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Physiology > Ch 1 - Skeletal Muscle > Flashcards

Ch 1 - Skeletal Muscle Flashcards

1
Q

Excitation-contraction coupling

A

AP -> binding of myosin and actin -> contraction

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2
Q

Myosin

A 
• Thick Filaments
• Large molecular weight
• Six polypeptides
  - 2 heavy-chain myosin
    • alpha-helical "tail"
  - 2 pairs of light chains
    • Globular "heads"
      >> Actin-binding site
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3
Q

Thin Filaments

A

• Anchored at the Z lines within the I bands
• Components
- Actin = has myosin-binding sites
- Tropomyosin = blocker

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4
Q

Tropomyosin

A
  • a filamentous protein
  • runs along the groove of each twisted actin filament
  • at rest, blocks the myosin-binding sites on actin, to prevent contraction
  • must be removed for contraction to occur
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5
Q

Troponin

A
  • Complex of 3 globular proteins:
    • Troponin T = attaches the troponin complex to tropomyosin
    • Troponin I = along w/ tropomyosin, inhibits the interaction of actin and myosin by covering the myosin-binding site on actin
    • Troponin C = Ca2+ binding site
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6
Q

Troponin T

A
  • T for tropomyosin

- Attaches the troponin complex to tropomyosin

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7
Q

Troponin I

A
  • I for inhibition

- along w/ tropomyosin, inhibits the interaction of actin and myosin by covering the myosin-binding site on actin

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8
Q

Troponin C

A
  • C for Ca2+

- Ca2+ binding site

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9
Q

Sarcomere

A
  • Basic contractile unit
  • Delineated by Z disks (goes from one z disc to another)
  • Full central A band with half of two I bands on either side
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10
Q

A bands

A
  • located in the center of the sarcomere
  • contain the thick (myosin) filaments
  • thick and thin filaments may overlap in the A band; potential sites of cross-bridge formation
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11
Q

I bands

A
  • located on either side of the A band

- contain the thin (actin) filaments, intermediate filamentous proteins, and Z disks

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12
Q

Z disks

A
  • Delineates the ends of each sarcomere
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13
Q

Bare Zone

A
  • NO POTENTIAL FOR CROSS-BRIDGING

- Center of each sarcomere w/ no overlapping filaments

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14
Q

M line

A
  • Bisects unit in 1/2; divides the sarcomere in half
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15
Q

Cytoskeletal Proteins of Sarcomere

A
  • Architectural proteins ensuring that the thick and thin filaments are aligned correctly and at proper distances w/ respect to each other
  • “Scaffold”
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16
Q

Titin

A
  • longitudinal cytoskeletal protein
  • associated w/ thick filaments
  • maintains the integrity of the sarcomere during contraction
  • center the thick filaments in the sarcomere
  • biggest protein in the body
  • extends from the M lines to the Z disks
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17
Q

Nebulin

A
  • longitudinal cytoskeletal protein
  • associated w/ thin filaments
  • serves as a “molecular ruler,” setting the length of thin filaments during their assembly
18
Q

alpha-Actinin

A

anchors the thin filaments to the Z disk

19
Q

Dystrophin

A

anchors the myofibrillar array to the cell membrane

20
Q

Duchenne Muscular Dystrophy

A
  • due to a deletion of the DMD gene (Xp21); dystrophin
  • symptoms usually appear in males before age 6
  • progressive proximal muscle weakness of the legs and pelvis associated with a loss of muscle mass is observed first
  • eventually weakness will spread to arms, neck, and other areas
  • Gowers’ maneuver
  • Pseudohypertrophy of calf = it increases in size and from the outside it looks like muscle growth but it is actually fat & connective tissue
  • few survive beyond 20
21
Q

Becker’s muscular dystrophy

A
  • milder form of DMD

- due to a mutation, not deletion

22
Q

Transverse Tubules

A
  • conducts AP to interior of fiber allowing release of Ca2+ from Sarcoplasmic Reticulum
  • communicates with the Sarcoplasmic Reticulum via the dihydropyridine receptor at the terminal cistern of the Sarcoplasmic Reticulum
23
Q

The transverse tubules communicate with the SR vi the

A

Dihydropyridine receptor at the terminal cistern of the SR

24
Q

Sarcoplasmic Reticulum

A
  • Site of storage and release of Ca2+
  • Ryanodine receptor = channel thru which Ca2+ is released
  • Calsequestrin = a low-affinity, high-capacity Ca2+ binding protein within the SR
  • Ca2+ ATPase (SERCA) = pumps Ca2+ into SR
25
Q

Steps in Excitation-Contraction Coupling in Skeletal Muscle

A

AP in the muscle cell membrane initiate depolarization of the T Tubules -> causes a critical conformational change in its voltage-sensitive dihydropyridine receptor opening the Ca2+ release channels (ryanodine receptors) on the nearby SR into the intracellular fluid -> Intracellular [Ca2+] rises to above 10^-7 M -> Ca2+ binds to troponin C on the thin filaments leading to cross bridging events

26
Q

Events in Cross Bridging

A

Ca2+ binds to Troponin C on the thin filaments -> Tropomyosin is moved out of the way -> actin & myosin bind, allowing thick and thin filaments to slide over each other, and ATP is hydrolyzed -> cross bridges break and a new molecule of ATP binds to the myosin head to begin a new cycle

27
Q

What happens during muscle cell relaxation?

