2 - Mechanical properties of the Heart 1

Question 1

what shape are ventricular myocytes?

Answer 1

rod shaped

Question 2

What is the calcium transient?

Answer 2

when the amount of calcium in the sarcoplasm increases for a short period of time

Question 3

What is the length of a single ventricular cell?

Answer 3

100 micrometres

Question 4

what is the width of a single ventricular cell?

Answer 4

15 micrometres

Question 5

what is the diameter of a t tubule?

Answer 5

200 nanometres

Question 6

What is the distance between 2 t-tubules?

How does this effect the structure of the cell?

Answer 6

about 2 micrometres

this ensures that the T-tubule lies alongside each z line of every microfibril

Question 7

What receptors on cardiomyocytes detects deplorisation?

What does this cause?

Answer 7

L type calcium channels (a type of VGCC) detects depolarisation and calcium from the outside enters the cell

Question 8

What is the effect of calcium entering the cell?

Answer 8

• some of it directly stimulates contraction

- the rest binds to Ryanodine receptors (on the SR), which causes the release of calcium from the sarcoplamic reticulum

Question 9

After the calcium has caused contraction, how are the concentrations restored?

Answer 9

(the calcium goes back to its origin in the same proportions)

• some of it goes back into the SR by Ca ATPase channels (SERCA- sarco/endoplasmic reticulum calcium ATPase)
• some of it is effluxed by a Sodium-Calcium exchanger (NOTE: this does not require energy)

Question 10

What is an isometric contraction?

Answer 10

the muscle doesn’t change length, it changes in tone

Question 11

What is the length tension relation in cardiac muscle?

Answer 11

an increase in muscle length= an increase in the force produced

Question 12

Explain the peak of the curve in the length-tension relation in cardiac muscle (i.e. the point where further stretching does not generate more force)?

Answer 12

there is not enough overlap between the filaments

Question 13

what is the passive force of a muscle?

Answer 13

is resistance to stretch of the muscle

Question 14

how does the passive force of skeletal and cardiac muscle differ?

Answer 14

skeletal - much less passive force produced
(can be overstreched)

cardiac - much more resilient to stretch (so exerts more passive force)
(cannot be overstretched)

Question 15

What impacts the resistance of a muscle?

Answer 15

the properties of the ECM and cytoskeleton

i.e. cardiac muscle is resistant because it is contained within its pericardium

Question 16

Describe the 2 forms of contraction with reference to the ventricles of the the heart

Answer 16

• isometric
  the muscle fibres do not change in length but pressure in both ventricle increases
• isotonic
  shortening of fibres and blood is ejected from the ventricles

Question 17

What is preload in the heart?

Answer 17

filling of the venticles with blood

Question 18

what is afterload in the heart?

Answer 18

the (back) pressure in the aortic valves

Question 19

Explain Starling’s Law in lay terms

Answer 19

an increase in preload/stretching leads to an increase in shortening and the speed of shortening

Question 20

What is stroke work in relation to work done?

Answer 20

the work done by the heart in one contraction

Question 21

Explain the Law of Laplace in lay terms

Answer 21

increased radius = increased tension

Question 22

How is the sarcoplasm arranged in cells?

Answer 22

it joins onto the t-tubules by wrapping around them

Question 23

What percentage of the cardiomyocytes are myofibrils?

Answer 23

50%