1. Unit 5 Notes

2. To Review… Muscle surrounded by epimysium
Fascicle surrounded by perimysium
Individual muscle fibers (cells) surrounded by endomysium

4. Let’s zoom in on an individual muscle fiber…
Note: The sarcolemma (cell membrane) is surrounded by endomysium (connective tissue).

6. Let’s zoom in on a myofibril…

7. Definitions
Sarcolemma: the specific name for the plasma membrane of a muscle cell; surrounded by endomysium.

8. Definitions
Myofibril: contractile organelles found in the cytoplasm of muscle cells.

9. Definitions
Sarcomere: tiny contractile units that make up the myofibril; aligned end-to-end.

10. Definitions
Myofilaments: filaments composing the myofibrils; the two types are actin (thin) and myosin (thick).

11. Sarcoplasmic reticulum: specialized smooth ER that surrounds myofibrils; stores calcium and releases on demand

12. There are also multiple nuclei and mitochondria scattered throughout the muscle fiber, surrounding the myofibrils.

13. So, in summary, as you work your way smaller in a muscle….
Muscle – surrounded by epimysium
Fascicle – surrounded by perimysium
Muscle fiber (cell) – surrounded by endomysium/sarcolemma
Myofibril – sections called sarcomeres, surrounded by sarcoplasmic reticulum, mitochondria, and nuclei
Myofilaments – 2 types are actin & myosin

14. The light band is also called the I band, and has a dark midline interruption called the Z disc.
The Z disc marks the end of the sarcomere, and is formed by a disc-like membrane.

15. The dark band is also called the A band, and has a lighter central area called the H zone (which contains the M line).
The H zone lacks thin filament, so it looks a bit lighter than the rest of the A band.

16. Notice that the I and A bands are responsible for the striations of skeletal muscle tissue!
So, striations are formed by the locations of actin and myosin filaments.

17. Zooming in on a sarcomere…
The bands of myofilaments are actually formed by many myofilaments packed together.

19. THICK FILAMENTS…
Contain myosin protein
ATPase help split ATP to generate power for muscle contraction
Form the “A” band
Have projections that are myosin “heads” – called cross bridges
Cross bridges link thick and thin filaments together in contraction of muscle

21. THIN FILAMENTS…
Contain actin protein
Also contain tropomyosin protein filaments (which block specific binding sites) and troponin protein
Thin filaments anchored to the Z disc (a disc-like membrane separating the sarcomeres)
The zone where thin filaments do not exist in the sarcomere is called the H zone

24. What functional properties allow a muscle to perform its duties?
Irritability
Ability to receive and respond to a stimulus
Contractility
Ability to shorten when adequate stimulus is received

25. What functional properties allow a muscle to perform its duties?
Conductivity
Ability for impulse to travel along plasma membrane of muscle cell
Elasticity
Ability to recoil and resume original length

26. What role does the nervous system play in muscle movement?
Motor Unit – one neuron and all the skeletal muscle cells it stimulates

27. Within a motor unit… Muscle Fibers Axons
Axon Terminals (neuromuscular junctions)

30. Nerve endings and muscle fibers don’t physically touch…
Neuromuscular juction – where axon terminals match up with muscle fibers
Snyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle

31. What steps occur to stimulate muscle movement?
1. Nerve impulse reaches axon terminals
2. Chemical Neurotransmitter (ACh – acetylcholine) released
3. ACh diffuses across synaptic cleft and attaches to receptors

32. What steps occur to stimulate muscle movement?
4. ACh causes the sarcolemma to become temporarily permeable to Na+
5. Na+ rush into the muscle cell
6. Excess of positive ions creates electric current (action potential)
7. Muscle contracts (another whole set of steps!)

33. So, we know how muscle contraction is stimulated… but now we need to know the steps that help muscle contraction to happen!
Called the Sliding Filament Theory

34. The Sliding Filament Theory
Muscle fibers activated by nervous system due to action potential
Calcium ions (Ca+2) happen to be released
Do you remember which structure releases those calcium ions???

35. The Sliding Filament Theory
Muscle fibers activated by nervous system due to action potential
Calcium ions (Ca+2) happen to be released
Do you remember which structure releases those calcium ions???
THE SARCOPLASMIC RETICULUM

37. The Sliding Filament Theory
Release of Ca+2 allows troposin & tropomyosin to stop blocking binding sites on the actin… which allows the cross-bridges on the thick filaments to attach to the binding sites on the thin filaments
Let the sliding begin!

