1. Muscle Contraction

2. Objectives
Describe the process of excitation and contraction coupling and muscle relaxation
Practical
In a muscle tracing, identify the following phenomena, muscle twitch, summation, tetanus, staircase phenomenon, muscle fatigue, effect of temperature on muscle contraction

4. Skeletal Muscle Fiber Structure
Pg. 401
Figure 12-3b: ANATOMY SUMMARY: Skeletal Muscle

5. Muscle Fiber Structure
Pg. 402
Figure 12-4: T-tubules and the sarcoplasmic reticulum

6. Muscle contraction
Depolarisation of the muscle membrane spreads through the muscle
Causes muscle contraction (mechanical event)

7. Muscle contraction Excitation - contraction coupling
Excitation : electrical event
Contraction : mechanical event

8. Important structural details
sarcolemma
conduct AP over the surface of the muscle fibre
t tubules
Invagination of sarcolemmal membrane
conduct AP deep into the muscle fibre

9. Important structural details
sarcoplasmic reticulum (SR)
ends dilated as terminal cisternae
contains abundance of Ca++ ions bound to calsequestrin
Release Ca++ in response to AP in t tubules
Remove Ca++ back in to SR (resequester)

11. AP spreads through t tubule into the muscle tissue
Close to the sarcoplasmic reticulum DHP receptor (dihydropyridine receptor) senses the membrane depolarization
alters its conformation
activates the ryanodine receptor (RyR)
that releases Ca2+ from the SR
Ca flows to the myoplasm in the vicinity of actin & myosin

13. THIN FILAMENT (Actin)
THICK FILAMENT (Myosin)

14. Troponin
Actin
Myosin
Tropomyosin
THIN FILAMENT
THICK FILAMENT

15. Troponin
Actin
Myosin
Tropomyosin
ATP
Actin
Composed of 3 different proteins: actin, tropomyosin & troponin
Actin has myosin binding sites
They are normally covered by tropomyosin
Troponin contains Ca++ binding sites
Myosin
Myosin contains protein chains with bent heads which forms the cross bridges
Myosin head contain ATPase. An ATP molecule is attached to it. ATP is broken down during sliding

16. Troponin shifts tropomyosin
Ca++ binds to troponin
Troponin shifts tropomyosin
Myosin binding sites in actin filament uncovered
Myosin head binds with actin
Cross bridges form
Filaments slide with ATP being broken down
Muscle shortens
New ATP occupies myosin head
Myosin head detaches
Filaments slide back
Cycling continues as long as Ca is available
Troponin
Actin
Myosin
Tropomyosin
ATP

17. Ca2+ Binding Ca2+ Detachment Sliding Ca2+ Ca2+ Actin Troponin
Myosin
Tropomyosin
Ca2+
Binding
Ca2+
Myosin
binding
sites
Detachment
Sliding
Ca2+
Ca2+

19. Actin Myosin Ca++ binds to troponin Tropomyosin exposes actin Ca++ ATP
myosin head binds to actin
& cross bridge forms
Filaments slide
ATP is broken down
New ATP comes, Ca is removed, ready to detach

21. ATP as the source of energy for contraction
the heads of the cross-bridges bind with ATP
ATPase activity of the myosin head immediately cleaves the ATP
Ca++binds with troponin-tropomyosincomplex, active sites on the actin filament are uncovered
bond between head of the cross-bridge & the active site of the actin filament

22. once the head of the cross-bridge tilts, this allows release of the ADP & phosphate ion
a new molecule of ATP binds
binding of new ATP causes detachment of the head from the actin
the new molecule of ATP is cleaved to begin the next cycle

23. The process by which depolarization of the muscle fiber initiates contraction is called excitation-contraction coupling.

24. Walk-along theory of contraction
Heads of myosin filament is known to forms cross bridges by attachment
Then head bends causing actin filament to slide
Then head detaches from the actin filament and walked to a new site in actin filament and attaches again
This process continue to happen
As if myosin head walk-along actin filament

25. Relaxation
This occurs when Ca++ is removed from myoplasm by Ca++ pump located in the sarcoplasmic reticulum
When Ca++ conc is decreased
Troponin returns to original state
Trpomyosin covers myosin binding sites
Cross-bridge cycling stops

26. Timing of Electrical & Mechanical Events
Myogram of Single Muscle Twitch

27. Dystrophin
Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane
It provides an anchoring function to the muscle proteins

29. Dystrophin deficiency causes rare muscle diseases known as muscular dystrophies

30. Muscle twitch

31. Muscle twitch
A single action potential causes a brief contraction followed by relaxation
This response is called a muscle twitch.
The twitch starts about 2 ms after the start of depolarization of the membrane, before repolarization is complete

32. Muscle twitch
Duration of the twitch varies with the type of muscle being tested
Fast muscle fibers, primarily those concerned with fine, rapid, precise movement, have twitch durations as short as 7.5 ms
Slow muscle fibers, principally those involved in strong, gross, sustained movements have twitch durations up to 100 ms
The strength of twitch depends on the number of motor units activated

33. Summation and tetanus

34. Summation
If 2 stimuli are delivered in rapid succession the second twitch will be greater than the first
This only occurs if repolarization is not complete
The tension developed during summation is considerably greater than that during the single muscle twitch

35. Tetanus
With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation has occurred
Individual responses fuse into one continuous contraction
Such a response is called a tetanus or tetanic contraction

36. Tetanus
It is a complete tetanus when there is no relaxation between stimuli and an incomplete tetanus when there are periods of incomplete relaxation between the summated stimuli
During a complete tetanus, the tension developed is about four times that developed by the individual twitch contractions

37. Staircase phenomenon (treppe)
When a series of stimuli is delivered to skeletal muscle, there is an increase in the tension developed during each twitch until, after several contractions, a uniform tension per contraction is reached
This phenomenon is known as treppe, or the "staircase" phenomenon
This is the basis of “warm up”
Treppe is believed to be due to increased availability of Ca2+ for binding to troponin C, accumulation of heat or effect of pH
It also occurs in cardiac muscle although cardiac muscles cannot be tetanised
It should not be confused with summation of contractions and tetanus

38. Staircase phenomenon (treppe)

39. Staircase effect, summation and tetanus

40. Tetanus

41. Isotonic contraction Produces movement
Most of the time movement is of this type
Used in
Walking
Running
Movement of a part of the body (eg. Hand movement)

42. Isotonic contraction

43. Isometric contraction
Muscular contraction involves shortening of the contractile elements, but because muscles have elastic and viscous elements in series with the contractile mechanism, it is possible for contraction to occur without an appreciable decrease in the length of the whole muscle
Such a contraction is called isometric ("same measure" or length)

44. Isometric contraction

45. Isometric contraction
Produces no movement
Used in
Standing
Sitting
Postural control