The molecular basis for myocardial contraction

Question 1

Myocardium: main components

Answer 1

• Contractile tissue,
• Connective tissue,
• Fibrous frame,
• Specialised conduction system

Question 2

What does the cardiac Myocyte do?

Answer 2

• The contractile proteins are activated by a signalling process called excitation-contraction coupling.
• Excitation-contraction coupling begins when the action potential depolarizes the cell and ends when ionized calcium (Ca2+) that appears within the cytosol binds to the Ca2+ receptor of the contractile apparatus.
• Movement of Ca2+ into the cytosol is a passive (downhill) process mediated by Ca2+ channels.
• The heart relaxes when ion exchangers and pumps transport Ca2+ uphill, out of the cytosol

Question 3

Myocardial metabolism

Answer 3

• Relies on free fatty acids during aerobic metabolism (efficient energy production)
• 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 4

A-band

Answer 4

the region of the sarcomere occupied by the thick filaments.

Question 5

I-band

Answer 5

is occupied only by thin filaments that extend toward the centre of the sarcomere from the Z-lines. It also contains tropomyosin and the troponins.

Question 6

Z lines

Answer 6

bisect each I-band

Question 7

The sarcomere

Answer 7

the functional unit of the contractile apparatus,
• The sarcomere is defined as the region between a pair of Z-lines,
• The sarcomere contains two half I-bands and one A-band

Question 8

sarcoplasmic reticulum

Answer 8

• The sarcoplasmic reticulum is a membrane network that surrounds the contractile proteins,
• The sarcoplasmic reticulum consists of the sarcotubular network at the centre of the sarcomere and the subsarcolemmal cisternae (which abut the T-tubules and the sarcolemma)

Question 9

transverse tubular system

Answer 9

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

Question 10

contraction

Answer 10

• Sliding of actin over myosin by ATP hydrolysis through the action of ATPase in the head of the myosin molecule.
• These heads form the crossbridges that interact with actin, after linkage between calcium and TnC, and deactivation of tropomyosin and TnI

Question 11

Myosin

Answer 11

• 2 heavy chains, also responsible for the dual heads.
• 4 light chains.
• The heads are perpendicular on the thick filament at rest, and bend towards the centre of the sarcomere during contraction (row.)
• Alpha myosin and beta myosin

Question 12

actin

Answer 12

• Globular protein.
• Double-stranded macromolecular helix (G).
• Both form the F actin
• Every 40 degrees there is an actin head

Question 13

Tropomyosin

Answer 13

• Elongated molecule, made of two helical peptide chains.
• It occupies each of the longitudinal grooves between the two actin strands.
• Regulates the interaction between the other three

Question 14

Troponin

Answer 14

• I: with tropomyosin inhibit actin and myosin interaction.
• T: binds troponin complex to tropomyosin.
• C: high affinity calcium binding sites, signalling contraction.
• The latter bond, drives Troponin I away from Actin, allowing its interaction with myosin

Question 15

control of the contractile cycle

Answer 15

• Calcium ions,
• Troponin phophorylation,
• Myosin ATPase