Excitation-contraction coupling in cardiac muscle. Mechanism of cardiac muscle contraction

Question 1

step 1

Answer 1

• Action potentials in cardiac muscle
• Refractory periods
• Rhythmical excitation of the heart

Question 2

step 2

Answer 2

DEPOLARISATION OF SARCOLEMMA

transverse T tubules carry the AP from sarcolemma to inside cell

causing open of voltage gated sodium ion channels
- influx of na+
- making MP inside more positive

Question 3

step 3

Answer 3

calcium induced calcium release

voltage gated calcium L type channels open
= Ca2+ influx

RyR2 receptors = Ca2+ influx

more calcium needed for muscle contraction

Question 4

step 4a

Answer 4

calcium binds to troponin C on actin = conformational changes on troponin-tropomyosin complex

Question 5

step 4b

Answer 5

(thick) myosin binding site on actin is now exposed

Question 6

step 4c

Answer 6

myosin head binds to exposed myosin binding site on actin = sliding filament = cross bridge formation

Question 7

step 4d

Answer 7

ATP hydrolysed releasing ADP + Pi which pulls actin towards centre = muscle shortens = power stroke = muscle contraction

Question 8

step 5

Answer 8

RELAXATION

• calcium taken back to SR by Ca2+/ATPase pump
• Na+/Ca2+ exchanger pumps calcium out of plasmamembrane
• tropomyosin to go back to myosin binding site on actin
• myocyte now relaxed

Question 9

step 6

Answer 9

repolarisation

voltage gated sodium ion channel close

voltage gated potassium ion channel open = RMP restored, next AP ready