Muscles Part 1

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Questions and Answers

What is primarily responsible for muscle contraction in skeletal muscle cells?

  • Myofibrils (correct)
  • Z disks
  • Actin filaments
  • Troponin

What constitutes the dark bands known as A bands in muscle fibers?

  • Regions occupied by thin filaments
  • Regions occupied by both myosin and actin filaments
  • Interdigitating filaments
  • Regions occupied solely by thick filaments (correct)

Which proteins are responsible for the regulatory function of thin filaments in skeletal muscle?

  • G-actin and F-actin
  • Calcium ions and actin
  • Troponin and tropomyosin (correct)
  • Myosin and actin

What is the role of Troponin I in muscle contraction?

<p>To bind to actin and inhibit contraction (A)</p>
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What initiates the release of Ca2+ from the sarcoplasmic reticulum in muscle fibers?

<p>Depolarization of the T-tubule membrane (B)</p>
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What defines a sarcomere in skeletal muscle?

<p>The repeating unit between adjacent Z disks (B)</p>
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How do myofibrils contribute to muscle cell function?

<p>By facilitating muscle contraction directly (C)</p>
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What is the function of the Z disk in a myofibril?

<p>To serve as an attachment point for actin filaments (C)</p>
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What type of assembly do thick filaments form in muscle fibers?

<p>Bipolar assemblies of multiple myosin molecules (B)</p>
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Which of the following structures is critical for action potential propagation in muscle fibers?

<p>Transverse tubule network (C)</p>
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What is the main factor that determines the amplitude of skeletal muscle contraction force?

<p>The number of motor units excited (A)</p>
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What occurs during the unfused (incomplete) tetanus in muscle fibers?

<p>The stimulation rate allows the fiber to relax fully between stimuli. (C)</p>
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Which type of motor unit is characterized by having high excitability and a small number of myofibers innervated?

<p>Type I (A)</p>
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What is the term used for the process of increasing muscle force through the recruitment of additional motor units?

<p>Multiple-fiber summation (D)</p>
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How does the frequency of muscle cell stimulation affect muscle contraction?

<p>Tension can summate with increased frequency of action potentials. (D)</p>
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In how many phases does an auxotonic contraction occur?

<p>Two phases (C)</p>
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What is the relationship between muscle fiber length and tension development?

<p>Initial fiber length determines the degree of overlap between actin and myosin filaments. (D)</p>
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What type of muscle contraction is referred to as isometric?

<p>Tension increases without muscle shortening (C)</p>
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Which of the following correctly describes fused tetanus?

<p>The muscle fiber exhibits a steady tension without relaxation. (C)</p>
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Which type of muscle fibers are primarily associated with Type II motor neurons?

<p>Mainly white / glycolytic myofibers (C)</p>
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What describes the effect of changing the frequency of muscle stimulation?

<p>It can lead to variations in tension through frequency summation. (C)</p>
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Which statement about the sliding filament theory is correct?

<p>Actin filaments pull the myosin filaments toward the sarcomere's center. (B)</p>
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What is a common characteristic of muscle fatigue?

<p>Temporary decline in muscle force generation (C)</p>
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What distinguishes isotonic contractions from isometric contractions?

<p>Isotonic contractions involve length change during tension development (D)</p>
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The term 'single muscle twitch' refers to what?

<p>A response from the muscle fiber to a single action potential. (A)</p>
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Which statement is true regarding pure isotonic contractions in the human body?

<p>They occur never in the human body (C)</p>
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In which condition does peak tension resemble a straight line on the tension graph?

<p>During fused tetanus. (D)</p>
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Which of the following best describes Type I motor units?

<p>They respond to low intensity activities due to high excitability (C)</p>
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Which factor does NOT influence the force of muscle contraction?

<p>The size of the muscle fiber. (A)</p>
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What is the primary characteristic of unfused (incomplete) tetanus in muscle fibers?

<p>The muscle fibers partially relax between stimuli. (A)</p>
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How does the initial length of a muscle fiber affect its ability to generate force?

<p>The initial length determines the degree of actin-myosin overlap, influencing tension development. (A)</p>
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What type of summation occurs when multiple action potentials occur close enough in time to increase tension beyond a single muscle twitch?

<p>Frequency summation (D)</p>
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Under what condition does fused tetanus occur in muscle fibers?

<p>When stimulation rate is high enough for continuous contraction without relaxation. (D)</p>
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Which of the following best describes the sliding filament theory?

<p>Actin filaments slide over myosin filaments, contributing to muscle contraction. (D)</p>
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What occurs during a single muscle twitch in isolated muscle fibers?

<p>A brief contraction in response to a single action potential. (B)</p>
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Which statement correctly describes the tension developed by individual muscle fibers?

