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Adv. Phys. > L4 - Skeletal Muscle Contraction (Chapters 6-7) > Flashcards

L4 - Skeletal Muscle Contraction (Chapters 6-7) Flashcards

1
Q

Be able to label this diagram

A
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2
Q

Myofiber

A
  • elongated muscle cell
  • size is due to fusion of myoblasts
  • many nuclei along the periphery
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3
Q

Myofibril

A
  • filament like structure that contains the contractile components actin and myosin
  • component of myofibers
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4
Q

Sarcolemma

A

plasma membrane of muscle cells

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5
Q

Sarcoplasm

A

cytoplasm of muscle cells

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6
Q

Sarcoplasmic reticulum

A

modified smooth endoplasmic reticulum, storage site for intracellular calcium

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7
Q

What other specializations to skeletal muscle cells possess?

A
  • many mitochondria (muscle need a lot of ATP to function)
  • glycosomes - cellular inclusion (molecules that form large complexes)
  • myoglobin - carries O2 (similar to hemoglobin, but can only carry 1 or 2 molecules of oxygen)
  • SR/T tubule system
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8
Q

Be able to label the structure of a myofibril

A
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9
Q

What is a sarcomere?

A
  • basic contractile unit of myofibrils
  • cause of striated appearance in skeletal and cardiac muscle
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10
Q

What are the parts of a sarcomere?

A
  • Z discs border the sarcomere
  • M line goes down the center of each sarcomere
  • H zone surrounds the M line
  • A band is isotropic
  • I band is anisotropic
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11
Q

What proteins are found in each part of the sarcomere? Which regions contain actin, which contain myosin, and which contain both? Can you draw a myofibril in cross-section? How many actin filaments surround each myosin filament?

A
  • thick filaments are made of myosin (motor protein)
  • thin filaments are made of actin
  • I band is made of actin
  • A band is made of both actin and myosin
  • H zone only has myosin
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12
Q

What is the structure of the myosin filaments? Can you draw one?

A
  • a.k.a. thick filaments
  • 6 total peptides - 2 heavy chains and 4 light chains
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13
Q

What are the two major parts of myosin filaments? Which part contains the actin binding site? Which part contains the ATPase?

A
  • Head - composed of 2 light chains and a heavy chain
  • Tail - contains two heavy chains wound helically around each other
  • Head contains both the actin binding site and ATPase
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14
Q

What is the purpose of the actin binding site and ATPase located in/on the head of myosin?

A
  • ATPase breaks down ATP, allows for phosphate via hydrolyzing to be freed and used to power functions
  • actin binding sites are for binding actin… duh
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15
Q

What are myosin crossbridges?

A
  • the arm and head of myosin that interact with each other during contraction
  • involved in the walk-along theory of contraction (ratcheting motion of head towards arm after binding to actin pulls the actin along)
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16
Q

What are the hinges of myosin crossbridges? What is their function?

A
  • extends away from myosin tail in the form of a hinge
  • connection point btwn head and arm of crossbridge
  • allows for dynamic action like a ratcheting motion
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17
Q

What is the structure of the actin filaments? Can you draw one?

A

also called thin filaments

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18
Q

F-actin

A
  • filamentous component of actin
  • found of microtubules
  • polymer of G-actin
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19
Q

G-actin

A
  • globular component of actin
  • reserve pool of actin used to make microtubules
  • contains binding sites for myosin, tropomyosin and troponin
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20
Q

How are G-actin and F-actin related?

A

G-actin is the monomer of F-actin, and F-actin is the polymer of G-actin

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21
Q

Where are the myosin binding sites located? How are they arranged along F-actin?

A
  • located on G-actin
  • offset to point out radially from F-actin helix
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22
Q

What two other proteins associate with F-actin in sarcomeres? How are these proteins arranged along F-actin?

A
  • Troponin is located on top of tropomyosin
  • Tropomyosin is located on the myosin binding sites
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23
Q

What is the function of troponin and tropomyosin on actin filaments?

