2-5. Skeletal Muscle Structure & Function Flashcards
1
Q
What is the hallmark of all living organisms?
A
Ability to use energy
2
Q
What is ATP? Where is it found? In what amounts?
A
- Energy currency found in all cells
- In small amounts ranging from 3-6 mmol/kg in skeletal muscle
3
Q
How many skeletal muscles are there in the body?
A
660
4
Q
What percentage of body weight does skeletal muscle make up?
A
45%
5
Q
What is the primary function of skeletal muscle?
A
Force generation
6
Q
What are the secondary functions of skeletal muscle?
A
- Heat production
- Glucose uptake and storage
- Regulation of metabolic rate
- Source of fuel (amino acids, esp in starvation)
7
Q
How does skeletal muscle aid in heat production?
A
- Body is inefficient at converting substrates into energy
- Vast majority is lost as heat
- Muscles help maintain body temp
8
Q
What happens when skeletal muscles don’t utilize glucose?
A
- Muscle is a primary utilizer of glucose
- When sedentary & not utilizing glucose, peeps get glucose regulatory conditions like diabetes
9
Q
What is a muscle composed of? Percentages?
A
- 75% water
- 20% protein
- 5% other
10
Q
List and describe the cellular components of muscle?
A
- Contractile tissue = myosin, actin
- Connective tissue = tendons, fascia
- Vascular tissue
- Neural tissue = motor innervation & sensory receptors
11
Q
What are the sensory receptors of muscle?
A
- Muscle spindle
- Golgi tendon organs
- Mechanoreceptors
- Nociceptors
12
Q
What are the cellular organelles contained in each myocyte?
A
- Multiple myonuclei
- Mitochondria
- Ribosomes, Golgi Apparatus, Endoplasmic Reticulum
13
Q
Why are nuclei generally lie on the external/peripheral surface of a muscle cell?
A
To avoid “machinery” pumping in the middle of muscle cells during contraction
14
Q
What are the unique features of skeletal muscle?
A
- Multinucleated
- Capable of regeneration
- Very adaptable
15
Q
Why are muscle cells multimucleated?
A
Diffusional efficiency
16
Q
How many nuclei per cubic mm are there in each muscle cell?
A
200-300
17
Q
How are muscle fibers adaptable?
A
- Can modify their metabolic properties in response to alterations in use
- Can alter their size throughout life in response to various stressors
18
Q
What muscle cells are capable of regeneration?
A
Satellite cells
19
Q
What are the 3 types of connective tissue sheathes of a muscle cell? What does each cover?
A
- Epimysium covers the whole muscle
- Perimysium covers the fascicles
- Endomysium covers the individual muscle fibers
20
Q
What are the 2 membranes that surround each muscle fiber? Which is semi-permeable?
A
- Sarcolemma = plasma membrane, semi-permeable lipid bilayer (inner)
- Basal lamina = basement membrane (outer)
21
Q
What is the cross-sectional area of a muscle fiber determined by? What is the range?
A
- Number of myofibrils
- 2000 - 7500
22
Q
What is the range of muscle fiber length?
A
3-4 mm to 30cm
23
Q
What is a satellite cell? Location? Allows for what following injury? Why is this property is lacking in the heart?
A
- Undifferentiated nuclei
- Between sarcolemma & basal lamina
- Provide regenerative potential
- Lacking in heart so it doesn’t grow too large b/c of training
24
Q
What percentage of all nuclei are satellite cells?
A
5-15%
25
What kind of nucleus are myonuclei? Location?
- Post-mitotic
- Beneath (inside) sarcolemma
- Periphery of muscle fiber
26
What are the functions of mitochondria?
- Production of ATP to drive cross-bridge formation
| - Role in maintenance of muscle force during prolonged contractions
27
Where are mitochondria located in a muscle fiber?
- Intermyofibrillar = between myofibrils
| - Subsarcolemmal = below sarcolemma
28
What are the key markers of mitochondria?
- Succinate dehydrogenase
- Citrate synthase
- Cytochrome C
29
What makes up the macrovasculature of skeletal muscle?
- Arteries
| - Veins
30
What makes up the microvasculature of skeletal muscle?
- Arterioles
- Venules
- Capillaries
31
What is an aspect of blood flow that only occurs at the levels of the capillaries?
Exchange of gases, nutrients and metabolites between the blood and the tissues
32
What makes up a muscle microvascular unit?
- One terminal arteriole
- 2 collecting venules
- 15-20 capillaries
33
How many muscle fibers does one microvascular muscle unit serve?
20-30
34
What do the major features of sarcomeres look like under light microscopy?
- A bands = dark, wide
- I bands = lighter, wide
- Z line = dark, thin, bisects I bands
35
What are the major features of a sarcomere?
- Z line = anchor for thin filaments
- M line = anchor for thick filaments
- A band = all myosin (w/ overlap)
- I band = only actin
- H band = only myosin
36
During a contraction, how do the sizes of bands in a sarcomere change?
