer 9

sigmoidal

Question 10

whats the relationship between muscle length and potential force energy?

Answer 10

an increase in muscle length causes in an increase in the potential force energy

Question 11

what happens if you stretch the muscle?

Answer 11

• at first the force is passive and there is no shortening the TENSION changes however if you keep stretching the muscle a point is reached where stretching does not generate any more force their is because there is not enough overlap between the filaments

Question 12

what is isometric contraction

Answer 12

• there is no shortening - just tension changes which causes force - occurs in systole - both the atria fill and the pressure increases which stresses and puts a pre -load on the cardiac cells

Question 13

what is isotonic contraction?

Answer 13

• there is shortening and this is what causes the force - this occurs in diastole - the cardiac cells contract and the fibres shorten to eject blood from the ventricles

Question 14

what are the length tension differences between cardiac muscle and skeletal muscle?

Answer 14

• skeletal muscle produces less passive force - cardiac muscle is more resilient to stretch

Question 15

whats a pre load?

Answer 15

a pre load is a weight that stretches a muscle before it is stimulated to contract - the pre load determines the stretching of the cardiac cell and is defined by blood pressure

Question 16

what is an after load?

Answer 16

an after load is a weight not apparent to the muscle in resting state it is only encountered when the muscle has started to contract - the afterload determines the contraction of the cardiac cell as is defined as the resistance of the blood flow

Question 17

what does more pre load result in? what does more after load result mean?

Answer 17

more pre load means more stretch which means more force is produced more after-load means there is a lesser shortening more after-load also means the lower velocity of shortening

Question 18

what is the concept of the pre load? what is the concept of the after load?

Answer 18

Preload is weight that makes the muscle stretch (blood going in)

Afterload is weight against which you have to push ( blood going out)

Question 19

how does pre load happen in the heart?

Answer 19

1. as blood fills the ventricles during diastole the walls stretch due to the pressure increase 2. the stretch determines the pre load 3. thus the pre load depends on how much blood fills the ventricles

Question 20

what are the measures of pre load?

Answer 20

• end diastole volume - end diastole pressure - the right atrial pressure

Question 21

how does after load happen in the heart?

Answer 21

1. the afterload is the force the heart muscle must overcome in ejecting blood from the ventricles

Question 22

what happens to the afterload if you are hypertensive?

Answer 22

there is more pressure therefore the after load is increased

Question 23

what are the measures of the afterload?

Answer 23

• diastolic arteriol blood pressure

Question 24

what does increased afterload in the heart mean?

Answer 24

= this means a decreased shortening and a decreased velocity of shortening

Question 25

what do we get for the same afterload with more pre load?

Answer 25

- you get an increase in shortening

Question 26

what is starlings law?

Answer 26

increased diastolic fibre length increases the ventricular contraction in simpler terms : the increase in stretching leads to an increase in shortening and speed of shortening

Question 27

what is the consequence of starlings law on a larger pre load?

Answer 27

blood coming in determines the strength of the ventricular contraction and therefore determines the volume of blood that leaves the ventricles.

Question 28

what is the starlings law dependent on?

Answer 28

1. changes in the number of myofilament cross bridges that interact 2. changes in the calcium sensitivity of the myofilaments

Question 29

why do changes in the number of myofilament cross bridges affect the starling law?

Answer 29

- at shorter lengths than optimal the actin filaments overlap therefore reducing the number of myosin cross bridges that can be made

Question 30

what happens to calcium sensitivity with increased length?

Answer 30

with increased length there is also an increase in calcium sensitivity

Question 31

what are the 2 possibilities for the mechanism responsible for the increase in calcium sensitivity with increased length ?

Answer 31

1. troponin is a thin filament that binds to calcium and regulates cross bridge formation

Question 32

what are the 2 possibilities for the mechanism responsible for the increase in calcium sensitivity with increased length ? number 1

Answer 32

1. troponin is a thin filament that binds to calcium and regulates cross bridge formation - at longer sarcomere lengths the affinity of troponin C for calcium could be increased due to a conformational change - therefore less calcium is required for the same force that is generated

Question 33

what are the 2 possibilities for the mechanism responsible for the increase in calcium sensitivity with increased length ? number 2

Answer 33

with stretch the space between myosin and actin filaments decreases - with decreasing myofilament lattice spacing the probability of forming strong cross bridges increases - this produces more force for the same amount of calcium

Question 34

what is stroke work?

Answer 34

stroke work is the work done by the heart to eject blood under pressure into the aorta and the pulmonary artery

Question 35

what is the equation for stroke work?

Answer 35

stroke work = stroke volume x pressure

Question 36

what is stroke volume affected by? what is pressure affected by?

Answer 36

- affected by pre load and after load - affected by the structure of the heart

Question 37

what is the law of laplace?

Answer 37

- when the pressure within a cylinder is held constant the tension on its walls increases with an increasing radius the force around the circumference is equal to pressure x radius

Question 38

how does the law of laplace relate to the ventricles of the heart

Answer 38

the radius of curvature of the walls of the left ventricle is less that that of the right ventricle therefore the right ventricle experiences more tension than the left

Question 39

what is the size of the radius of the ventricle in giraffes and in frogs?

Answer 39

giraffes : wall stress is kept low by a small radius to generate high pressure and have low tension frogs: the pressure is kept lower so the ventricles have a larger radius but experience greater pressure

Question 40

what ventricle radius do failing hearts show?

Answer 40

they display a large radius which has large wall stress and low pressure is generated