A
  • Tropomyosin blocks the binding of myosin to actin bc it has a higher affinity for myosin binding sites – imp for when muscle is relaxed, there is no binding
  • Troponin moves tropomyosin out of the way when muscle needs to contract so that myosin can bind to actin
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24
Q

Titin

A
  • also called connectin
  • largest know protein
  • extends from Z disc to M line
  • more likely to collect mutations bc the gene sequence of this protein is large
  • present in striated muscle only
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25
Q

What does a mutation of titin cause?

A
  • dilating cardiomyopathy - dilation of ventricles that can result in an overall weakening of the myocardium
  • can also cause skeletal myopathies
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26
Q

Myomesin

A
  • located in M line
  • holds the phosphorylzed tails of proteins to connect adjacent thick filaments
  • contributes to how electron dense a membrane is
27
Q

Alpha actinin

A
  • located in Z disc
  • anchors thin filaments and helps their arrangement
28
Q

Nebulin

A
  • located in I band
  • binds and regulates actin filament assembly
29
Q

Specifically, what are some of the functions of the protein titin that we discussed?

A
  • Regulates tension
  • Involved in mechanotransduction/hypertrophy
  • Prevents atrophy of muscles via mechanosensing
  • Quality control of proteins in the cell
30
Q

What are the different regions of titin? What are functions related to the regions?

A
  • A band portion – thought to have a role in the assembly of thick filaments
  • I band portion – thought to act like a molecular spring, elastic portion that regulates tension across sarcomere
31
Q

How does titin regulate muscle remodeling?

A
  • mechanosensing/mechanotransducing
    • transduces mechanical signal into chemical changes that will lead to hypertrophic signaling
  • titan kinase (TK) regulates this
32
Q

Hypertrophic signaling

A
  • response to mechanical signal in muscles that will either decrease degradation of proteins or increase production of proteins
33
Q

What is turnover of proteins?

A
  • degradation or destruction of old or damaged proteins
  • titin regulates this
34
Q

What two titin systems are involved in protein turnover and degradation?

A
  • Autophagosome system – autophagosome pairs with a lysosome that contains digestive hydrolytic enzymes in order to degrade proteins
  • Ubiquitin proteasome system – ubiquitin attaches to proteins through ubiquitination, which causes them to be targeted for destruction
35
Q

What is the sliding filament model of contraction?

A

a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle

36
Q

What happens to the A band during sarcomere contraction?

A
  • remains the same no matter degree of stretch or shortening of sarcomere
37
Q

What happens to the I band during sarcomere contraction?

A
  • decreases in size or disappears as is slides over the A band
38
Q

What happens to the H zone during sarcomere contraction?

A
  • disappears as actin and myosin slide over each other
39
Q

What happens to the Z disks during sarcomere contraction?

A
  • get closer to each other
  • at maximum contraction, they will bump into the A bands
40
Q

What is a costamere?

A
  • connects sarcomeres to the sarcolemma, bridge the inside of the cell to the outside of the cell
  • transfer tension laterally from the sarcolemma to the muscle
41
Q

What are the different parts of the costamere?

A
  • glycoprotein dystroglycan complex
  • integrin-talin complex
42
Q

What are some types of muscular dystrophy that we discussed? What causes them?

A
  • Beckers - reduces motor function, causes weakness and muscular atrophy
  • Duchenne - more severe, motor function depleted completely
  • Beckers is due to poorly functioning dystrophin
  • Duchenne is due to no functional dystrophin
43
Q

Which costameric protein is involved in muscular dystrophies?

A
  • dystrophin - serves as a weak link btwn costamere and the sarcoglycan complex bc it easily picks up mutations
44
Q

How is skeletal muscle excited?

A

Function in cooperation with motor neurons in a group called the motor unit (a motor neuron and all of the muscle fibers it innervates)

45
Q

What are (lower) motor neurons? Where are they located specifically?

A
  • Specifically control somatic motor function
  • Located in the ventral horn of the spinal cord, enter the spinal cord through the ventral nerve root
46
Q

What does damage to the ventral horn cause? The dorsal horn?

A
  • weakness or muscle paralysis
  • loss of sensory function
47
Q

What forms the spinal nerve?

A

the junction of the ventral and dorsal nerve roots

48
Q

What are motor units?