<p>It is directly influenced by the initial length of the muscle fiber. (C)</p>
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What characterizes the peaks in the tension trace during unfused tetanus?

<p>The peaks fluctuate due to the individual muscle twitches present. (D)</p>
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What results in increased muscle contraction force through the recruitment of additional motor units?

<p>Each motor unit recruited contributes equally to the force generated. (A)</p>
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Which statement about muscle contraction is true regarding action potentials?

<p>Higher rates of action potentials directly result in increased muscle tension. (A)</p>
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What is the primary structural component of myofibrils responsible for muscle contraction?

<p>Myofilaments (A)</p>
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What defines the repeating unit of a myofibril?

<p>Sarcomeres (D)</p>
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What role does Troponin C play in muscle contraction?

<p>Binds to Ca2+ (C)</p>
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What causes the propagation of action potentials into muscle fibers?

<p>Depolarization of T-tubules (C)</p>
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What is the function of tropomyosin in muscle cells?

<p>Regulates interactions of actin and myosin (A)</p>
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What structural feature is visible as a dark line at the center of the I band?

<p>Z disk (B)</p>
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What initiates the release of Ca2+ from the sarcoplasmic reticulum?

<p>Action potentials in the transverse tubules (A)</p>
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What are the two types of filaments found within myofibrils?

<p>Thick and thin filaments (D)</p>
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What is the result of acetylcholine signaling at the neuromuscular junction?

<p>Conversion to electrical signals (A)</p>
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Which component of the muscle cell acts as a storage for calcium ions?

<p>Sarcoplasmic reticulum (C)</p>
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What best describes the concept of multiple-fiber summation in muscle contraction?

<p>It is the increase in tension through the contraction of multiple motor units. (C)</p>
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Which statement about the characteristics of Type II motor units is inaccurate?

<p>They are associated with slow-twitch muscle fibers. (A)</p>
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In an auxotonic contraction, which sequence of events is most accurate?

<p>Tension is consistent while the muscle shortens. (B)</p>
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What triggers muscle fatigue during physical activity?

<p>Temporary decline in muscle's capacity to generate force. (C)</p>
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Which factor does NOT affect the degree of muscle tension during contraction?

<p>The type of myofibers predominately present in the muscle. (C)</p>
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Which of the following is true regarding isometric contractions?

<p>They are characterized by a constant length despite tension change. (D)</p>
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What distinguishes the conduction characteristics of small motor neurons from larger motor neurons?

<p>They conduct impulses at a slower speed than larger neurons. (C)</p>
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In terms of muscle contraction phases, which statement is most accurate?

<p>Auxotonic contractions are the most prevalent type of skeletal muscle contraction. (A)</p>
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Which best describes the firing thresholds of motor neurons in Type I versus Type II motor units?

<p>Type I neurons fire at lower thresholds compared to Type II neurons. (C)</p>
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What is an accurate description of isometric contraction relative to muscle dynamics?

<p>It triggers an increase in internal muscle tension while length does not change. (B)</p>
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Flashcards

Myofibril

A cylindrical element within a muscle cell, responsible for contraction and comprising smaller myofilaments.

Myofilament

A smaller filament in a myofibril, consisting of thick and thin filaments.

Thick Filament

A filament in a myofibril composed of myosin molecules.

Thin Filament

A filament in a myofibril composed of actin, tropomyosin, and troponin.

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Sarcomere

Functional unit of a myofibril, the repeating segment between two Z discs.

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I band

The light band in a myofibril, containing only thin filaments.

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A band

The dark band in a myofibril, containing both thin and thick filaments.

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Z disc

A dark perpendicular line in the center of the I band, dividing sarcomeres.

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Neuromuscular junction

The point where a motor neuron and muscle fiber meet, initiating muscle contraction.

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Action potential

An electrical signal that travels along the muscle fiber, triggering muscle contraction.

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Sliding Filament Theory

The myosin cross-bridges pull actin filaments towards the center of the sarcomere, making it appear as if the filaments slide past each other during muscle contraction.

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Muscle Twitch

A single muscle contraction caused by a single action potential in a muscle fiber.

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Frequency Summation

Increasing the frequency of action potentials in a muscle fiber to increase the tension developed during muscle contraction.

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Unfused Tetanus

Muscle contraction where the stimulation rate is not fast enough for complete relaxation between stimuli, resulting in a bumpy graph.

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Fused Tetanus

Muscle contraction where the stimulation rate is fast enough to prevent relaxation, leading to constant maximum tension.

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Muscle Fiber Length

The length of a muscle fiber influences the tension it can develop.

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Motor Unit

A motor neuron and all the muscle fibers it controls.

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Calcium Ions

Essential for the muscle contraction process.

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Tension Development

The force a muscle fiber can generate.

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Multiple-Fiber Summation

Increasing muscle force by activating more motor units.

