Muscle contraction Flashcards
What is the plasma membrane of skeletal muscle called?
Sarcolemma
What is the cytoplasm of skeletal muscle called?
Sarcoplasm
What is the endoplasmic reticulum of skeletal muscle called?
Sarcoplasmic reticulum
What is the mitochondria of skeletal muscle called?
Sarcosomes
Muscle fibre organisation full (large - small)
Muscle fibre - myofibrils - repeating units of sarcomeres - regular arrays of thick and thin filaments
What is the diameter of muscle fibre?
> 100µM
What is the diameter of myofibrils?
1µm
What is the diameter of thick filaments?
18nm
What is the diameter of thin filaments?
5-8nm
What is the length of thick filaments?
1.6µm
What is the length of thin filaments?
1µm
What area is the sarcomere?
The area between Z-lines
What is the H-zone?
Central thick filament i.e. non-overlapped thin filament
What is the A band?
Thick filaments, overlapping with thin filaments at end
What is the I band?
Thin (actin) filament (non-overlapped) - terminates at Z line
What is the resting length of I band?
2.5µm
Thin filament is composed of …
Actin
Thick filament is composed of …
Myosin
How many thin filaments can a single thick filament cross bridge to?
Six
What are the contractile proteins of the sarcomere?
- Actin
- Myosin
What are the structural proteins of the sarcomere?
- Titin
- Dystrophin
What are the regulatory proteins of the sarcomere?
- Troponin-C
- Tropomyosin
How many myosin molecules are on thick filaments?
300
What are the two sites on the heads of the thick filament (myosin)?
- ATPase site
- Actin binding site
What is the molecular weight of the 2 identical chains of the thick filament?
200,000 kD
What is the percent of central bare zone in thick filaments?
10
What is the molecular weight of G-actin?
42,000
What does each G-actin molecule have?
A myosin binding site
What type of protein is G-actin?
A globular protein
What is thin filament connected through Z-line by?
Alpha-actinin
What is F-actin (relative to G-actin)?
An alpha-helical polymer
What is the thin filament surrounded by?
Surrounded by regulatory proteins
- Tropomyosin
- Troponin complex
Tn-C, Tn-T, Tn-I
What is the function of the structural proteins?
- Maintain sarcomere/myofibril structure
- Provide framework for myofilament arrangement
What is the function of titin?
- Template for myosin assembly
- Stability at long sarcomere lengths
- Helps prevent sarcomere popping & Z-line streaming
Where does titin run from?
Z-line to M-line (1mm)
What does tropomyosin span?
Spans 7 G-actin monomers
What is the function of tropomyosin at rest?
To block myosin binding sites on actin
How many troponin complex per tropomyosin, and what do they control?
1 per tropomyosin - controls 7 G-actin monomers
What is the function of troponin C?
Binds to Ca2+ to produce a conformational change in TnI
What is the function of troponin T?
Binds to tropomyosin, to form a troponin-tropomyosin complex
What is the function of troponin I?
Binds to actin to hold the troponin-tropomyosin complex in place
What occurs when troponin receives Ca2+?
Destabilisation of tropomyosin actin binding - Cross Bridge Formation
What occurs when troponin is lacking Ca2+?
Stabilisation of tropomyosin actin binding
What is the basic cycle of cross bridging and sarcomere shortening?
1) Actin-myosin binding
2) Power stroke
3) Detachment
4) Binding
Give the steps of what occurs in cross bridging and sarcomere shortening.
- Myosin cross bridge binds to molecule
- Cross bridge bends, pulling thin myofilament inward
- Cross bridge detaches at end of power stroke and returns to original conformation.
- Cross bridge binds to move distal actin molecule; cycle repeats.
What happens to the H zone during contractions?
Shortens
What happens to the I band during contractions?
Shorter
What happens to the A band during contractions?
Same width - no change
Give details of the role of ATP (Ca2+ present - excitation, no ATP)
- Energised: ATP split by myosin ATPase; ADP and P remain attached to myosin; enegry stored in cross bridge.
- Binding: Ca2+ released one excitation; removes inhibitory influence from actin, enabling it to bind with cross bridge.
- Bending: Power stroke of cross bridge triggered on contact between myosin and actin; P released during and ADP released after power stroke.
- Rigor complex: If no fresh ATP available, actin and myosin remain bound in rigor complex.
Give details of the role of ATP (Ca2+ present - excitation, fresh ATP)
- Energised: ATP split by myosin ATPase; ADP and P remain attached to myosin; enegry stored in cross bridge.
- Binding: Ca2+ released one excitation; removes inhibitory influence from actin, enabling it to bind with cross bridge.
- Bending: Power stroke of cross bridge triggered on contact between myosin and actin; P released during and ADP released after power stroke.
- Detachment: Linkage between actin and myosin broken as fresh molecule of TP binds to myosin cross bridge; cross bridge assumes original conformation; ATO hydrolyzed
Give details of the role of ATP (no Ca2+ present)
- Energised: ATP split by myosin ATPase; ADP and P remain attached to myosin; enegry stored in cross bridge.
- Resting: No excitation; no Ca2+ released; actin and myosin prevented from binding; no cross-bridge cycle; muscle fibre remains at rest.
Give details of the cross bridge cycle
At rest myosin is energised
ATP has been cleaved to ADP, Pi & energy
Energy is stored within the myosin protein
Ca2+ binds to troponin-C
Myosin binding site on actin becomes exposed
Myosin becomes bound to actin
Pi is released from myosin upon binding with actin
Myosin releases energy through the power stoke
Actin is dragged toward the interior of the sarcomere
Appearance provides the Sliding Filament Theory
Sarcomere length decreases, translating to whole muscle shortening (CONTRACTION)
After power stroke myosin binds new ATP molecule
Actin and myosin detach
ATP hydrolysis occurs to re-energise myosin
Myosin head returns to resting position
If [Ca2+]i is maintained:
Crossbridge cycle continues
However [Ca2+] is rapidly
pumped back into the SR by the SR/ER Ca2+ ATPase
(SERCA).
What drives the contractile apparatus?
Calcium
Where is the calcium from in the cross bridge cycle?
Sarcoplasmic reticulum
What is the signal for calcium release?
When an action potential, travels through a motor neuron.
What must bind to troponin for the cross bridge cycle?
Calcium
What is the first step towards elevating intracellular free calcium in the return to the NMJ?
The transfer of the action potential to invaginations in the sarcolemma
Where do voltage-dependent calcium release?
Via the ryanodine receptor
PG
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