A
  • motor neuron plus all of the muscle fibers that it innervates
  • differ in size depending on amount of muscle fibers that are being innervated
  • allow for precision control regarding recruitment
49
Q

What happens when a single motor neuron fires?

A
  • an AP is created that propagates bidirectionally along the muscle fiber to the site of the synapse
  • causes the release of ACh from the NMJ
50
Q

What direction does an AP propagate on a myofiber?

A

bidirectionally

51
Q

What is an NMJ?

A

junction btwn a neuron and a muscle cell that performs chemical synaptic transmission

52
Q

Chemical synaptic transmission

A
  • allows for individual control over neurons
  • release of chemicals like neurotransmitter from presyn. cell to receptors on postsyn. cell
53
Q

What are the steps of chemical transmission?

A
  1. AP depolarizes the presyn. membrane
  2. voltage gated Ca+ channels open, allowing Ca+ to enter the presyn cell
  3. Ca+ activates the VSNARE synaptotagmin, which causes the release of vesicles
  4. fusion pore is created with the VSNARE on the vesicle and the TSNARE on the presyn. membrane
  5. neurotransmitter is released into synaptic cleft
  6. nicotinic ACh receptor on postsyn. cell receives 2 ACh molecules
  7. EPP is generated, causing an inward current of ACh into the postsyn. cell
  8. voltage gated Na+ channels are activated by the EPP, which allows for the generation of an AP
54
Q

What causes the termination of chemical transmission in the NMJ?

A
  • Ca+ channels close on the presyn. cell, and Ca+ pumps remove Ca+ from the cell
  • acetylcholinesterase breaks down ACh in the postsyn. cell
55
Q

What happens if ACh is not broken down?

A

muscle cell loses the ability to depolarize, essentially paralyzing the muscle

56
Q

When does EC coupling begin?

A

when synaptic transmission ends

57
Q

What are the steps of EC coupling?

A
  1. AP propagates down sarcolemma toward end of myofiber
  2. AP simultaneously travels down t-tubules (storage site of Ca+)
  3. Ca+ voltage gated channels are opened
  4. Ca+ is released from terminal cisternae of sarcoplasmic reticulum
  5. Ca+ binds to troponin
  6. Ca+-troponin complex moves tropomyosin from actin binding sites
  7. myosin binds to actin and pulls actin filaments toward center of sarcomere
  8. Ca+ is taken up by the sarcoplasmic reticulum when there is no longer an AP
  9. tropomyosin covers actin binding sites
58
Q

What controls the voltage gated Ca+ channels during EC coupling?

A
  • DHP receptors and RYR channels
    1. depolarization of the t-tubule triggers a conformational change of the DHP channel
    2. DHP pulls the plug from the Ca+ release channel (RYR) on the SR
    3. Ca+ diffuses out of sarcoplasmic reticulum
59
Q

What protein helps hols Ca+ inside the SR?

A

calsequestrin acts like a sponge and soaks up extra Ca+

60
Q

Can you explain the binding of Ca+ to troponin?

A
  • troponin is a trimeric protein, with subunits TnT, TnC, and TnI
    1. TnI, which inhibits myosin heads from interacting w/ actin is only moved after Ca+ binds with TnC
    2. once Ca+ binds with TnC and pulls it out of the way, TnI slides out of the way, and tropomyosin slides into the groove btwn actin filaments
    3. myosin can then bind to actin
61
Q

Can you explain the steps of cross-bridge cycling?

A
  1. myosin head attached to actin, forming a cross bridge
  2. an inorganic phosphate that was made during the previous cycle is released, causing the power stroke (myosin head pivots)
  3. ADP is released
  4. ATP breaks the cross bridge
  5. ATP used to break the cross bridge is then hydrolyzed into ADP and a free phosphate
  6. cycle repeats
62
Q

Rigor mortis

A
  • stiffness after death
63
Q

What causes rigor mortis?

A
  • ATP is no longer being produced, causes:
    1. Ca+ is no longer being pumped back into SR
    2. cross bridges are no longer being broken

Adv. Phys. (11 decks)

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