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Skeletal Muscle Flashcards

(61 cards)

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0
Q

Organization of skeletal muscle

A
  • each fiber is a long multinucleated cell (formed by fusion of myoblasts during development)
  • fibers vary in size
  • each fiber contains myofibril for contraction
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1
Q

Skeletal muscle general characteristics

A
  • striated
  • multinucleated
  • attaches to bone
  • somatic NS
  • isolated from neighboring cells
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2
Q

Myofibrils

A

thin rod-like structures used for contraction

composed of many sarcomeres end to end

-organelles, many per muscle fiber (cell)

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3
Q

Muscle fibers attach to plasma membrane via:

A

adherens junctions which attach to tendons via the basal lamina

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4
Q

Fascicles

A

bundles of muscle fibers visible to the naked eye

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5
Q

epimysium

A

fascia (connective tissue) that surrounds the muscle and separates it from its neighbors

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6
Q

endomysium

A

surrounds each muscle fiber

very thin septa

separates fibers electrically

constitutes the BM and some CT

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7
Q

perimysium

A

medium thickness

surrounds the fascicles

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8
Q

Sarcomere

A

structural and functional unit of striated muscle contraction

contains overlapping thin (actin) and thick (myosin) filaments

the barbed ends of these filaments are anchored in the z-disk

the myosin (thick) binds to actin (thin)

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9
Q

M Line

A

central link between bipolar myosin filaments

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10
Q

Z disk

A

crosslinks the thin filaments

polarity on one end of the z line is the same

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11
Q

Z disk (line) contains

A

alpha actinin

Cap Z

proteins

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12
Q

What happens in a sarcomere during muscle contraction

A

thin filaments slide past the thick filaments

distance between z disks increases

cyclic interaction of myosin and actin powers contraction

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13
Q

I Band

A

no thick filament overlapping the thin

space between the z disks

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14
Q

Tension depends on

A

the number of myosin heads overlapped by thin filaments

-FORCE

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15
Q

Myosin

A

from a superfamily (39 genes in humans)

Myosin II is the molecular motor for muscle contraction

polymerizes at tails to form bipolar filaments

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16
Q

Myosin II structure

A

total 6 polypeptide chains: composed of 2 heavy chains, 4 light

each heavy chain has a regulatory light chain and an essential light chain

3 structural domains

1) motor
2) subfragment (allows swing)
3) Light meromyosin (LMM)

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17
Q

Myosin motor structure

A

regulatory domains on the heavy chain are alpha helices that extend from the motor domain and at as lever arm

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18
Q

Actin

A

second most abundant protein on earth

DNA sequence is highly conserved

monomeric actin (g-actin) polymerizes to form filaments (f-actin)

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19
Q

G Actin

A

binds ADP/ATP

hydrolysis is slow

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20
Q

G Actin polymerization

A

upon polymerization ATP rapidly hydrolyzed

associate/disassociate only at the ends

the filament is POLARIZED

pointed end - minus end

barbed end - plus end (anchored in z-disk)

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21
Q

Myosin/Actin binding

A
  • tight in the absence of ATP (rigor)
  • weakened by ATP
  • actin accelerates myosin ATPase
  • myosin heads bind actin at an angle at barbed end, moves towards the barbed ends via powerstroke
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22
Q

Energy for motility

A

1) myosin binding to ATP releases actin
2) myosin ATP hydrolysis, ADP/P stay tightly bound and the myosin filament cocks (reverse power stroke)
3) myosin reattaches to actin
4) P leaves and myosin returns to uncocked state (force generating power stroke)

