Muscle Physiology Flashcards
1
Q
Describe the structure of a skeletal muscle.
A
- Connected to two or more bones by tendons
- The tissue surrounding muscle (epimysium) and tendon connective tissue are continuous
- Perimysium extends into the muscle body, dividing muscle into bundles (fascicles) of muscle cells
- Fascicles contain 100s-1000s of muscle cells—muscle fibers, which extend the length of the muscle
2
Q
What are the components of a muscle fibre?
A
- Muscle fibers surrounded by connective tissue (endomysium)
- sarcolemma (plasma membrane)
- multinucleated
- sarcoplasm (cytoplasm)
- Contains many myofibrils
- Sarcoplasmic reticulum (Smooth ER)
- Contains many mitochondria—high energy
- Transverse tubules (T Tubules)
- Lateral sacs (terminal cisternae) (Ca2+)
3
Q
What is a triad?
A
T tubule + 2 lateral sacs
4
Q
What gives skeletal and cardiac muscle striated appearance?
A
- myofibrils
- due to thick and thin filaments that run parallel to the long axis
5
Q
What are the 2 types of filaments?
A
- actin
- myosin
6
Q
Filaments form ______.
A
sarcomeres
7
Q
Components of a sarcomere include:
A
- A band
- H zone
- M line
- I band
- Z line
8
Q
What is the A band?
A
- dark band
- thick filaments
9
Q
What is the H zone?
A
- thick filaments
- no overlap
10
Q
What is the M line?
A
links thick filaments
11
Q
What is the I band?
A
- light band
- thin filament
- no overlapping
12
Q
What is the Z line?
A
links thin filaments
13
Q
What is the sarcomere?
A
- functional unit
- Z line to Z line
14
Q
Name the components of a thin filament.
A
- actin
- tropomyosin
- troponin
15
Q
Describe actin.
A
- contractile protein
- each G actin has a binding site for myosin
16
Q
Describe tropomyosin.
A
- regulatory protein
- overlaps binding sites on actin for myosin
17
Q
Describe troponin.
A
- Regulatory protein
- Complex of three proteins
- Ca2+ binding to troponin regulates skeletal muscle contraction
18
Q
The complex of three proteins of troponin attach to what?
A
- Attaches to actin
- Attaches to tropomyosin
- Binds Ca2+ reversibly
19
Q
In the thick myofilament, myosin tail is towards …
A
the M line
20
Q
In the thick myofilament, myosin head is towards …
A
the I band
21
Q
In the thick myofilament, myosin head has 2 binding sites:
A
- actin binding site
- nucleotide binding site for ATP and ATPase
22
Q
Describe titin.
A
- Is a very elastic protein
- Supports protein in muscle
- Anchors thick filaments between the M line and the Z line
- Provides structural support and elasticity
23
Q
What is muscle contraction?
A
shortening of muscle
24
Q
Describe the sliding filament model.
A
- Thick and thin filaments overlap
- Neither thick nor thin filaments shorten
- Filaments slide past each other
25
What happens within a sarcomere during contraction?
- A band stays the same length
- I band shortens
- H zone shortens
- Sarcomere shortens
26
What is the cross bridge cycle?
Cyclical formation of links between actin and myosin, resulting in the sliding of thin filaments toward the M line of a sarcomere
27
_____ _____ undergoes conformation changes, swiveling back and forth.
Myosin head
28
What are the 2 energy forms of myosin during the cross bridge cycle?
- high-energy form
| - low-energy form
29
Describe high-energy form.
ADP and Pi bound to myosin
| High affinity for actin
30
Describe low-energy form.
ATP bound to myosin
| Low affinity for actin
31
The cross bridge cycle relies on...
hydrolysis of ATP
32
In a analogy of the cross bridge cycle to rowing a boat, the oar paddle is...
crossbridge
33
In a analogy of the cross bridge cycle to rowing a boat, oar contact with water is...
linking of thick filament to thin filament
34
In a analogy of the cross bridge cycle to rowing a boat, the power stroke (movement of oar propelling boat) is ...
myosin head moves, propelling thin filament toward center of muscle
35
In a analogy of the cross bridge cycle to rowing a boat, the oar breaking contact with water is...
thick and thin filaments detach
36
In a analogy of the cross bridge cycle to rowing a boat, oar moved to new position is...
