Skeletal Muscle Structure and Function
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Questions and Answers

What is the average diameter of muscle fibers?

  • 250 μm
  • 100 μm (correct)
  • 50 μm
  • 200 μm

Which type of protein is classified as a contractile protein in muscle fibers?

  • Myosin (correct)
  • Dystrophin
  • Tropomyosin
  • Actin

What are the light bands in muscle fibers called?

  • Z bands
  • H bands
  • A bands
  • I bands (correct)

How many actin filaments surround one myosin filament in a sarcomere?

<p>6 (B)</p> Signup and view all the answers

Which component reflects the myosin filament length within a sarcomere and does not change size during muscle contraction?

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

What happens to I bands during muscle contraction?

<p>Decrease in size (D)</p> Signup and view all the answers

Which of the following proteins is the largest known protein in muscle fibers?

<p>Titin (B)</p> Signup and view all the answers

What defines muscle contraction?

<p>Generation of force (A)</p> Signup and view all the answers

What is the primary component of skeletal muscle in adults by body weight percentage?

<p>Skeletal muscle = 40% (C)</p> Signup and view all the answers

Which of the following is NOT a function of skeletal muscle?

<p>Constricting blood vessels (B)</p> Signup and view all the answers

What is the role of ATP in the cross-bridge cycling of skeletal muscle?

<p>It provides energy for muscle contraction. (D)</p> Signup and view all the answers

What is the primary role of calcium in skeletal muscle contraction?

<p>It facilitates the activation of cross-bridges. (D)</p> Signup and view all the answers

What is one common type of muscular dystrophy mentioned?

<p>Duchenne Muscular dystrophy (C)</p> Signup and view all the answers

Which type of muscle is responsible for eye movement?

<p>Skeletal muscle (A)</p> Signup and view all the answers

What is thermogenesis in the context of skeletal muscle?

<p>Heat production during muscle contraction. (B)</p> Signup and view all the answers

What is the average diameter of a muscle fiber in skeletal muscle?

<p>100 μm (B)</p> Signup and view all the answers

What does muscle contraction refer to in the context of skeletal muscles?

<p>The activation of cross-bridges within muscle fibers (A)</p> Signup and view all the answers

What proteins compose the thick myofilaments in skeletal muscle?

<p>Myosin and troponin (C)</p> Signup and view all the answers

What role does calcium play in muscle contraction?

<p>It binds to troponin C, allowing myosin to bind to actin (C)</p> Signup and view all the answers

Which statement describes the role of tropomyosin in muscle contraction?

<p>It prevents myosin binding at rest (D)</p> Signup and view all the answers

What happens during rigor mortis in relation to muscle contraction?

<p>Cross-bridges cannot be broken due to the lack of ATP (B)</p> Signup and view all the answers

What is the purpose of the cross-bridge cycle in muscle fibers?

<p>To split ATP into ADP and Pi for energy (D)</p> Signup and view all the answers

Which of the following describes the interaction between myosin heads and actin during muscle contraction?

<p>About 50% of myosin heads make contact with actin at a time (C)</p> Signup and view all the answers

What prevents myosin from binding to actin in a relaxed muscle fiber?

<p>Low calcium ion concentrations (C)</p> Signup and view all the answers

Flashcards

Muscle Fiber

A single muscle cell, long and cylindrical, responsible for muscle contraction. It's covered by a plasma membrane called sarcolemma and contains multiple nuclei.

Myofibrils

Densely packed subunits within a muscle fiber, running its entire length. They are responsible for muscle contraction and are composed of thick and thin myofilaments.

Thick Myofilaments

Made of the protein myosin, responsible for pulling actin filaments during muscle contraction. They are located in the center of the sarcomere.

Thin Myofilaments

Made of the protein actin, they slide past myosin filaments during muscle contraction. They are located on either side of the thick filaments.

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Sarcomere

The fundamental contractile unit of a muscle fiber, containing both thick and thin filaments, arranged in an organized pattern. Each sarcomere is responsible for generating force.

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Striations

Alternating light (I bands) and dark (A bands) bands within a muscle fiber. They are caused by the arrangement of thick and thin myofilaments.

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Sliding Filament Theory

Explains muscle contraction as the sliding of thin actin filaments past thick myosin filaments, shortening the sarcomere, leading to muscle contraction.

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A Bands

The dark bands within a muscle fiber, representing the area where both actin and myosin filaments overlap. They remain the same size during contraction.

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Muscle Contraction Definition

Muscle contraction doesn't always mean shortening. It refers to activation of force-generating sites within muscle fibers, the cross-bridges. Holding a dumbbell steady involves contraction without shortening.

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What are myofilaments?

Myofilaments are the building blocks of muscle fibers. There are thin filaments composed of actin and thick filaments composed of myosin.

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What's special about myosin?

Myosin has globular heads containing binding sites for actin and ATP. These heads have ATPase activity, allowing them to break down ATP and use the energy for muscle contraction.

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What do tropomyosin and troponin do?

These proteins are part of the thin filament and act like a switch. They prevent myosin binding to actin at rest, ensuring the muscle is relaxed.

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How are cross-bridges formed?

