BIO 1130 Lecture Notes - Lecture 2: Tetanus, Cellular Respiration, Sarcolemma

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7 Aug 2016
ANP1105 – Lecture 2
Ultrastructure & Molecular Composition of Myofilaments:
--> Thick (myosin) filaments (16 nm)_
- Rod like tail (2 heavy chains) + 2 globular heads (ends of heavy chains + 4 light chains
= cross bridges)
- Cross bridges are the business ends of the thick filaments_
- Each thick filaments = -200 myosins; arrayed so central part smooth & each and
studded with many myosin heads_
- Myosin heads contain ATPase
--> Thin filaments (7-8 nm)
- Binding sites for myosin heads (cross bridges)
- Strands are F-actin; 2 strands wound as a helix
- Tropomyosin: 2 strands
- Stiffen filament
- Blocks cross bridge bonding
- Troponin: 3-polypeptide complex
Sarcoplasmic Reticulum:
• Elaborate web for ER around each myofibril inside PM_
• Most SR tubules run longitudinally; pairs of terminal cisternae across A-I junctions_
• Role: regulate intracellular Ca++ storage depot. Releases it when muscle stimulated to
- At A-I junction sarcolemma penetrates cytoplasm to form hollow elongated tubes (T-
tubules = Transverse Tubules)
- Triad: group of 3 structures: 2 terminal cisternae + 1 T-tubule in the middle
- T-System: thousands (2/sarcomere) of T-Tubules in a single muscle cell_
- Role: increase surface area
• Muscle fiber shortens because sarcomeres shorten; filaments remain same length
• Think filaments slide over thick filaments_
- Relaxed: only slight overlap of thick and thin filaments_
- Contracted: think filaments penetrate more deeply into A band - Z discs pulled toward
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