Muscle contraction

1. Muscle Contraction
Prof. Vajira Weerasinghe
Dept of Physiology
Faculty of Medicine
University of Peradeniya

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

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

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)

10. Ca++Ca++

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. •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
Troponin
Actin
Myosin
Tropomyosin
ATP

16. •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. Ca
2+
Troponin
Actin
Myosin
Tropomyosin
Ca
2+
Myosin
binding
sites
Ca
2+
Ca
2+
Detachment
Sliding
Binding

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

23. 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

24. •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

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

26. 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

28. •Dystrophin deficiency causes rare muscle
diseases known as muscular dystrophies

29. Muscle twitch

30. 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

31. 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

32. Summation and tetanus

33. 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

34. 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

35. 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

36. 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

37. Staircase phenomenon (treppe)

38. Staircase effect, summation and
tetanus

39. Tetanus

40. 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)

41. Isotonic contraction

42. 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)

43. Isometric contraction

44. Isometric contraction
•Produces no movement
•Used in
–Standing
–Sitting
–Postural control