NJN 2. Musculoskeletal
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Questions and Answers

What is the primary function of the M-line within a sarcomere?

  • Blocking the active sites on actin molecules.
  • Stabilizing the thick filament and connecting it to titin. (correct)
  • Anchoring the thin filaments to the Z discs.
  • Storing calcium ions for muscle contraction.

During muscle contraction, what role does tropomyosin play in the process?

  • It anchors the thin filaments to the Z discs.
  • It binds calcium ions, initiating the contraction.
  • It directly binds to myosin heads, causing the filaments to slide.
  • It blocks the active sites on actin, preventing myosin binding in the resting state. (correct)

What is the key structural difference between G-actin and F-actin?

  • F-actin is found in thick filaments, while G-actin is in thin filaments.
  • G-actin is a monomeric form, while F-actin is a filamentous polymer. (correct)
  • F-actin is responsible for ATP hydrolysis, while G-actin provides structural support.
  • G-actin contains calcium binding sites, while F-actin does not.

How does titin contribute to the structure and function of the sarcomere?

<p>It anchors thick filaments to the Z disc and contributes to sarcomere elasticity. (B)</p> Signup and view all the answers

What is the functional significance of the troponin complex in muscle contraction?

<p>It regulates the position of tropomyosin on actin, controlled by calcium binding. (A)</p> Signup and view all the answers

What distinguishes the I-band from the A-band in a sarcomere?

<p>The I-band appears lighter and contains only thin filaments, while the A-band is darker and contains thick filaments. (B)</p> Signup and view all the answers

Which protein is responsible for anchoring the thin filaments to the Z disc?

<p>Nebulin (D)</p> Signup and view all the answers

What are the key structural components of the myosin protein?

<p>Head, neck, and tail regions, with the head containing ATPase activity and actin-binding sites. (B)</p> Signup and view all the answers

How does dystrophin contribute to maintaining muscle fiber integrity?

<p>It connects the actin cytoskeleton to the extracellular matrix, stabilizing the sarcolemma. (A)</p> Signup and view all the answers

Which of the following best describes the H zone?

<p>The area within the A band where only thick filaments are present. (C)</p> Signup and view all the answers

What is the underlying genetic cause of Duchenne muscular dystrophy (DMD)?

<p>A nonsense or frameshift mutation in the dystrophin gene, resulting in little to no functional protein. (A)</p> Signup and view all the answers

How does Becker muscular dystrophy (BMD) differ genetically and clinically from Duchenne muscular dystrophy (DMD)?

<p>BMD is caused by a missense mutation leading to a partially functional dystrophin protein, resulting in a milder course compared to DMD. (D)</p> Signup and view all the answers

In the context of myosin structure, what role do the light chains play?

<p>They support the head and neck region, and can undergo regulatory modification. (A)</p> Signup and view all the answers

What is Alpha-actinin's primary function?

<p>The main protein making up the Z disk. (D)</p> Signup and view all the answers

Which of the following events is directly triggered by calcium binding to troponin C?

<p>Conformational change in troponin, leading to the exposure of actin's active sites. (B)</p> Signup and view all the answers

Flashcards

Endomysium

Connective tissue covering the muscle fiber.

Sarcomere

Basic functional unit of a muscle fiber, from Z disc to Z disc.

Alpha-actinin

Protein forming the Z disc, anchoring thin filaments.

A band

Region with thick filaments; appears dark under a microscope.

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I band

Region with only thin filaments; appears light under a microscope.

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M line

Line in the middle of the sarcomere, connecting thick filaments.

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Titin

Structural protein that anchors thick filaments to the Z disc.

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H zone

Region in the sarcomere with no thin filaments (only thick).

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Nebulin

Thin filament protein that links to the Z disc.

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G-actin

Monomeric form of actin.

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F-actin

Polymerized form of actin forming the thin filament.

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Tropomyosin

Blocks myosin-binding sites on actin during muscle rest.

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Troponin

Binds calcium ions and regulates muscle contraction.

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Myosin

A protein with a tail, neck, and head that binds actin and causes filaments to slide.

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Dystrophin

Structural protein linking actin to the sarcolemma and extracellular matrix.

