Myocardial mechanics

Question 1

What kind of signalling leads to myocardial contraction

Answer 1

Electrical

Question 2

What two structures lead to rapid transmission of action potentials in the myocardium

Answer 2

Intercalated discs and T-tubules

Question 3

When does calcium concentration rise during the myocardial potential

Answer 3

During the plateau phase - 100nm to 1-10um

Question 4

By what mechanism does calcium rise intracellularly

Answer 4

Calcium induced calcium release - Calcium intracellularly is able to activate calcium release from intracellular stores in the sarcoplasmic reticulum

Question 5

How does calcium interact with the troponin/tropomyosin complex with actin

Answer 5

1. Troponin I moves away from the actin/tropomyosin filament, allowing tropomyosin molecule to move away from the myosin binding site
2. Transmission through troponin T, tropomyosin is pushed away from the myosin binding site into the actin groove - myosin head is able to interact with actin to form cross bridge linkage

Question 6

What is muscle tension proportional to?

Answer 6

The number of crossbridges formed

Question 7

What is the number of crossbridges formed proportional to and why

Answer 7

Sarcomere length - If the sarcomere is too short the thin filaments overlap creating less tension
If they are too long then there are reduced areas for crossbridge formation between thick and thin filaments

Question 8

Define isometric contraction

Answer 8

Contraction where the length of the muscle is fixed but the amount of tension is changed due to contraction

Question 9

Define isotonic contration

Answer 9

The amount of tension on the muscle is fixed but upon contraction, the length of the muscle shortens - provided that the force of contraction is greater than the preload

Question 10

At what length of the sarcomere is the max tension produced and why

Answer 10

2.2um because the max number of crossbridge formation occurs at this length

Question 11

What produces the 2.2um sarcomere length in the heart

Answer 11

10-12mmHg filling pressure - pre-systole

Question 12

How is papillary muscle used to investigate force-velocity relationship

Answer 12

Tension transducer connected to papillary muscle
Papillary muscle initially stretched from pre load - represents sarcomere length after diastole
It’s then stimulated to lift an afterload - Represents the force in which ventricles act against to eject blood (arterial blood pressure)

Question 13

Why is Vmax a constant

Answer 13

Indicates the cardiac muscles contractility which stays the same - investigation into speed

Question 14

What happens to speed of contraction as the afterload is increased and what is the physiological reason behind this

Answer 14

Slows down - Higher afterload represents higher blood pressure in the arteries so the slower the heart is able to eject blood

Question 15

What happens to any given afterload if you increase the preload

Answer 15

There is an increased velocity - increasing preload increases max force

Question 16

what causes a change in contractility

Answer 16

When the heart changes the output per beat when the end diastolic volume is constant

Question 17

How does contractility increase

Answer 17

Due to increased calcium concentrations intracellularly causing more crossbridge formations per stimulus

Question 18

What is a chronotropic effect

Answer 18

A change in rate

Question 19

What is an inotropic effect

Answer 19

A change in contractility

Question 20

What effect does noradrenaline have on contraction

Answer 20

Positive inotropic and chronotropic effects (increased max force and max velocity of contraction)

Question 21

How does frequency of contraction also influence the force of contraction

Answer 21

An increase in frequency causes an increase in contractility - because the intracellular calcium builds up due to there not being enough time between beats for it to be removed to its pre original concentration.