1. Excitation Contraction Coupling
Dr. Atif Mahmood
May 14, 2010

2. EXCITATION- CONTRACTION COUPLING
(SEQUENCE OF EVENTS)

3. EXCITATION- CONTRACTION COUPLING

4. EXCITATION- CONTRACTION COUPLING (cont’d)

5. EXCITATION- CONTRACTION COUPLING (cont’d)

6. Muscle Contraction and Relaxation
Four actions involved in this process
Excitation = nerve action potentials lead to action potentials in muscle fiber
Excitation-contraction coupling = action potentials on the sarcolemma activate myofilaments
Contraction = shortening of muscle fiber
Relaxation = return to resting length
Images will be used to demonstrate the steps of each of these actions

7. Nerve Activation of Individual Muscle Cells (cont.)

8. Excitation/contraction coupling
Action potential along T-tubule causes release of calcium from cisternae of TRIAD
Cross-bridge cycle

9. Begin cycle with myosin already bound to actin

10. 1. Myosin heads form cross bridges
Myosin head is tightly bound to actin in rigor state
Nothing bound to nucleotide binding site

11. 2. ATP binds to myosin
Myosin changes conformation, releases actin

12. 3. ATP hydrolysis ATP is broken down into:
ADP + Pi (inorganic phosphate)
Both ADP and Pi remain bound to myosin

13. 4. Myosin head changes conformation
Myosin head rotates and binds to new actin molecule
Myosin is in high energy configuration

14. 5. Power stroke
Release of Pi from myosin releases head from high energy state
Head pushes on actin filament and causes sliding
Myosin head splits ATP and bends toward H zone. This is Power stroke.

15. 6. Release of ADP
Myosin head is again tightly bound to actin in rigor state
Ready to repeat cycle

16. THE CROSS-BRIDGE CYCLE Relaxed state
Crossbridge energised
Crossbridge attachment
A + M l ADP l Pi
Ca2+ present
A – M l ATP
AlMlADPlPi
Crossbridge detachment
Tension develops
ADP + Pi
ATP
AlM
A, Actin; M, Myosin

17. Cross Bridge Cycle

18. Rigor mortis
Myosin cannot release actin until a new ATP molecule binds
Run out of ATP at death, cross-bridges never release

19. Need steady supply of ATP!
Many contractile cycles occur asynchronously during a single muscle contraction
Need steady supply of ATP!

20. Regulation of Contraction
Tropomyosin blocks myosin binding in absence of Ca2+
Low intracellular Ca2+ when muscle is relaxed

21. Ca+2 binds to troponin during contraction
Troponin-Ca+2 pulls tropomyosin, unblocking myosin-binding sites
Myosin-actin cross-bridge cycle can now occur

22. How does Ca2+ get into cell?
Action potential releases intracellular Ca2+ from sarcoplasmic reticulum (SR)
SR is modified endoplasmic reticulum
Membrane contains Ca2+ pumps to actively transport Ca2+ into SR
Maintains high Ca2+ in SR, low Ca2+ in cytoplasm

23. The action potential triggers contraction
How does the AP trigger contraction?
This question has the beginning (AP) and the end (contraction) but it misses lots of things in the middle!
We should ask:
how does the AP cause release of Ca from the SR, so leading to an increase in [Ca]i?
how does an increase in [Ca]i cause contraction?

24. Structures involved in EC coupling
A band
(myosin)
I band
(actin)
Z disc
Contractile proteins in striated muscle are organised into sarcomeres
T-tubules and sarcoplasmic reticulum are organised so that Ca release is directed toward the regulatory (Ca binding) proteins
The association of a t-tubule with SR on either side is often called a ‘triad’ (tri meaning three)
Z disc
M line
Z disc
sarcoplasmic
reticulum
t-tubules
Triad
junctional feet

25. Structures involved in EC coupling - Skeletal Muscle -
T-tubule
sarcolemma
out in
sarcoplasmic
reticulum
voltage sensor?
junction foot

26. Ca2+ Controls Contraction
Ca2+ Channels and Pumps
Release of Ca2+ from the SR triggers contraction
Reuptake of Ca2+ into SR relaxes muscle
So how is calcium released in response to nerve impulses?
Answer has come from studies of antagonist molecules that block Ca2+ channel activity 19

28. Dihydropyridine Receptor
In t-tubules of heart and skeletal muscle
Nifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubules
In heart, DHP receptor is a voltage-gated Ca2+ channel
In skeletal muscle, DHP receptor is apparently a voltage-sensing protein and probably undergoes voltage-dependent conformational changes 20