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Isometric Contraction

Muscle tension increases but there's no shortening.

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Isotonic Contraction

Muscle shortens while maintaining constant tension.

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Auxotonic Contraction

A type of skeletal muscle contraction that involves both isometric and isotonic phases.

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Muscle Fatigue

A temporary reduction in a muscle's ability to generate force, due to previous exertion.

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Small Motor Neurons

Motor neurons with high excitability and fast conduction speed, innervating primarily red/oxidative muscle fibers.

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Large Motor Neurons

Motor neurons with low excitability and very fast conduction speed, innervating primarily white/glycolytic muscle fibers.

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Red/Oxidative Myofibers

Muscle fibers specialized for endurance activities, using oxygen.

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White/Glycolytic Myofibers

Muscle fibers specialized for short bursts of intense activity, not relying heavily on oxygen.

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Muscle Fiber Length and Tension

The initial length of a muscle fiber influences the amount of tension it can generate.

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How Does Actin and Myosin Overlap Affect Tension?

The amount of overlap between actin and myosin filaments determines the tension a muscle fiber can generate.

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What is a Single Muscle Twitch?

A single contraction of a muscle fiber caused by a single action potential.

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How are Motor Units Involved in Regulating Muscle Force?

Varying the number of activated motor units within a muscle controls the force of contraction.

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What is the Relationship Between Stimulation Frequency and Tension?

The maximal tension a muscle fiber can develop is directly proportional to the frequency of action potentials in the fiber.

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What Does a Muscle Contraction Graph Look Like?

A muscle contraction graph shows the tension developed over time, with different shapes representing different types of contractions.

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Spatial Summation

Another term for Multiple-Fiber Summation, where increased force is achieved by engaging more motor units.

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Myosin

A protein that forms thick filaments in muscle fibers. It has a head that binds to actin and a tail that interacts with other myosin molecules.

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Actin

A protein that forms thin filaments in muscle fibers. Myosin heads bind to actin, initiating contraction.

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Troponin

A protein complex that controls the interaction of actin and myosin. Binds to calcium ions, initiating contraction.

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Tropomyosin

A protein that wraps along actin and blocks myosin binding sites when the muscle is relaxed.

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Study Notes

Muscles Part 1

  • Skeletal muscles are responsible for movement.
  • Muscle tissue consists of many layers with different connective tissue components.
  • Connective tissue components of muscle tissue include: epimysium, perimysium, and endomysium.
  • Myofibrils are cylindrical elements within muscle cells.
  • Myofibrils account for approximately 80% of the intracellular volume.
  • Myofibrils are made of myofilaments.
  • Myofilaments are composed of thick filaments (myosin) and thin filaments (actin).
  • A sarcomere is the functional unit of a myofibril.
  • A sarcomere runs from one Z line to another.
  • The sliding filament theory describes how muscle contraction occurs.
  • During contraction, myosin filaments pull on actin filaments, causing them to slide past each other.
  • The Z line is where the thin filaments attach.
  • The H zone is located in the center of the sarcomere.
  • The I band contains only thin filaments.
  • The A band contains both thick and thin filaments.
  • Thick filaments are bipolar assemblies of multiple myosin molecules.
  • A myosin molecule has a tail region and two heads.
  • Thin filaments are composed of G-actin arranged in a helical filament called F-actin, along with tropomyosin and troponin.
  • Troponin and tropomyosin are regulatory proteins and govern muscle contraction.
  • The T-tubules allow action potentials to propagate deep within muscle fiber.
  • Sarcoplasmic reticulum stores and releases calcium ions.
  • Muscle contraction begins at the neuromuscular junction.
  • Acetylcholine is a neurotransmitter involved in muscle contraction and it is released from the motor neuron to transmit the signal to the muscle fiber.
  • The conversion of an acetylcholine signal into an electrical signal in a muscle is essential for muscle contraction.
  • Depolarization of the T-tubule membrane causes calcium release from the sarcoplasmic reticulum.
  • Cross-bridge cycling is the process by which myosin heads bind to and pull on actin filaments, creating muscle contractions.
  • The sliding filament theory describes this process.
  • Muscle force can be regulated by stimulating frequency, initial length of a muscle fiber, and varying the number of motor units excited in the muscle.
  • Unfused (incomplete) tetanus occurs when stimulation rate is not high enough for complete relaxation. Twitch components are still visible..
  • Fused (complete) tetanus occurs at a high stimulation rate, where no relaxation occurs and the tension remains constant.
  • The force developed in a muscle fiber is proportional to the rate of action potentials.
  • Muscle fatigue is a temporary decline in muscle's ability to generate force due to previous activity.
  • Causes of muscle fatigue include: depletion of energy stores and accumulation of lactic acid.
  • Other factors in muscle fatigue: nervous system fatigue, and end plate fatigue.

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