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Questions and Answers
What is the primary role of troponin in muscle contraction?
What is the primary role of troponin in muscle contraction?
Which structure moves during muscle contraction according to the sliding filament theory?
Which structure moves during muscle contraction according to the sliding filament theory?
What role do calcium ions play in muscle contraction?
What role do calcium ions play in muscle contraction?
Which component of a motor unit connects to muscle fibers?
Which component of a motor unit connects to muscle fibers?
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What is the primary function of tropomyosin in muscle fibers?
What is the primary function of tropomyosin in muscle fibers?
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What initiates the contraction process in muscle fibers?
What initiates the contraction process in muscle fibers?
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What role does troponin play during muscle contraction?
What role does troponin play during muscle contraction?
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During muscle contraction, which event occurs after the release of ADP and Pi from the myosin head?
During muscle contraction, which event occurs after the release of ADP and Pi from the myosin head?
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Which of the following statements about the sliding filament theory is true?
Which of the following statements about the sliding filament theory is true?
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What is the primary function of calcium ions in muscle contraction?
What is the primary function of calcium ions in muscle contraction?
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Study Notes
Mechanism of Muscle Contraction/Sliding Filament Theory
- Muscle fiber contraction occurs through the sliding of thin filaments over thick filaments.
- A motor neuron and the muscle fibers it connects to make up a motor unit.
- Skeletal muscles are primarily involved in locomotion and body posture changes.
Myofilaments and Structure of Contractile Proteins
- Each myofibril has dark and light bands due to the distribution of actin and myosin, giving it a striated appearance.
Thin/Actin Filament
- Bisected by the Z-line.
- Composed of contractile protein actin.
- Actin is a globular protein ('G'-actin) that polymerizes into a helical arrangement called F-actin.
- Tropomyosin runs along the length of F-actin.
- Troponin is distributed at regular intervals on tropomyosin.
Thick/Myosin Filament
- Held by the M-line.
- Composed of the contractile protein myosin.
- Myosin has a fibrous membrane and a globular head (HMM - Heavy Meromyosin).
- The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
- The globular head projects outwards from each other at regular distances and angles from the surface of polymerized myosin filament.
- This projecting globular head is known as the 'cross arm'.
- Myosin has a tail (LMM – Light Meromyosin).
- Troponin masks the active binding sites on actin for myosin.
Motor Neuron Activity
- The central nervous system (CNS) via a motor neuron stimulates muscle contraction through the release of the neurotransmitter acetylcholine.
Muscle Contraction Process
- At the neuromuscular junction, an action potential in the motor end plate stimulates the release of Calcium ions (Ca2+) from the sarcoplasmic reticulum (SR) into the sarcoplasm.
- This increase in Ca2+ concentration in the sarcoplasm causes Ca2+ ions to bind to troponin, leading to a conformational change.
- This change in conformation exposes the active sites on the actin filament for myosin binding.
- Energetic myosin (myosin-ADP + Pi) binds to actin forming a cross bridge (actin-myosin-ADP + Pi).
- This binding pulls the thin filaments towards the center, shortening the sarcomere, bringing Z-lines closer together, and reducing the length of the I-band. The length of the A-band remains consistent.
- ADP + Pi are released from the myosin head.
- A new ATP molecule binds to the myosin head, breaking the cross bridge.
- ATP hydrolysis occurs on the myosin head.
- The cycle repeats.
- This process continues as long as Ca2+ ions remain in the sarcoplasm.
- Once Ca2+ is pumped back into the sarcoplasmic cisternae, the Z-lines will return to their original position.
- The reaction time of muscle fibers varies depending on the muscle.
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Description
Explore the mechanism of muscle contraction through the sliding filament theory. Understand the structure and roles of actin and myosin in muscle fibers, and learn about motor units and their function in locomotion and posture. This quiz covers the essential concepts and components involved in skeletal muscle contraction.