- Overlap increases → lose areas that are single filament type → I band (actin), H zones (myosin) decrease
- A band doesn’t change b/c length of filaments doesn’t change
37
What is the ratio of myosin to actin where they overlap in a sarcomere?
6 actin : 1 myosin
38
What are the proteins associated w/ the sarcomere?
- Contractile proteins
- Regulatory contractile proteins
- Structural & costameric proteins
39
What is the role of contractile proteins? Examples?
- Force generation
| - Myosin, actin
40
What is the role of regulatory contractile proteins? Examples?
- Turn the contractile apparatus on/off
- Modulate activity of MHC
- Tropomyosin, troponins, MLC
41
What are the functions of structural and costameric proteins?
- Maintain adjoining sarcomeres in alignment
- Maintain alignment of thick and thin filaments
- Mechanical linkage between sarcomere and extracellular matrix (endomysium)
42
What is the protein makeup of a thick filament?
- Mostly myosin
- 2 MHC
- 4 MLC
43
What are the 2 functions of the MHC globular head.
- Actin binding site
| - ATPase activity
44
What are the proteins of thin filaments? Which are regulatory?
- Actin = only non-regulatory
- Troponin
- Tropomyosin = inhibitory (TnI), calcium (TnC), tropomyosin (TnT)
45
Why can't a sarcomere expand outwardly toward the Z line with force?
Myosin heads (MHC) only contract towards the center of a sarcomere
46
In a sarcomere, how do the regulatory proteins (troponins & tropomyosin) control exposure of the action-myosin binding site?
- Tropomyosin directly blocks binding sites
- TnI = keeps whole troponin complex in place
- TnC = binds calcium to move tropomyosin out of the way when needed
- TnT = keeps tropomyosin in place
47
What 2 sarcomeric proteins co-ordinate the “tracking” of actin and myosin?
- Nebulin
| - Titin
48
What 2 structural proteins maintain “registry” and force transfer between sarcomeres? Location?
- Alpha-actinin
- Desmin
- Both anchored @ Z line
49
Which protein contributes to passive (recoil) tension in muscle?
Titin
50
What are the functions of the cytoskeleton?
- Has additional structures that facilitate the force generating function of the myofilaments
- Involved in the alignment of the thick and thin filaments
- Transmission of force from the sarcomere to the skeleton
51
What are costameres?
- Sites of mechanical linkage between sarcolemma and basal lamina
- Contains dystrophin --> lack of which causes muscular dystrophy (cells rip themselves apart b/c not connected to one another)
52
Describe the phenomenon of lateral force transmission in endomysial matrix.
EXPERIMENT A:
-anchored single fiber extending from muscle on both ends
-untethered fibers remain unanchored
-stimulated anchored fiber
-single fiber tetanic tension
EXPERIMENT B:
-anchored one end of extended muscle fiber and 2 ends of untethered fibers
-stimulated extended fiber only anchored on one end
-get the exact same force production as experiment A
53
What are the results of endomysial matrix? Significance?
- Force transmission occurs not only through serial connections of muscle fibers to their tendons at the ends of muscle fibers, but also through parallel (lateral) connections among adjacent muscle fibers contained w/in the endomysial matrix
- Allows for greater redundancy in force transmission
54
What does the Sliding Filament Theory propose?
- A muscle shortens or lengthens b/c the thick and thin filaments slide past each other, w/o changing length of filaments
- Sarcomere length changes
- The myosin cross bridges, which cyclically attach, rotate and detach from the actin filaments w/ energy from ATP hydrolysis
- This provides the molecular motor to drive fiber shortening
55
How does the length of the sarcomere change according to the Sliding Filament Theory?
- Decreased length of I band, H zone
| - No change of length of A band
56
Explain the events that must occur before the cross bridge cycle.
-Ca2+ ions released from SR bind to troponin --> troponin changes shape --> tropomyosin unblocks binding site
57
Explain the cross bridge cycle
1. Cross bridge formation = myosin head binds to actin
2. Powerstroke = ADP released --> activated myosin head pivots --> thin filaments slides toward center of sarcomere
3. Cross bridge detachment = ATP binds to myosin head --> link between actin & myosin weakens --> myosin head detaches from actin
4. Reactivation of myosin head = ATP binds to myosin head --> ATP hydrolyzed to ADP & inorganic phosphate --> energy liberated from hydrolysis of ATP activates myosin head --> head assumes cocked position
58
What are sarcoplasmic reticulum cisterns? Relation to the T tubules?
- Invaginations of sarcolemma
- Located on both sides of T tubule
- Store calcium
59
How is the triad positioned relative to internal structure of a sarcomere?
Triad positioned where actin & myosin interact
60
What makes up the triad?
T tubule sandwiched between 2 sarcoplasmic reticulum cisterns
61
How is the level of intra-cellular calcium controlled within a muscle cell? How is it elevated? How is it lowered?
LOOKUP
62
Describe the process of excitation-contraction coupling.
1. ACh released --> depolarizes muscle cell & triggers AP
2. AP propagates into T tubule
3. Depolarization of T tubule --> Ca2+ release from SR
4. Ca2+ binds to troponin --> cross bridge cycle
5. Contraction terminates when Ca2+ is pumped back into SR via ATP pump
63
How is the depolarization of the T tubule translated into release of calcium from the SR?
