L12 - Myocardial Mechanics

Question 1

What describes the electrical excitation leading to contraction

Answer 1

Excitation - contraction - coupling

Question 2

What does excitation contraction coupling describe

Answer 2

The electrical excitation leading to the contraction

Question 3

What is the role of the T-tubules

Answer 3

Cause activation of many sarcomeres at one time

Leads to a faster contraction

Question 4

What is the structure of the T-tubules - how does this relate to their function

Answer 4

Invaginations of the sarcolemma

They allow the depolarisation to spread deep into the cell

Question 5

What do intercalated discs contain

Answer 5

Ion channels

Question 6

What is the role of the intercalated discs

What does this form?

Answer 6

Between the ion channels - this ionically links the myofibrils forming a functional syncitium

Question 7

Describe the sarcoplasmic reticulum

Answer 7

Fluid filled membranous sac - acts a as Ca store

Question 8

Where are the cisterns found

Answer 8

At the end of the SR

Question 9

What do T-tubules and cisterns form

Answer 9

Triads

Question 10

Describe the sliding filament theory / state for a relaxed muscle

Answer 10

Ca in sarcoplasm low
Ca pumps sequester Ca in SR
Tropomyosin obscures the actin-myosin binding site preventing cross bridge formation

Question 11

What is the role of calsequesterin

Answer 11

Binds to free Ca in the SR - gives impression that Ca is SR is low so pumps can work faster

Question 12

Describe sliding filament theory when muscle is contraction

Answer 12

Propagation of AP along T-tub activation of VGCC
CA SR rises
Ca binds to Tn-C
Conf change in tropomysosin - actin mysoing binding site exposed
Cross bridge can form and cycling can occur if ATp/ADP/Pi available

Question 13

What is muscle tension proportional to

Answer 13

The number of cross bridges

Question 14

What is the number of cross bridges proportional to

Answer 14

The length of the sarcomere

Question 15

What produces a maximum tension generation

Answer 15

An optimum resting length

Question 16

Why isnt max tension gen. with short sarcomeres

Answer 16

Because of overlapping thin filaments

Question 17

What isnt max tension gen. with long sarcomeres

Answer 17

Reduced area for cross bridge formation so less tension is generated

Question 18

What differs between the heart and other muscles - what are the implications of this

Answer 18

Heart is not bound by any joints so it is possible for it to go beyond the optimum length
This leads to less force ==> so more blood left in with each contraction and can lead to heart failure

Question 19

What is the optimum sarcomere length for cardiomyocytes

Answer 19

2.2 - 2.6 um

Question 20

Describe the process that would be undertaken to measure length-tension

Answer 20

Clamped muscle held fixed at one end - other end attached to a force transducer
Electrical stimulation via electrode
Record l of muscle and force gen by the contraction
Lengthen muscle and repeat over a range of muscle lengths

Question 21

What happens to the active force as sarcomere length increases

Answer 21

Starts increasing to a maximum and then decrease

Question 22

What happens to passive force as sarcomere length increases

Answer 22

At 0 until high sarcomere length then increases

Question 23

What happens to total force as sarcomere length increases

Answer 23

Increases - peaks - begins to decrease as active force decreases BUT passive force increases so total froce begins to increase

Question 24

Describe how an engineer would model a cardiomyocyte

Answer 24

One contractile element with a critcal damper to prevent oscilations
One spring in parallel - representing the neck of the cross bridge
One spring in series - representing the accessory protein of the sarcomere (titin)

Question 25

What length sarcomere is produced by a 10-12 mmHg filling pressure

Answer 25

2.2. um | this is the presystolic volume

Question 26

What is a key difference in the regulation of tension between cardiac and skeletal myocyte

Answer 26

In cardiac greater regulation at the cellular level In skeletal muscle to cause greater tension more fibres are recruited In caridac the force produced by each myocyte is changed

Question 27

What is isotonic contraction

Answer 27

Where tension does not change but the length does change

Question 28

What is isometric contraction

Answer 28

Length is unchanged but the tension does change

Question 29

Describe the relationship of the muscle, pre load and the afteload

Answer 29

Muscle is streched by the preload which stimulates it to lift the afterload

Question 30

What is the preload

Answer 30

Initial streching of the sarcomere length | The ventricular end diastolic volume

Question 31

What is the afterload

Answer 31

The force which the ventricles have to act against in order to eject the blood -- essentially the arterial blood pressure and vascular tone

Question 32

For an isotonic contraction what can be said about contraction for a heavier load

Answer 32

It will be slower

Question 33

What is the relationship between shortening velocity and the afterload

Answer 33

There is an inverse relationship between shortening velocity and afterload

Question 34

What are the 3 key results from a graph of initial velocity of shortening against the load

Answer 34

A high preload gives a high maximal force (high preload = high P0) For any given afterload a high preload gives a high velocity of shortening Maximum initial velocity of shortening is constant

Question 35

Describe 3 varying definitions of contractility

Answer 35

Change occurs when a heart changes its output per beat with the EDV remaining constant Changes can occur when more crossbridges form per stimulus May reflect the qualitative state of the actin/myosin cross bridges

Question 36

What is the effect of sympathetic stimulation by NAdr on contractility

Answer 36

Increases the Vmax (maximum initial velocity of shortening) | Increase the P0 (maximum force which can be produced)

Question 37

What are the effects of sympathetic stimulation said to be ...

Answer 37

Positive chronotropic (time) and ionotropic (force)

Question 38

What does the interbeat duration influence

Answer 38

The force of contraction

Question 39

Why does an increased frequency of beats lead to a increase in contractility

Answer 39

With faster beats - less time for Ca to be resequestered | So Ca accululates with each beat