Questions and Answers
What is the length of a sarcomere in resting muscle?
Which protein binds to Ca2+ during muscle contraction?
What characterizes the A band within a sarcomere?
What is the core structural feature of thin filaments in skeletal muscle?
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Which component of myosin is primarily involved in forming cross-bridges with actin filaments?
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What is the role of myomesin located at the M line in muscle fibers?
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Why do I bands stain more lightly compared to A bands in muscle tissue?
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Which statement about titin is correct?
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What is the primary function of creatine kinase in muscle tissue?
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Which protein is largest in the body and provides support to thick myofilaments?
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Study Notes
Organization of Muscle Fibers
- Skeletal muscle fibers show cross-striations consisting of alternating light (I bands) and dark (A bands) bands.
- A bands are anisotropic (birefringent under polarized light), while I bands are isotropic (do not alter polarized light).
- Each I band is bisected by a Z disc, visible in Transmission Electron Microscopy (TEM).
Sarcomere Structure
- The basic functional unit of muscle contraction is the sarcomere, spanning from Z disc to Z disc, measuring approximately 2.5 μm in length in resting muscle.
- The arrangement of thick (myosin) and thin (actin) myofilaments produces the distinct A and I banding patterns.
Myosin Filaments
- Thick myosin filaments are 1.6 μm long and 15 nm wide, located in the A band at the sarcomere's center.
- Myosin is a complex protein (500 kDa) comprised of two heavy chains and two pairs of light chains; heavy chains form long, rod-like tails.
- Myosin heads are globular projections that bind to actin and ATP, facilitating energy transfer during contraction.
Actin Filaments
- Thin actin filaments are 1.0 μm long and 8 nm wide, situated between thick filaments.
- Each G-actin monomer has a binding site for myosin and is anchored to the Z disc by α-actinin.
Regulatory Proteins
- Thin filaments interact with two key regulatory proteins:
- Tropomyosin: A 40-nm-long coil of two polypeptide chains that resides in the grooves of twisted actin strands.
- Troponin: A complex with three subunits—TnT (attaches to tropomyosin), TnC (binds Ca²⁺), and Tnl (regulates actin-myosin interaction). Troponin complexes are regularly spaced along tropomyosin.
A Band and H Zone
- A bands consist of thick filaments and overlapping thin filaments, with a central lighter zone termed the H zone, showcasing only the myosin rod portions.
- The M line, found at the center of the H zone, comprises myosin-binding protein myomesin which stabilizes thick filaments and contains creatine kinase for ATP supply during contraction.
Protein Composition
- Myosin and actin collectively account for over half of the total protein content in striated muscle.
- I bands, bisected by Z discs, consist of thin filament portions that do not overlap thick filaments, causing lighter staining.
- Titin: The largest protein in the body (3700 kDa), exhibits elastic properties and connects thick filaments to Z discs.
- Nebulin: A large accessory protein (600-900 kDa), binds thin myofilaments laterally to help anchor them to α-actinin.
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Description
Explore the intricate structure of muscle fibers, focusing on the role of troponin complexes and their attachment to tropomyosin molecules. Understand the significance of the A band, H zone, and M line in muscle contraction mechanics.