A
  • Ca2+ is reaccumulated in the SR by the Ca2+ ATPase of the SR membrane (SERCA)
  • Ca2+ concentration decreases to less than 10^-7 M -> insufficient Ca2+ for binding to troponin
  • Ca2+ is released from troponin C
  • Tropomyosin returns to its resting position preventing myosin interaction w/ actin
28
Q

Length-Tension Relationship

A

Refers to the effect of muscle fiber length on the amount of tension the fiber can develop

29
Q

Isometric contraction

A

A contraction in which a muscle exerts force (tension) but does not change in length
The muscle is allowed to develop tension at a preset length (called preload) but is not allowed to shorten
Yoga & stretching

30
Q

Isotonic contraction

A

Shortening of muscle at a constant afterload

Actually doing reps with the barbell

31
Q

Passive Tension

A

Tension developed by simply stretching a muscle to different lengths
Generate greatest tension at its resting length; tension of an unstimulated muscle

32
Q

Active Tension

A

Represents the active force developed during cross-bridge cycling
Active tension = total tension - passive tension
Proportional to the number of cross-bridges that cycle

33
Q

Force-Velocity Relationship

A

The velocity of shortening reflects the speed of cross-bridge cycling
As the afterload on the muscle increases, the velocity will be decreased b/c cross-bridges can cycle less rapidly against the higher resistance
As the afterload increases to even higher levels, the velocity of shortening is reduced to zero

34
Q

Smooth Muscle

A
  • unstriated; thick & thin filaments, while present, are not organized in sarcomeres
  • found in GI Tract, bladder, uterus (strongest muscle in body), bronchioles, etc.
35
Q

Types of Smooth Muscle

A

Unitary Smooth Muscle: located in GI tract, bladder, uterus; contain gap junctions which allows for the coordinated fashion of organ contraction, permits electrical coupling between cells. Slow waves which sets a characteristic pattern of APs within an organ, which then determines the frequency of contractions
Multiunit Smooth Muscle: located in iris, ciliary muscles of the lens, and in the vas deferens. Each muscle fiber behaves as a separate motor unit (similar to skeletal muscle), and there is little or no coupling between cells. Densely innervated by postganglionic fibers of the para & sympathetic nervous systems

36
Q

What is the regulating factor of the myosin-actin bridges in smooth muscle?

A

Calmodulin rather than troponin is the regulating factor of the myosin-actin bridges

  • Ca2+ binds to calmodulin instead of troponin C
  • In turn, Ca2+ - calmodulin regulates myosin-light-chain kinase, which regulates cross-bridge cycling
37
Q

Steps in Excitation-Contraction Coupling in Smooth Muscle

A
  1. AP occur in the smooth muscle cell membrane
  2. Opening of voltage-gated Ca2+
  3. Ca2+ flows down its gradient into the cell (intracellular Ca2+ concentration increases)
  4. Ca2+ binds to calmodulin
  5. The Ca2+ - calmodulin complex binds to & activates myosin-light-chain kinase
  6. Myosin becomes phosphorylated
  7. Cross-bridge formation
38
Q

Relaxation of smooth muscles occurs when

A

The SR re-accumulates Ca2+, via the Ca2+ ATPase, and lowers the ICF [Ca2+] below the level necessary to form Ca2+ - calmodulin complexes

39
Q

Mechanisms that Increase Intracellular Ca2+ Concentration in Smooth Muscle

A
  1. Voltage-gated Ca2+ channels are sarcolemmal Ca2+ channels that open when the cell membrane potential depolarizes
  2. Ligand-gated Ca2+ channels also are present in the sarcolemmal membrane
  3. IP3-gated sarcoplasmic reticulum Ca2+ channels also are opened by hormones & neurotransmitters
40
Q
  • associated w/ thick filaments
  • maintains the integrity of the sarcomere during contraction
  • center the thick filaments in the sarcomere
A

Titin

41
Q
  • associated w/ thin filaments

- serves as a “molecular ruler,” setting the length of thin filaments during their assembly

A

Nebulin

42
Q

Calsequestrin

A

A low affinity, high capacity Ca2+ binding protein within the SR
At rest, by binding to Ca2+, intracellular Ca2+/free Ca2+ is maintained low

Physiology (13 decks)

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