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Questions and Answers
What characterizes a motor unit?
Which factor primarily influences the graded increase in muscle contraction strength?
What occurs during tetanus in skeletal muscle?
How does the length-tension relationship affect contraction strength?
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Which best describes the mechanism of force variation in skeletal muscle?
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What limitation exists when sarcomeres are overstretched?
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What is the result of recruiting larger motor units first during muscle contraction?
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Which muscle type does NOT rely on action potentials generated from autonomic nerves for contraction?
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What happens to muscle units during prolonged contractions?
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Which of the following best differentiates smooth muscle from skeletal and cardiac muscle?
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Study Notes
Muscle Components
- Each muscle fiber contains multiple myofibrils, which are composed of repeating units called sarcomeres.
Muscle Strength and Contraction
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Length-Tension Relationship: The force a muscle generates during contraction is directly related to its resting length.
- Optimal Resting Length: There is an optimal resting length for the sarcomere where maximum strength of contraction can be achieved.
- Overstretched Sarcomere: When a sarcomere is overstretched, there is no overlap between actin and myosin filaments, leading to reduced tension.
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Force/Strength of Contraction: The force of muscle contraction can be varied through two mechanisms:
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Tetanus: When a muscle is stimulated repeatedly at a high frequency, it leads to a sustained contraction known as tetanus.
- Tetanus is caused by temporal summation of action potentials, which results in an increase in intracellular calcium concentration.
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Motor Unit Recruitment: A motor unit consists of a motor neuron and all the muscle fibers it innervates.
- The size of a motor unit determines the precision of control over a muscle.
- Smaller motor units are recruited first to generate smaller forces, while larger motor units are recruited as more force is needed.
- During prolonged contractions, motor units are recruited in a rotating pattern, allowing some units to rest while others contract, delaying fatigue.
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Tetanus: When a muscle is stimulated repeatedly at a high frequency, it leads to a sustained contraction known as tetanus.
Cardiac Muscle Contraction
- The cardiac muscle action potential has a prolonged plateau phase, which ensures a long duration of contraction, allowing for the efficient pumping of blood.
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Mechanism of Contraction:
- Cardiac muscle contraction is initiated by calcium influx from the extracellular space.
- Calcium binds to troponin, triggering the exposure of binding sites on actin filaments.
- Myosin heads attach to actin, causing sliding of filaments and muscle contraction.
- Relaxation occurs when calcium is pumped back out of the cell.
Smooth Muscle Contraction
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Mechanism of Contraction:
- Smooth muscle contraction is initiated by calcium influx from the extracellular space or internal stores.
- Calcium binds to calmodulin, activating myosin light chain kinase (MLCK).
- MLCK phosphorylates myosin, allowing it to bind to actin and initiate the sliding filament mechanism.
- Relaxation occurs when calcium is pumped back into storage and MLCK activity decreases.
Differences in Muscle Types
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Skeletal Muscle:
- Voluntary control
- Striated appearance due to sarcomere organization
- Fast contraction speed
- Fatigue prone
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Cardiac Muscle:
- Involuntary control
- Striated appearance
- Intermediate contraction speed
- Fatigue resistant
-
Smooth Muscle:
- Involuntary control
- Non-striated appearance
- Slow contraction speed
- Fatigue resistant
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Description
Test your understanding of muscle components and the mechanics of muscle contraction. This quiz covers key concepts such as the length-tension relationship, optimal resting length, and the mechanisms behind tetanus and motor unit recruitment. Enhance your knowledge of how muscles generate force and perform actions.
