1. MUSCLE TISSUE

2. Joint: a location where two or more bones articulate

3. Skeletal Muscles

4. Structure of Skeletal Muscle
Connective tissue components
Skeletal muscles are surrounded by a fibrous epimysium
Connective tissue called perimysium subdivides the muscle into fascicles
Each fascicle is subdivided into muscle fibers surrounded by endomysium

5. Muscle Fiber Structure
Have many of the organelles found in other cells
Have plasma membranes called sarcolemma
Are multinucleated; form a syncytium
Are striated
I bands: light bands
A bands: dark bands
Z-lines (discs): dark lines in the middle of the I bands

6. Skeletal Muscle Fibers
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Nuclei
Muscle fiber
© Ed Reschke

7. Muscles and Bones
Generally, both ends of a muscle are attached to bone by tough tendons
When a muscle contracts, it shortens.
This places tension on tendons connecting it to a bone.
This moves the bone at a joint.
The bone that moves is attached at the muscle insertion.
The bone that does not move is attached at the muscle origin.
Movement is toward the insertion

8. Skeletal Muscle Actions

9. Mechanisms of Contraction

10. Neuromuscular Junction

11. Motor Unit
A motor unit is a single motor neuron and all the muscle fibers it innervates; all the muscle fibers in a motor unit contract at once
Graded contractions – varied contraction strength due to different numbers of motor units being stimulated
Neuromuscular junction: site where a motor neuron stimulates a muscle fiber
Motor end plate: area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter, acetylcholine

12. Motor Unit Somatic motor neuron Motor unit Spinal cord Motor unit
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Somatic motor neuron
Motor unit
Spinal cord
Motor unit
Somatic motor neuron
(a)
Motor unit
Neuromuscular
junctions
Somatic
motor axon
Skeletal
muscle fibers
(b)

13. So how does this contraction work?

14. Muscle Fiber Binding
Each fiber has densely packed subunits called myofibrils that run the length of the muscle fiber
Stacked in register so that the dark and light bands align
Composed of thick and thin myofilaments

15. Z line: partitions the sarcomeres
A Band: dark band where thick and thin filaments overlap
I band: composed of thin filaments – these shorten during contraction
H zone: center of A band where only thick filaments are present

17. Sarcomere Unit of contraction From one Z line to the next Z line
Contains overlapping of thick and thin filaments

18. Arrangement of Thick and Thin Filaments
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Sarcomere
A band
I band
H band
I band
Thin
filament
Thick
filament
(a)
Z disc
Z disc
Myofibril
Sarcomere
Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, W.H. Freeman, 1979

19. Arrangement of Thick and Thin Filaments
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Z disc
Z disc
(b) M SR
Myofibril M
Copyright by Dr. R.G. Kessel and R.G. Kardon, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, W.H. Freeman, 1979

20. Cross Bridges More about myofilaments
Thick: composed of the protein myosin
Each protein has two globular heads with actin-binding sites and ATP-binding sites.
Thin: composed of the protein actin
Have proteins called tropomyosin and troponin that prevent myosin binding at rest.

21. Action of Sliding
Sliding is produced by several cross bridges that form between myosin and actin.
The myosin head serves as a myosin ATPase enzyme, splitting ATP into ADP + Pi.
This allows the head to bind to actin when the muscle is stimulated.

22. Cont’d
Release of Pi upon binding cocks the myosin head, producing a power stroke that pulls the thin filament toward the center.
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Actin 1
ADP Pi 2 Pi
Power
stroke

23. Cont’d After the power stroke, ADP is released and a new ATP binds.
This makes myosin release actin.
ATP is split.
The myosin head straightens out and rebinds to actin farther back.
Continues until the sarcomere has shortened

24. Activation of the Myosin Head
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Troponin
Actin
Tropomyosin
Myosin binding site
Thin
filament
Actin-binding
sites
ATP-binding
site
ADP 2 1 Pi
Myosin head
ATP
Myosin tail
Thick
filament