When muscle cells receive a signal, calcium is released. It binds to troponin C, causing a shift in tropomyosin, exposing the actin binding sites. Myosin heads can then attach to actin, forming the cross-bridge.

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What is the cross-bridge cycle?

The cross-bridge cycle describes the repeated cycle of attachment, pulling, detachment, and re-attachment of myosin heads on actin. This movement allows the thin filaments to slide past the thick filaments, generating the muscle contraction.

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What happens in rigor mortis?

Rigor mortis occurs after death when ATP is depleted. Without ATP, myosin remains attached to actin in a fixed state, causing muscle stiffness.

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How much myosin interacts with actin?

Around 50% of myosin heads are attached to actin at any given time, forming the protein complex actomyosin. This continuous interaction ensures a smooth and powerful muscle contraction.

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What are the 3 main types of muscle?

Skeletal, cardiac, and smooth muscle. These types differ in structure, function, and control.

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What makes muscles 'excitable'?

Muscles are excitable because their membrane potential (electrical charge) is linked to their function, contraction in this case.

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What is the function of the diaphragm?

The diaphragm is a skeletal muscle that plays a key role in respiration, helping us breathe in and out.

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What are the 2 types of thermogenesis?

Shivering (muscle contraction) and non-shivering thermogenesis (brown fat) both generate heat.

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What is Duchenne Muscular Dystrophy?

A genetic disorder causing progressive muscle weakness due to a lack of a protein needed for muscle fiber stability.

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What is Myasthenia Gravis?

An autoimmune disease where antibodies attack acetylcholine receptors at the neuromuscular junction, leading to muscle weakness.

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What is Sarcopenia?

Muscle loss that happens naturally with aging, leading to decreased strength and function.

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How does skeletal muscle contribute to blood glucose regulation?

Skeletal muscle cells take up glucose from the bloodstream, especially when stimulated by insulin.

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Study Notes

Skeletal Muscle Structure and Function

  • Skeletal muscle is made of muscle fibers, averaging 100 µm in diameter and up to several centimeters long.
  • Muscle fibers have plasma membranes called sarcolemma.
  • Muscle fibers are multinucleated, containing multiple nuclei.
  • Muscle fibers are striated (striped), with alternating light (I bands) and dark (A bands) regions.
  • Myofibrils are densely packed subunits within muscle fibers.
  • Myofibrils are composed of thick (myosin) and thin (actin) myofilaments.
  • Myofilaments are organized into sarcomeres, the functional units of muscle contraction.
  • A sarcomere extends from Z-line to Z-line.
  • Sarcomeres contain thick and thin filaments that overlap to create the striations.
  • The I band contains only thin filaments, while the A band contains both thick and thin filaments.
  • The H zone is the central region of the A band where only thick filaments are present.
  • The M line is the center of the sarcomere, connecting the thick filaments.
  • Muscle fibers contain myofibrils arranged in a parallel manner, allowing for coordinated contraction.
  • Periosteum covers the bone, and tendons connect muscle to bone.

Skeletal Muscle Proteins

  • Contractile proteins: Myosin and Actin
  • Regulatory proteins: Troponin and Tropomyosin
  • Structural proteins: Titin, Nebulin, Alpha-actin, Myomesin, Dystrophin
  • Titin (3700 kDa) is the largest protein in the sarcomere and is responsible for providing elasticity.
  • Dystrophin (427 kDa) is vital for muscle cell stability.

Sliding Filament Mechanism

  • Muscle contraction is not necessarily shortening; it's the activation of force-generating sites within muscle fibers (cross-bridges).
  • During muscle contraction, sarcomeres shorten.
  • The I band shortens.
  • The H zone shortens.
  • The A band remains the same length.
  • During the sliding filament mechanism, thin filaments slide towards the H zone.

Cross-Bridge Cycle

  • Myosin heads act as ATPase enzymes, splitting ATP into ADP and phosphate.
  • This allows the myosin head to bind to actin when the muscle is stimulated.
  • The myosin head then undergoes a power stroke, pulling the thin filament.
  • The myosin head releases ADP and phosphate, returning to its original conformation.
  • A new ATP molecule binds to the myosin head, causing detachment from actin.

Regulation of Contraction

  • Tropomyosin physically blocks cross-bridges.
  • Troponin complex:
    • Troponin I inhibits myosin binding.
    • Troponin T binds to tropomyosin.
    • Troponin C binds to calcium.
  • Calcium is released inside muscle fiber when stimulated.
  • Calcium binding to troponin C causes a conformational change in troponin and tropomyosin.
  • This allows myosin access to form cross-bridges.

Role of ATP and Calcium

  • ATP is essential for the detachment of myosin from actin.
  • Calcium is crucial for initiating the cross-bridge cycle.
  • When calcium levels are low, tropomyosin blocks actin's binding sites, preventing myosin from binding and stopping contraction.

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MD137 Muscle Lecture 1 PDF

Description

Explore the intricate structure and function of skeletal muscle in this quiz. Discover the roles of muscle fibers, myofibrils, and the organization of sarcomeres in muscle contraction. Test your knowledge on the components that make skeletal muscle unique!

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