Myosin head returns to the initial position, cycle starts again
37
What is excitation-contraction coupling?
Sequence of events whereby an action potential in the sarcolemma causes contraction
38
Excitation-contraction coupling is dependent on...
neural input from the motor neuron
39
Excitation-contraction coupling requires...
Ca2+ release from the sarcoplasmic reticulum
40
Describe the role of the neuromuscular junction in excitation-contraction coupling.
- Each motor neuron innervates several muscle cells
- Each muscle fiber receives input from a single motor neuron
- Similar to ordinary synapse
- Acetylcholine released
- Motor end plate
- End-plate potential
- Motor neuron AP always creates a muscle cell AP
41
Describe the motor end plate of the neuromuscular junction in excitation-contraction coupling.
- High density of acetylcholine receptors
| - Highly folded
42
What happens in if there is no Ca2+?
- troponin holds tropomyosin over myosin binding sites on actin
- No crossbridges form between actin and myosin
- Muscle relaxed
43
What happens if there is Ca2+ present?
- binds to troponin, causing movement of troponin, causing movement of tropomyosin, exposing binding sites for myosin on actin
- Crossbridges form between actin and myosin
- Cycle occurs; muscle contracts
44
What are the steps of excitation-contraction coupling?
1. Action potential in sarcolemma
2. Action potential down T tubules
3. DHP receptors of T tubules open Ca2+ channels (ryanodine receptors) in lateral sacs of SR
4. Ca2+ increases in cytosol
5. Ca2+ binds to troponin, shifting tropomyosin
6. Crossbridge cycling occurs
45
How does the termination of contraction occur?
Ca2+ must leave troponin, allowing tropomyosin to cover myosin binding sites on actin
46
How is Ca2+ removed from cytosol?
- Ca2+-ATPase in the sarcoplasmic reticulum
| - Transports Ca2+ from cytosol into the sarcoplasmic reticulum
47
ATP is required by _____ to generate work.
Muscle
48
Production of ATP is dependent on:
- intensity
| - duration
49
_____ drives cross bridge cycling.
ATP
50
Name 3 sources of ATP.
- Phosphorylation of ADP by creatine phosphate
- Oxidative phosphorylation of ADP in mitochondria
- Anaerobic glycolysis
51
What energy source is used most in the first second of light to moderate exercise?
creatine phosphate
52
What energy source is used most in seconds 1-2 of light to moderate exercise?
anaerobic glycolysis
53
What energy source is used most from 2 seconds on of light to moderate exercise?
oxidative phosphorylation
54
What is the role of the creatinine/creatine phosphate system?
- At rest, small store of ATP
| - Must quickly increase ATP synthesis
55
Creatine phosphate + ADP =
Creatine + ATP
56
How does the law of mass action play a role in the creatinine/creatine phosphate system?
- Use of ATP drives the reaction to the right
- Can supply up to 5 times the quantity of resting ATP
- "On tap"
57
How does muscle cell metabolism change with exercise intensity (light to moderate)?
- Oxidative phosphorylation
- Initially, glycogen stores supply glucose
- Up to 30 min, glucose and fatty acids in blood
- O2 supply must be kept adequate
- Transient increase in GLUT4
58
Why must O2 supplies be kept adequate during exercise?
Increases ventilation
Increases heart rate and contraction
Dilates vessels to muscle
59
Describe what happens with exercise of heavy intensity.
- Anaerobic glycolysis
- Lactate
- Only 2 ATP molecules per glucose molecule
60
What energy source is used most in the first second of intense exercise?
creatine phosphate
61
What energy source is used most in the first 3 seconds of intense exercise?
anaerobic glycolysis
62
What energy source picks up in seconds 3-4 on of intense exercise?
oxidative phosphorylation
63
How is skeletal muscle classified?