- At rest, DHPR on T tubule has a "plunger" component that mechanically blocks RyR on SR
- When activated by AP from motor neuron, T tubule becomes depolarized --> conformational change in DHPR --> RyR channel opens --> Ca2+ diffuses from SR to cytosol
- SR reuptake of Ca via ATPase
64
What are the 2 primary membrane proteins involved in calcium release from the SR?
- Dihydropyridine receptors = voltage sensitive calcium release channel
- Ryanodine receptors = calcium release channel
65
What are the roles of calcium in muscle cells?
- Initiation of muscle contraction
- Activation of cytoplasmic enzymes (GLUT4)
- Activation of mitochondrial enzymes (krebs cycle)
- Cell signaling for glucose uptake and regulation of fiber types
66
What makes up the total force developed by a muscle?
Sum of the forces generated by all of the cycling cross bridges
67
What does the number of simultaneously cycling cross bridges depend on?
- Frequency of muscle stimulation
| - Initial length of the muscle fiber
68
What is tension? What does it do?
- Force exerted when a muscle is stimulated to contract
| - Tends to pull attachments toward each other
69
Isometric vs. Concentric vs. Lengthening contraction
- Isometric = constant length, occurs when both ends of the tendons are fixed and immovable
- Concentric = shortening
- Eccentric = lengthening
70
What is a twitch?
Smallest contractile response to a single stimulus
71
How does the frequency of stimulation of a muscle control force generation?
- Frequency-tension relationship
| - Higher frequency of stimulation = increased force generation
72
What is power?
Power = force x velocity
73
What muscular property is affected by altering the number of sarcomeres in series?
Velocity
74
What muscular property is affected by altering the number of sarcomeres in parallel?
Force
75
What is tetanus?
Response of muscle to repeated stimuli
76
Fused vs. Unfused tetany
- Fused = smooth tetany; no relaxation between stimuli
| - Unfused = relaxation between stimuli
77
Explain the frequency-tension relationship.
- Force production depends on frequency of stimulation
| - Higher frequency of stimulation = increased muscle tension
78
Explain the length-tension relationship. What does the graph look like? What happens at optimal length? Before? After?
- Relationship between length of muscle and force output
- Inverted U graph
- Optimal length = most force produced b/c maximal overlap of actin & myosin
- Before optimal length = actin overlaps in the middle of sarcomere w/o presence of myosin heads
- Past optimal length = no overlap between actin & myosin
79
What is passive force? When does it occur in a muscle?
- Occurs when muscle goes beyond optimal length
| - Elastic component of muscle resists lengthening
80
What makes passive force different than active force?
- Doesn't require neural activation
| - Doesn't come from cross bridge formation
81
What kind of movements do the force-velocity relationship describe?
Movement that aren't at a constant muscle length
82
What kind of movements do the length-tension relationship describe?
Isometric contractions (constant length)
83
Explain the force-velocity relationship. Example?
- Describes force generated during isotonic contractions
- Velocity of muscle contraction depends on the force resisting the muscle
- Force & velocity inversely related
- Ex) biking uphill vs. downhill
84
What are isotonic contractions? When do they occur? Delay?
- Constant load
- Occurs when the whole muscle can shorten and lift a load of constant mass
- Latency period between when a muscle is stimulated and when the load moves
85
How does a muscle increase the number of fibers it can pack into a fixed volume?
Angle fibers in a pennate fashion
86
What is the result of having a greater number of fibers in a muscle?
More fibers in parallel = more force generated
87
On a force-velocity curve, what do we call the point where the curve crosses the y-axis? x-axis?
- y = Max. isometric force
| - x = Vmax (max velocity of shortening)
88
For a force-velocity relationship, is eccentric maximal force greater than, less than, or equal to isometric Fmax?
Greater than
89
What factors control the max amount of force a muscle can produce?
- PCSA of fibers in parallel
- Length-tension relationship = overlap between actin & myosin
- Frequency of stimulation
90
What factors control the max velocity of shortening of a muscle?
- Myosin ATPase activity
| - Length of muscle fibers (sarcomere in series)
91
Longitudinal vs. Pennate muscle. What is each specialized for?
```
LONGITUDINAL:
-longer fibers w/ more sarcomeres in series
-fibers run in direction of tendons
-specialized for speed of contraction
PENNATE:
-shorter fibers w/ more sarcomeres in parallel
-fibers run at angle to the tendons
-greater PCSA
-specialized for force production
```
92
Which muscle design has the greatest CSA?
Bipennate
93
Anatomical vs. Physiological CSA
- Anatomical = perp to line of pull of a muscle
| - Physiological = circular area produced by length of muscle and A-CSA
94
How do you increase P-CSA?
Increase angle of muscle fibers
95
Fast vs. Slow twitch muscles w/ regards to myosin ATPase activity.
- Fast = high myosin ATPase activity
| - Slow = slower rate of ATP hydrolysis by myosin ATPase