29. The "foot structure" in terminal cisternae of SR
Ryanodine Receptor
The "foot structure" in terminal cisternae of SR
Foot structure is a Ca2+ channel of unusual design
Conformation change or Ca2+ -channel activity of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels
Many details are yet to be elucidated! 21

30. Skeletal muscle The AP: moves down the t-tubule
voltage change detected by DHP （双氢吡啶） receptors
DHP receptor is essentially a voltage-gated Ca channel
is communicated to the ryanodine receptor which opens to allow Ca out of SR
activates contraction
out in
voltage sensor
(DHP receptor)
junctional foot
(ryanodine receptor)
sarcoplasmic
reticulum
sarcolemma
T-tubule

31. Cardiac muscle The AP: moves down the t-tubule
voltage change detected by DHP receptors (Ca channels) which opens to allow small amount of (trigger) Ca into the fibre
Ca binds to ryanodine receptors which open to release a large amount of (activator) Ca (CACR)
Thus, calcium, not voltage, appears to trigger Ca release in Cardiac muscle!
out in
voltage sensor
& Ca channel
(DHP receptor)
junctional foot
(ryanodine receptor)
sarcoplasmic
reticulum
sarcolemma
T-tubule

32. The Answers! Skeletal Cardiac
The trigger for SR release appears to be calcium (Calcium Activated Calcium Release - CACR)
The t-tubule membrane has a Ca2+ channel (DHP receptor)
The ryanodine receptor is the SR Ca release channel
The ryanodine receptor is Ca- gated & Ca release is proportional to Ca2+ entry
Skeletal
The trigger for SR release appears to be voltage (Voltage Activated Calcium Release- VACR)
The t-tubule membrane has a voltage sensor (DHP receptor)
The ryanodine receptor is the SR Ca release channel
Ca2+ release is proportional to membrane voltage

33. Transverse tubules connect plasma membrane of muscle cell to SR

34. Ca2+ release during Excitation-Contraction coupling
Action potential on motor endplate travels down T tubules
Ryanodyne R
Ca-release ch.

35. Voltage -gated Ca2+ channels open, Ca2+ flows out SR into cytoplasm
Ca2+ channels close when action potential ends. Active transport pumps continually return Ca2+ to SR
Ca ATPase
(SERCA)

36. Excitation-Contraction Coupling
Depolarization of motor end plate (excitation) is coupled to muscular contraction
Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca2+ from SR
Ca2+ binds to troponin and causes position change in tropomyosin, exposing active sites on actin
Permits strong binding state between actin and myosin and contraction occurs
ATP is hydrolyzed and energy goes to myosin head which releases from actin

37. Summary: Excitation-Contraction Coupling

38. Sliding Filament Model I:
Actin myofilaments sliding over myosin to shorten sarcomeres
Actin and myosin do not change length
Shortening sarcomeres responsible for skeletal muscle contraction
During relaxation, sarcomeres lengthen

39. Sliding filament model II:

40. Sarcomere Shortening

41. Sliding Filament Theory for Muscle Contraction
Muscle contraction occurs when actin and myosin, the major proteins of the thin and thick filaments, respectively, slide past each other in an ATP-driven enzymatic reaction.
Huxley AF and Niedergerke R, Nature 173, (1954)
Structural changes in muscle during contraction; interference
Microscopy of living muscle fibers.
Huxley HE and Hanson J, Nature 173, (1954) Changes in cross-strations of muscle during contraction and stretch and their structural interpretation.

43. Structure of Actin and Myosin

45. Myosin structure:

46. Thick filament structure:

47. Structure of the M-line:

49. Structure of thin filament:

50. Cross-bridge formation:

54. Sliding Filament Theory for Muscle Contraction
Coupling of chemical reactions with vectorial motion.
ATP (Crossbridge) ADP + Pi + Energy
Cross-bridge hypothesis of muscle contraction:
Sliding of thin and thick filaments is caused by the cross-bridges that extend from the myosin filament, attach to actin, pull the thin filaments toward the center of the sarcomere and detach. This cyclic interaction is coupled with the hydrolysis of ATP.

55. Initiation of Contraction, Ca2+ release:

58. The micrograph shows myosin bound to actin
EM evidence for sliding filament theory
The micrograph shows myosin bound to actin
Contraction of one sarcomere

59. Mechanism of muscle contraction