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CTB > Skeletal Muscle > Flashcards

Skeletal Muscle Flashcards

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
Q

tropomodulin

A

-capping protein regulates actin polymerization/depolymerization at the pointed ends

25
Q

CapZ

A

capping protein that regulates actin polymerization/depolymerization at the barbed end

26
Q

Titin

A

giant protein that forms elastic connections between z disks and myosin

27
Q

Z disks connect to the plasma membrane via

A

intermediate filaments

28
Q

What stabilizes the plasma membrane of skeleton muscle?

A

dystrophin and proteins

issues in these can cause muscular dystrophy

29
Q

Innervation of muscle fiber

A

at single motor end plate

each fiber has its own, separate innervation

30
Q

Excitation/contraction coupling

A
  • 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
Q

Motor Unit

A

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
Q

activation of one motor unit

A

weak but distributed contraction

33
Q

activation of multiple motor units

A

stronger contraction

34
Q

Twitch

A

mechanical response due to a single action potential

briefest normal contraction of a skeletal muscle

35
Q

Tetanus

A

fused twitches

36
Q

Skeletal muscle action potential has a positive after twitch because:

A
  • t tubular and surface membranes are electronically coupled

- action potential in the t tubular membrane occurs slightly after

37
Q

Depolarization of plasma membrane (sarcolemma) causes

A

calcium release from sarcoplasmic reticulum

38
Q

Transverse tubules

A
  • 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
Q

Mechanisms of calcium removal from the cytoplasm

A

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
Q

Muscle can be found in three states:

A
  • relaxed
  • contracted
  • rigor
41
Q

Relaxation

A
  • regulatory proteins prevent actin/myosin interaction
  • few heads are bound to actin
  • sarcomere can be stretched passively
42
Q

Contraction state

A
  • muscle activated by calcium release
  • thousands of sarcomeres shorten in series causing the muscle to shorten
  • ATP is hydrolyzed and force is produced
43
Q

Rigor State

A
  • ATP is depleted
  • all myosin heads are bound to actin
  • strong actin/myosin interaction prevents stretching
44
Q

Nerve stimulation determines the contractile force in two ways:

A

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
Q

Muscle contraction activated by

A

calcium

46
Q

Muscle contraction is regulated by

A

thin filaments - tropomyosin and troponin

47
Q

Tropomyosin

A
  • calcium sensitive regulatory protein
  • 2 alpha helice polypeptides
  • at low calcium (relaxation) it is bound to actin and blocks myosin
48
Q

Troponin

A
  • 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
Q

Length-tension relation

A

force production is proportional to the number of site that crossbridges (actin/myosin interactions) can form

50
Q

Total tension

A

active + passive

51
Q

Active tension

A

due to contraction (actin and myosin)

52
Q

Passive tension

A

due to other elastic elements parallel to contractile elements

-example: titin

53
Q

Force production

A
  • 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
Q

Range of fiber =

A

range of sarcomere x sarcomeres in series

55
Q

Velocity of shortening of a muscle fiber

A

velocity of shortening of one fiber x sarcomeres in series

-longer muscle fibers have LARGER shortening range and FASTER shortening rates

56
Q

Advantage of long fiber

A

-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
Q

Disadvantage of long fibers

A

does not increase the maximum force but does increase the amount of energy required

58
Q

Isometric contraction

A

the muscle develops force at a constant length

59
Q

Isotonic contraction

A

the muscle shortens under a constant load

60
Q

Force-velocity relationship

A

-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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