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23
Q

Myosin ATPase

A

activity increased by actin

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24
tropomodulin
-capping protein regulates actin polymerization/depolymerization at the pointed ends
25
CapZ
capping protein that regulates actin polymerization/depolymerization at the barbed end
26
Titin
giant protein that forms elastic connections between z disks and myosin
27
Z disks connect to the plasma membrane via
intermediate filaments
28
What stabilizes the plasma membrane of skeleton muscle?
dystrophin and proteins issues in these can cause muscular dystrophy
29
Innervation of muscle fiber
at single motor end plate each fiber has its own, separate innervation
30
Excitation/contraction coupling
- contraction due to acetyl choline release at end plate causing depolarization of surface membrane - depolarization spreads via action potential - calcium acts as second messenger to allow the disinhibition of the actin/myosin system and causes contraction
31
Motor Unit
single neuron innervates many muscle fibers through branching (all the same type) skeletal muscles contain numerous motor units fibers of a motor unit are dispersed and intermingle with other fibers small motor units reach firing threshold easier
32
activation of one motor unit
weak but distributed contraction
33
activation of multiple motor units
stronger contraction
34
Twitch
mechanical response due to a single action potential briefest normal contraction of a skeletal muscle
35
Tetanus
fused twitches
36
Skeletal muscle action potential has a positive after twitch because:
- t tubular and surface membranes are electronically coupled | - action potential in the t tubular membrane occurs slightly after
37
Depolarization of plasma membrane (sarcolemma) causes
calcium release from sarcoplasmic reticulum
38
Transverse tubules
- invaginations of the plasma membrane - membrane is continuous with plasma membrane, lumen with the extracellular space - between the cisterna of the sarcoplasmic reticulum (feet where Ca released from SARCOPLASMIC RETICULUM NOT ECM)
39
Mechanisms of calcium removal from the cytoplasm
Out to Extracellular: - Ca/H pump (ATP linked) - Na/Ca pump Back to SR: - H/Ca pump (ATP linked) - bound to calcireticulin and calcisequestrin
40
Muscle can be found in three states:
- relaxed - contracted - rigor
41
Relaxation
- regulatory proteins prevent actin/myosin interaction - few heads are bound to actin - sarcomere can be stretched passively
42
Contraction state
- muscle activated by calcium release - thousands of sarcomeres shorten in series causing the muscle to shorten - ATP is hydrolyzed and force is produced
43
Rigor State
- ATP is depleted - all myosin heads are bound to actin - strong actin/myosin interaction prevents stretching
44
Nerve stimulation determines the contractile force in two ways:
1) the NUMBER of motor units determines how many muscle cells produce force 2) the RATE of stimulation adjusts the force produced by active cells
45
Muscle contraction activated by
calcium
46
Muscle contraction is regulated by
thin filaments - tropomyosin and troponin
47
Tropomyosin
- calcium sensitive regulatory protein - 2 alpha helice polypeptides - at low calcium (relaxation) it is bound to actin and blocks myosin
48
Troponin
- 3 proteins - TN-C binds calcium - TN-I inhibits actin/myosin interaction - TN-T binds tropomyosin * When bound to calcium it causes the release of tropomyosin
49
Length-tension relation
force production is proportional to the number of site that crossbridges (actin/myosin interactions) can form
50
Total tension
active + passive
51
Active tension
due to contraction (actin and myosin)
52
Passive tension
due to other elastic elements parallel to contractile elements -example: titin
53
Force production
- depends on the number of myosin molecules/area and the fraction of their ATPase cycle - proportional to fiber diameter (fibers in series have the same tension as a single fiber) - maximum force depends on the number of fibers in PARALLEL - Exercise increased diameter of a fiber (and the force)
54
Range of fiber =
range of sarcomere x sarcomeres in series
55
Velocity of shortening of a muscle fiber
velocity of shortening of one fiber x sarcomeres in series -longer muscle fibers have LARGER shortening range and FASTER shortening rates
56
Advantage of long fiber
-stretch/shortening spread out over many sarcomeres so that there is a smaller change in length of a single sarcomere (smaller decrease of tension)
57
Disadvantage of long fibers
does not increase the maximum force but does increase the amount of energy required
58
Isometric contraction
the muscle develops force at a constant length
59
Isotonic contraction
the muscle shortens under a constant load
60
Force-velocity relationship
-the steady state velocity of shortening depends hyperbolically on the load IE: at isometric contraction the load is so heavy that the myosin are all involved in resisting the load and there are none available to shorten -at very light loads there are more myosin available for shortening

CTB (5 decks)

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