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Study Notes

  • Myofibrils consist of many connections of thick and thin filaments, as well as accessory proteins.
  • The notes zoom in on a portion of a myofibril to discuss its components.

Z Disks

  • Pink zigzag proteins called Z disks delineate the sarcomere.
  • Sarcomere: The functional unit of the muscle fiber, spanning from one Z disk to the next.

Sarcomere Structures

  • Thick Filament: A large red rectangular structure.
  • Thin Filament: Contains actin, and other proteins.
  • M Line: A line running down the middle of the sarcomere.

Titin

  • Blue proteins, are an important elastic structural protein.
  • Anchors the thick filament to the Z disk.
  • Connects the thick filament to proteins of the M line.
  • Stabilizes the thick filament.

M Line

  • Accessory protein structure at the center of the sarcomere.
  • Connects to titin, which in turn connects the thick filament to the Z disk.
  • Helps stabilize the thick filament.
  • Composed of Myomesin, C proteins, and Creatine Kinase.
  • Myomesin and C proteins are structural, while Creatine Kinase has a functional role in muscle metabolism.

Bands

  • A Band (anisotropic): Represents the entire length of the thick filament, appearing as a dark band.
  • I Band (isotropic): Extends from the end of one thick filament to the beginning of the next in an adjacent sarcomere, appearing lighter.

Thin Filament

  • Connects directly to the Z disk via nebulin.

Nebulin

  • A black protein that anchors the thin filament to the Z disk.

Actin

  • Exists in two main forms: G-actin and F-actin
  • G-actin: A monomer of actin.
  • F-actin: A polymer of G-actin molecules.
  • Multiple F-actin molecules form a supramolecular helix.

Tropomyosin

  • An orange ropelike protein.
  • Blocks the active sites on actin in a resting muscle, preventing myosin binding.

Troponin

  • A green protein complex with three binding sites:
    • Troponin T: Binds to tropomyosin.
    • Troponin C: Binds to calcium.
    • Troponin I: Binds to actin and has an inhibitory function.
  • Troponin, specifically troponin T, troponin C, and troponin I, is a marker for heart damage.

Thick Filament

  • Includes myosin molecules with a tail, neck, and head.
  • Myosin Head: Binds to actin's active sites, facilitating the sliding of myofilaments.
    • Possesses ATPase activity, cleaving ATP into ADP and inorganic phosphate.
  • Myosin Neck: Contains light chains that support the head and regulate myosin activity.
    • Regulatory Light Chain: Can be phosphorylated to control myosin activity.
    • Essential Light Chain: Provides structural stability to the myosin head and neck.

Z Disk Proteins

  • Primarily composed of alpha-actinin.

Bands Review

  • A Band: Dark, consists of the entire thick filament length within a sarcomere.
  • Sarcomere: Defined by the region between two Z disks.
  • I Band: Lighter, consists of thin filaments, titin, and portions of the Z disk.
  • M Line: Made of myomesin, C proteins, and creatine kinase, linking to titin to stabilize the thick filament.

H Zone

  • The distance between thin filaments on the same sarcomere.

Dystrophin

  • Links actin to the sarcolemma (muscle cell membrane) through protein complexes.
  • Connects to the extracellular matrix.
  • Mutations in the dystrophin gene can cause muscular dystrophy.

Muscular Dystrophy

  • Umbrella term for conditions arising from dystrophin protein mutations.
  • Duchenne Muscular Dystrophy (DMD): More severe, develops by age 5-6 years.
  • Becker Muscular Dystrophy (BMD): Less severe, develops by age 10-20 years.
  • Typically an X-linked recessive disorder, more common in males.

Genetic Difference Between DMD and BMD

  • DMD: Often caused by a nonsense mutation (no dystrophin produced).
  • BMD: Usually results from a missense mutation, leading to a misfolded, but partially functional, dystrophin protein.

Consequences

  • Absence or dysfunction of dystrophin leads to membrane instability, causing muscle cell damage.
  • Muscle cells are replaced with fibrous and fatty tissue.
  • Leads to muscle weakness, respiratory failure (due to diaphragm involvement), strained gait, and potential dilated cardiomyopathy.
  • Treatment: Limited, focuses on physical therapy and symptom management.

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