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39. The Sliding Filament Theory
Energized by energy from ATP, cross-bridges attach and detach from thin filaments
Works like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!)

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42. Net entry of Na+ Initiates an action potential which
ADP Pi
Net entry of Na+ Initiates
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
T tubule
Sarcolemma
SR tubules (cut)
Synaptic
cleft
vesicle
Axon terminal
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Calcium ions bind to troponin;
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Removal of Ca2+ by active transport
into the SR after the action
potential ends. SR
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+ 1 2 3 4 5 6

43. The Sliding Filament Theory
As this process is happening in every sarcomere throughout the muscle, the muscle itself is contracting!
The whole series of events (beginning with the nervous system signal) takes just a few thousandths of a second!!!

44. The Sliding Filament Theory
Notice in the contracted muscle, the H zone has disappeared
The I band has shortened significantly (all that’s left is the Z disc)
The A band (the dark striations!) have stayed the same thickness

45. Where’s all this energy coming from?
As a reminder, energy comes from ATP because of breaking a phosphate bond
Breaking a bond releases energy
When this energy is used by your body, it releases heat
Because ATP is the only energy source that can be used to move the cross-bridges back and forth (which contract the muscle), ATP must be regenerated continuously

46. ATP Regeneration – 3 Sources
Direct phosphorylation of ADP by creatine phosphate
When ATP used, changes to ADP
Creatine phosphate adds that missing phosphorous back on!
PROBLEM: only makes 1 ATP at a time… so not very much. And, only supplies energy for seconds of activity!
Your body will always do this, but it’s not very effective. Therefore, we have to have other ways of supplying energy…..

47. ATP Regeneration – 3 Sources
Aerobic respiration
Occurs in the mitochondria
Glucose broken down to pyruvic acid (releasing 2 ATP), and then into carbon dioxide and water (releasing 34 ATP)
36 ATP made for 1 glucose! A lot of energy! And, can supply energy for hours at a time!
PROBLEM: NEEDS OXYGEN
But what if you’re out of oxygen??? Then your muscles will begin……..

48. ATP Regeneration – 3 Sources
Anaerobic glycolysis and lactic acid formation
Glucose broken down to pyruvic acid, releasing 2 ATP
If oxygen present, process continues to the rest of aerobic respiration…
BUT… if oxygen is inadequate, or muscle activity is intense, pyruvic acid is instead changed to lactic acid
PROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!

49. ATP Regeneration 95% of ATP produced through aerobic respiration
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATP
If you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respiration
If they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!

50. Muscle Fatigue
A muscle is fatigued when it is unable to contract even though it is being stimulated – means you don’t have ATP to move the cross-bridges!
Lack of oxygen can cause…
Lactic acid buildup (anaerobic glycolysis)
ATP supply low (production can’t keep up with usage)
Muscle will contract less and less effectively, eventually stopping contraction completely

51. Muscle Fatigue
FYI: When you breathe heavy after physical activity, your muscles are trying to get enough oxygen for aerobic respiration to replace all of the ATP you used!

52. Muscle Contraction Isotonic contractions
Myofilaments slide, shortening the muscle
Movement occurs – bending knee, rotating arms, smiling

53. Muscle Contraction Isotonic contractions
Myofilaments slide, shortening the muscle
Movement occurs – bending knee, rotating arms, smiling
Isometric contractions
Myofilaments trying to slide, but can’t – just building up tension (crossbridges are “rowing”, but actin is not moving together)
Movement doesn’t occur – object too heavy to lift, push against wall

54. Muscle Tone
Muscle tone – sustained partial contraction of a muscle; muscle stays firm, healthy, and ready for action
Muscle inactivity can lead to muscle weakness and wasting (this is why Range of Motion exercises on bedridden people is important!)

55. Effect of Exercise on Muscles
Aerobic exercises include…
Running
Jogging
Biking
Elliptical

56. Effect of Exercise on Muscles
Increase endurance of muscles because muscle cells will form more mitochondria and store more oxygen (meaning more energy for the muscles)
Also – improves body metabolism, improve digestion, enhance coordination, strengthens skeleton, heart & lungs more efficient
Muscles do NOT increase in size!

57. Effect of Exercise on Muscles
Resistance exercises include…
Pushing against wall
Contracting muscles (likes gluteus maximus)
Lifting weights
Does increase muscle size!
Due to enlargement of individual muscle cells (makes more myofilaments)
You don’t add more muscle cells – you just bulk up the ones you already have!!!
NEED BOTH TYPES OF EXERCISES IN ANY TRAINING PROGRAM!