Motor Units and Force Generation
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Questions and Answers

Which of the following best characterizes a motor unit?

  • A single muscle fiber and all the neurons that innervate it.
  • A group of muscle fibers that contract independently.
  • A sensory neuron and the muscle fibers it stimulates.
  • An anterior motor neuron and all the specific muscle fibers it innervates. (correct)

    • What determines the degree of precision in a muscle's movement?

  • The size of the muscle itself.
  • The speed at which the muscle contracts.
  • The number of motor neurons in the muscle.
  • The ratio of muscle fibers to motor neurons. (correct)

    • What happens when a motor neuron receives a stimulus strong enough to trigger an action potential?

  • The accompanying muscle fibers in the motor unit contract asynchronously.
  • All of the accompanying muscle fibers in the motor unit contract synchronously. (correct)
  • Only some of the muscle fibers in the motor unit contract.
  • The action potential spreads to other motor units.

    • Why can't a stronger action potential produce a stronger contraction?

    <p>Because motor units exert force according to the 'all or none' principle. (D)</p> Signup and view all the answers

    What immediately determines the amount of force generated in a single muscle fiber?

    <p>The number of crossbridges formed. (D)</p> Signup and view all the answers

    Which of the following is NOT a key factor in the force generation of a group of muscles?

    <p>The size of the individual muscle fibers. (A)</p> Signup and view all the answers

    How does repetitive stimuli influence muscle tension?

    <p>Repetitive stimuli that reach a muscle before it relaxes increases the total tension. (B)</p> Signup and view all the answers

    Which type of motor unit will typically generate greater force?

    <p>Larger, Type II motor units. (C)</p> Signup and view all the answers

    Which of the following best describes the force production during static (isometric) muscle action?

    <p>Myosin cross-bridges form and recycle, but there is no sliding of filaments. (C)</p> Signup and view all the answers

    What is the primary characteristic of isokinetic contractions?

    <p>The muscle changes length at a constant rate of speed. (B)</p> Signup and view all the answers

    Which of the following is true regarding eccentric muscle actions?

    <p>Cross-bridges form, but the sarcomere lengthens. (B)</p> Signup and view all the answers

    Which of the following is a characteristic of dynamic muscle action?

    <p>It includes both isotonic and isokinetic actions. (A)</p> Signup and view all the answers

    What happens when the frequency of stimuli to a muscle fiber increases?

    <p>Fibers do not have time to relax, leading to summation and increased force production. (C)</p> Signup and view all the answers

    In concentric muscle actions, what occurs at the sarcomere level?

    <p>Sarcomeres shorten as filaments slide toward the center. (C)</p> Signup and view all the answers

    Which type of muscle action involves a muscle producing force without a change in muscle length?

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

    Consider a scenario where you are slowly lowering a heavy box to the floor. Which type of muscle action is primarily involved in controlling the descent?

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

    Flashcards

    Sustained Contractions

    Continuous muscle contractions that produce force without complete relaxation.

    Static Muscle Action

    Muscle produces force without changing its length or joint angle.

    Isotonic Contraction

    Muscle action where the force remains constant while muscle changes length.

    Isokinetic Contraction

    Muscle action where the speed of movement remains constant throughout the contraction.

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    Dynamic Muscle Action

    Muscle action involving changes in length and joint angle, including isotonic and isokinetic actions.

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    Concentric Contraction

    Type of contraction where muscle shortens while producing force.

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    Eccentric Muscle Action

    Muscle lengthens while producing force, often against gravity.

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    Force-Velocity Relationship

    Describes how the force a muscle can produce varies with its contraction speed.

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    Motor unit

    A motor unit consists of a motor neuron and the muscle fibers it innervates.

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    Muscle fiber innervation

    Each muscle fiber typically receives input from one neuron, but one neuron can control many fibers.

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    Precision in muscle function

    Muscles needing precision have fewer fibers per motor neuron compared to less precise muscles.

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    Action potential in motor units

    A strong enough stimulus activates all muscle fibers in a motor unit to contract together.

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    Force gradation

    Motor units do not create force gradation; an impulse either triggers contraction or not.

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    Force generation factors

    Five factors influence muscle force: motor units recruited, frequency of discharge, type of motor unit, preloading, and speed of contraction.

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    Motor unit recruitment

    Muscles generate more force with activation of all their motor units.

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    Type II motor units

    Larger motor units (Type II) produce greater force compared to smaller units.

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    Study Notes

    Motor Units

    • Motor units consist of anterior motor neurons and specific muscle fibers they innervate.
    • Each muscle fiber usually receives input from only one motor neuron.
    • One motor neuron can innervate many muscle fibers.
    • The number of muscle fibers per motor neuron relates to the muscle's function (precision).
    • Muscles needing less precision might have hundreds of fibers per neuron, while those needing more precision might have only one.

    All-or-None Principle

    • A stimulus strong enough to trigger an action potential in a motor neuron activates all fibers in the motor unit at once.
    • The motor unit either contracts fully or not at all.
    • There is no partial activation for motor units.
    • A stronger stimulus does not affect the force generated.

    Gradation of Force (Acute)

    • Force generation in a group of muscles depends on these factors:
      • Number of motor units recruited: More recruited units, more force. All units working together create maximal strength.
      • Frequency of motor unit discharge: Higher frequency, more tension as muscle does not fully relax, builds up tension.
      • Type of motor units recruited: Larger/Type II motor units generate greater strength.
      • Preloading the muscle: Activating stretch reflex to build force.
      • Speed of contraction: Different speeds generate optimal force and power output.

    Summation of Force

    • Normal body movement involves sustained contractions, not simple twitches.
    • Rapid stimuli do not allow time for fibers to relax and thus summate into higher tension for more forceful movements.
    • More frequent stimuli leads to summation of force.

    Muscle Actions

    • Static (isometric): Muscle produces force, but length doesn't change, joint angle does not change. Examples: Holding a heavy weight, static plank.
    • Isotonic: Muscle produces force AND changes length; joint angle changes.
      • Concentric: Muscle shortens; force generation while shortening (e.g., bicep curl).
      • Eccentric: Muscle lengthens, force generation while lengthening; e.g., lowering a weight during a bicep curl.
    • Isokinetic: Movement speed remains constant, but force can vary; involves specialized equipment.

    Force-Velocity Curve

    • The force-velocity curve is a graphical representation showing the relationship of force generated against velocity of muscle shortening/lengthening. An important way to visualize and understand the concept of concentric/concentric/isometric muscle force generation.

    Review Questions

    • Five ways in which muscles increase force output
    • Differentiating types of muscle actions
    • Relationship between force & velocity of contraction: What speed = max force/power?
    • Force generation/contraction abilities for concentric/eccentric/isometric muscle actions.

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    Related Documents

    Motor Units PDF

    Description

    Explore the essential concepts of motor units, including the relationship between motor neurons and muscle fibers, the all-or-none principle, and how force generation varies with the recruitment of motor units. This quiz will help solidify your understanding of muscle function and the mechanics behind muscle contractions.

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