CVS 2: Mechanical Properties of the Heart I

Question 1

What are the three stages of a ventricular cell contraction?

Answer 1

1, Electrical Event

2. Calcium Transient (amount of Ca2+ in the sarcoplasm increases for a short period of time)
3. Contractile event

Question 2

What are the dimensions of a single cardiac cell?

Answer 2

100 microms long and 15 microms wide

Question 3

What is the diameter of a T-tubule?

Answer 3

200nm

Question 4

Where are T- tubules located?

Answer 4

They are spread 2microms parts so that they lie alongside each Z-line of every microfibril

Question 5

Describe the stages of excitation- contraction coupling in the heart?

Answer 5

1. Depolarisation is sensed by the L-type calcium channel and channel opens causing influx of Ca2+ into the cell
2. Some of this Ca2+ directly causes contraction
3. There rest of the Ca2+ binds to the RyR which causes the release of Ca2+ from the SR

Question 6

What are the two ways that the IC levels of Ca2+ return to normal after excitation?

Answer 6

1. Calcium taken back up into SR by Ca2+ ATPase channel

2. Some of the Ca2+ is effluxes by a Na+/ Ca2+ exchanger (passive- uses Na+ conc gradient)

Question 7

Describe the relationship between cytoplasmic Ca2+ concentration and the force produced

Answer 7

• Sigmoidal
• 10 micromolar IC conc Ca2+ –> Maximum force
• small increase in Ca2+ conc ==> massive force generation

Question 8

Describe the length-tension relation in cardiac muscle

Answer 8

• An increase in muscle length => an increase in force
• But you get to a point where further stretching doesn’t generate more force
  (N.B. kind of like a spring but with its variable switched around)

Question 9

How is passive force generated?

Answer 9

Cardiac muscle tissue generates passive force by isometric contraction (where it doesn’t shorten) but pulls on the force transducer

Question 10

Why do you get to a point where cardiac muscle does not generate more force?

Answer 10

Not enough overlap between the filaments to produce more force

Question 11

What happens when you overstretch a muscle?

Answer 11

You get a decrease in force

this is what happens when you pull a muscle

Question 12

What is passive force?

Answer 12

The ability of muscle to resist stretching

Question 13

Compare the length-tension relation in cardiac and skeletal muscle

Answer 13

1. Cardiac Muscle more resistant to stretching ==> more passive force exerted
2. Cardiac muscle less compliant than skeletal muscle

Question 14

Why is it that only the ascending limb of the length- tension relationship is important for cardiac muscle?

Answer 14

Because you can’t overstretch cardiac muscle since the pericardium restricts its stretching

Question 15

What determines the resilience of cardiac muscle stretching?

Answer 15

Properties of:

• the ECM
• its cytoskeleton

Question 16

What are the two forms of contraction used by the heart? And when are these two types of contraction used by the heart?

Answer 16

ISOMETRIC- resists the high pressure

ISOTONIC- shortening of fibres when blood is ejected from the ventricles

Question 17

What is preload?

Answer 17

Weight that stretched the muscle before it is stimulated to contract (e.g. filling go the ventricles makes it stretch before it contracts)

Question 18

What is afterload?

Answer 18

Weight that is not apparent to the muscle in the resting state- only encountered once muscle has started to contract