25. Cross Bridges 1 Resting fiber; cross bridge is not attached to actin 6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1
Resting fiber; cross bridge is
not attached to actin 6
ATP is hydrolyzed and
phosphate binds to
myosin, causing cross
bridge to return to its
original orientation
Thin
filament
ADP Pi
Myosin head
Cross bridge
Thick filament 2
Cross bridge
binds to actin
ATP 5
A new ATP binds to
myosin head, allowing
it to release from actin 3
Pi is released from myosin
head, causing conformational
change in myosin 4
Power stroke causes
filaments to slide; ADP
is released

27. So what do we need for skeletal muscle contraction?

28. Role of Calcium
When muscle cells are stimulated, Ca2+ is released inside the muscle fiber.
Some attaches to troponin C, causing a conformational change in troponin and tropomyosin.
Myosin is allowed access to form cross bridges with actin.

29. Regulation of Contraction
F-actin is made of G-actin subunits, arranged in a double row and twisted to form a helix
Tropomyosin physically blocks cross bridges

30. Role of Calcium Actin Tropomyosin Relaxed muscle: ADP tropomyosin
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Actin
Tropomyosin
Relaxed
muscle:
tropomyosin
blocks the
binding site
ADP
Troponin Pi
Binding
site
Cross bridge
Myosin
Ca2+
Contracting
muscle:
myosin
head binds
to actin
Ca2+
ADP
Ca2+ Pi

31. Excitation-Coupling Contraction
Sarcoplasmic reticulum (SR)
SR is modified endoplasmic reticulum that stores Ca2+ when muscle is at rest.
Most is stored in terminal cisternae.
When a muscle fiber is stimulated, Ca2+ diffuses out of calcium release channels (ryanodine receptors).
At the end of a contraction, Ca2+ is actively pumped back into the SR.

32. Sarcoplasmic Reticulum

33. Stimulating a Muscle Fiber
Acetylcholine is released from the motor neuron
End plate potentials are produced
Action potentials are generated (All-or-none event)
Voltage-gated calcium channels in transverse tubules change shape and cause calcium channels in SR to open
Calcium is released and can bind to troponin C

34. Stimulating a Muscle Fiber
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Axon terminal
Sarcolemma 2 1
Ca2+ 3
Transverse
tubule 4
Sarcoplasmic
reticulum 1
Nicotinic acetylcholine receptor 3
Transverse tubule voltage-gated
calcium channel 2
Skeletal muscle voltage-gated
sodium channel 4
Sarcoplasmic reticulum calcium
release channel

35. Muscle Relaxation Action potentials cease
Calcium release channels close
Ca2+-ATPase pumps move Ca2+ back into SR (active transport)
No more Ca2+ is available to bind to troponin C
Tropomyosin moves to block the myosin heads from binding to actin

36. Energy Requirements of Skeletal Muscles

37. Where do muscles get energy?
At rest and for mild exercise: from the aerobic respiration of fatty acids
For moderate exercise: from glycogen stores
For heavy exercise: from blood glucose
As exercise intensity and duration increase, GLUT4 channels are inserted into the sarcolemma to allow more glucose into cells.

38. Aerobic Respiration Provides 95% of the ATP
Mitochondria absorbs oxygen, ADP, phosphate ions from cytoplasm
Molecules in the Krebs Cycle  oxidation of acetly coA into C02 and ATP

39. But what happens if you need a quick burst of energy?

40. Metabolism of Skeletal Muscle
Anaerobic for the first seconds of moderate to heavy exercise
Does not require oxygen
Breaks down glycogen reserves
Allows time to increase oxygen supply

41. So we can function without oxygen?
Yes, and no

42. Oxygen Debt
When a person exercises, oxygen is withdrawn from reserves in hemoglobin and myoglobin.
To create cross bridges in muscle contraction and pump calcium back into SR at rest
To metabolize lactic acid
Breathing rate continues to be elevated after exercise to repay this debt.

43. Labeling