Molecular mechanics of cardiac contraction

Question 1

What are the four main components of the myocardium?

Answer 1

Contractile tissue
Connective tissue
Fibrous frame
Specialised conduction system

Question 2

What is the pumping action of the myocardium dependent on? (function)

Answer 2

Interaction between contractile proteins and muscular wall

Question 3

What energy is required for mechanical work in the heart? (function)

Answer 3

ATP derived energy required for the mechanical work that move blood under pressure from great veins into pulmonary artery and from pulmonary veins into aorta

Question 4

How are contractile proteins activated?

Answer 4

By a signalling process called excitation (electrical)-contraction (mechanical) coupling

Question 5

How does excitation-contraction coupling work?

Answer 5

• It begins when the action potential depolarises the cell membrane
• As cell membrane depolarises, influx of Ca2+ into cytosol (passive)
• It binds to the Ca2+ receptor of the contractile apparatus
• When cell has enough ATP energy to push Calcium back into cytoplasm this leads to heart relaxing

Question 6

What myofibrils are in the myocardial cell?

Answer 6

Filled with cross-striated myofibrils

Question 7

What does the plasma membrane of the myocardial cell do? (3)

Answer 7

• Plasma membrane regulates excitation-contraction coupling and relaxation
• Plasma membrane produces part of the t tubule
• Separates cytosol from extra-cellular space

Question 8

How does the myocardium have energy during aerobic metabolism?

Answer 8

Through free fatty acids (FFA) - which is an efficient energy production

Question 9

How does the myocardium have energy during anaerobic metabolism?

Answer 9

• During hypoxia, there is no FFA metabolism, thus anaerobic metabolism ensues
• This relied on metabolising glucose (anaerobically) producing energy sufficient to maintain the survival of the affected muscle without contraction

Question 10

What is a sarcomere?

Answer 10

basic contractile unit of muscle fibre

Question 11

How are contractile proteins arranged in the sarcomere?

Answer 11

In a regular array of thick and thin filaments (myofibrils)

Question 12

What is contained in the A-Band?

Answer 12

The region of the sarcomere occupied by the thick filaments (myosin filament).
(there is also an overlap between thick and thin filament)
- Large area of sarcomere is A-Band

Question 13

What is contained in the I-Band?

Answer 13

• Contain thin filament (actin filament) that extend towards centre and binds to Z disc
• Also contain tropomyosin and troponin

Question 14

What is titin responsible for?

Answer 14

Responsible for the elasticity of muscle. Allows muscles to recoil to its normal form. Prevents muscle fibre from being overstretched.

Question 15

What is the sarcoplasmic reticulum?

Answer 15

A membrane network that surrounds the contractile protein

Question 16

What does the sarcoplasmic reticulum consist of?

Answer 16

• the sarcotubular network at the centre of the sarcomere
• the subsarcolemmal cisternae (which are next to the T-tubules and the sarcolemma).

Question 17

What are T-tubules?

Answer 17

Transverse tubules: extensions of cell membrane that penetrates into the centre of skeletal and cardiac muscle cells.

Question 18

What are transverse tubules lined by?

Answer 18

By a membrane that is continuous with the sarcolemma (plasma membrane), so that the lumen of the T-tubules carries the extracellular space toward the centre of the myocardial cell.

Question 19

How does the sarcoplasmic reticulum membrane help with contraction?

Answer 19

• They h ave ATPase on the membrane
• The hydrolysis of ATP into ADP and P allows conformation change of protein and lets Ca++ inside the organelle
• Upon activation there is a release of the stored Ca++ which bind to the troponin and inducing the actin-myosin complex- contractile proteins

Question 20

How many heavy and light chains does the myosin (thick) filament have?

Answer 20

• 2 heavy chains
• 4 light chains
• 2 globular heads on myosin end
• Heads are perpendicular at rest
• Globular heads present 40 degrees from each other to maximise chance of connecting with the actin during contraction

Question 21

What are some features of the actin (thin) filament?

Answer 21

• Globular protein
• Double stranded macromolecular helix
• Both form the F actin
• Actin has a myosin binding site which is partially covered by tropomyosin and held in place by troponin (clamp)

Question 22

What is tropomyosin?

Answer 22

Tropomyosin is a wire like structure that occupies the longitudinal grooves between the 2 actin strands.
- Made of 2 helical peptide chains

Question 23

What are the 3 subunits of troponin?

Answer 23

TnI- inhibitory: inhibits actin and myosin interaction
TnT- tropomyosin binding. Troponin binding to tropomyosin
TnC- Calcium binding- high affinity calcium binding sites which signal contraction and drives TnI from the myosin binding site allowing interaction between the actin and myosin.

Question 24

What does troponin do?

Answer 24

Regulates Ca2+ for contraction

Question 25

Describe contraction in terms of sarcomere and troponin

Answer 25

• Action potential arrives along the sarcolemma and passes down the t tubule and depolarises the membrane.
• Leads to release of Ca++ from sarcoplasmic reticulum into the cytosol
• Ca++ binds to TnC which pulls the TnI away from the groove along with the tropomyosin
• This allows for the globular head of the myosin to interact with the groove of the thin-actin filament. Crossbridge formation.
• ATP is required for the contraction and ATP hydrolysis (ATPase on myosin) breaks apart the the Ca++-TnC bond and the groove of the actin is partially blocked by the tropomyosin and TnI subunits.
• The power stroke action by the myosin leads to the sliding action of actin along myosin, thus shortening of the sarcomere and causes contraction.
• The crossbridge formation and release of Ca++ from TnC requires ATP hydrolysis.
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