- velocity of contraction
| - energy source
64
Describe how skeletal muscles are classified based on velocity of contraction.
fast vs slow
65
Describe how skeletal muscles are classified based on energy source.
oxidative vs glycolytic
66
Skeletal muscle speed of contraction is dependent on..
- Dependent on myosin ATPase activity
- ATP hydrolysis
- Higher rate = faster crossbridge cycling
67
ATP hydrolysis =
rate-limiting step of cycle
68
Describe fast skeletal muscle fibres.
- Myosin with fast ATPase activity
| - fast contraction speed
69
Describe slow skeletal muscle fibres.
- Myosin with slow ATPase activity
| - long contraction duration
70
What are the properties of slow oxidative fibres (type I)?
- Low myosin ATPase
- High oxidative capacity—aerobic
- Small diameter
- Fatigue slowly
71
What does high oxidative capacity mean?
- Mitochondria
- Rich blood supply
- Myoglobin (red)
72
What does small diameter mean?
- Little tension
| - Small diffusion barrier
73
What are the properties of fast oxidative fibres?
- Intermediate myosin ATPase activity
- High oxidative capacity—aerobic
- Myoglobin
- Slow to fatigue, but more rapid than slow oxidative fibers
- Intermediate diameter
74
What are the properties of fast glycolytic fibres?
- High myosin ATPase activity
- High glycolytic capacity
- No myoglobin (so they appear white)
- Large diameter
- Fatigue rapidly
75
What does high glycolytic capacity mean?
- High glycogen stores
| - Many glycolytic enzymes
76
What does large diameter mean?
greater tension
77
The response of the three muscle fibre types to exercise depends on ______.
function
78
Postural muscles have higher counts of what type of muscle fibre?
type I
79
1 muscular unit has ...
the same fibre type
80
What is the recruitment order for muscle fibres?
1. Type I
2. Type IIa
3. Type IIx
81
What are the 2 types of fibres in muscle spindles?
- intrafusal fibres
| - extrafusal fibres
82
Describe intrafusal fibres.
- Contractile cells of the muscle spindle
- Adjust the sensitivity of the muscle to stretch
- Innervated by gamma motor units
83
Describe extrafusal fibres.
- Contractile cells of the muscle
- Responsible for skeletal muscle contraction
- Innervated by alpha motor neurons
84
Muscle spindles detect...
changes in muscle length
85
What are the 2 types of sensory endings in muscle spindles?
- annulospiral endings
| - flower-spray endings
86
Annulospiral endings are for...
type Ia afferent fibres
87
Flower-spray endings are for...
type II afferent fibres
88
Sensitivity of muscle spindles can be adjusted by...
action of intrafusal fibres
89
What are Golgi tendon organs (GTOs)?
- Sensory capsules within tendons
| - Tendon stretch activates the GTO
90
Describe how tendon stretch activates the GTO.
- Reflex inhibition of muscle via type Ib afferent neurons
| - Protection against overactivity of muscle
91
Describe skeletal muscle fibre resistance to fatigue for high-intensity exercises.
- Glycolytic fibers
- Buildup of lactate
- Strong contractions cause compression of blood vessels
- Neuromuscular fatigue → depletion of acetylcholine
- Altered enzyme activity
- Recovery within minutes to hours
92
Describe skeletal muscle fibre resistance to fatigue for low-intensity exercises.
- Depletion of energy reserves (glycogen)
| - Long time to recover
93
Name the long term responses of muscles to aerobic exercise.
- increased oxidative capacity
- some fast glycolytic fibres can be converted to fast oxidative fibres
- increase in size and number of mitochondria
- decrease in fiber diameter
- increase in number of capillaries surrounding muscle fibres
94
Name the long term responses of muscles to high-intensity exercise.
- decreased oxidative capacity
- some fast oxidative fibres can be converted to fast glycolytic fibres
- decrease in size and number of mitochondria
- increase in fibre diameter
- reduced resistance to fatigue
95
What is disuse atrophy?
- Decrease in size (lose myofibrils)
| - Muscular dystrophy, sedentary-ism
96
What is sarcopenia?
loss of muscle tissue as a natural part of the aging process
