Sarcomere Structure Flashcards
What is whole muscle made up of?
Bundles of fascicle
What is each fascicle made up of?
Series of fibres
What is each muscle fibre made up of?
Myofibrils
What is each myofibril made up of?
Repeating units (sacromeres)
What cells does muscle start developing from?
Myosatellite cells - mesenchymal cells in each myotome
What do myosatellite/mesenchymal cells give rise to?
yoblasts - immature muscle cells
What are my oblasts like?
Mononuclear
Skeletal muscle precursors
What do myoblasts mature into?
Myocytes
What are myocytes like?
Multinucleated cells
Muscle development:
Myosatellite cells -> myoblasts -> myocytes
What do myotubes form?
Myofilaments by synthesising myosin and actin (contractile proteins)
What type of process is the formation of myofilaments?
Regulated process of differentiation
What inhibits formation of myofilaments?
Myostatin
How are sacromeres organised?
Within skeletal and cardiac muscle
Apparently of sacromeres:
Striped appearance - striated
What is a sacromere?
Portion of myofibril between two successive z discs
What’s the difference from one z disc to another z disc called?
One sarcomere
What do contractile proteins consist of?
Thin filaments (actins)
Thick filaments (myosin)
What does I band represent?
Actin filaments
What does A band represent?
Myosin filaments
What’s a H band?
Gap between successive actin filaments
What makes up a Z-disc?
a-actinin
What is a-actinin?
Actin-binding protein
Why is a-actinin important?
Binds ends of thin filaments to Z-discs
What’s Titin?
Largest protein
Why is Titin important?
Binds ends of thick filaments (M line) to Z discs
What do different Titin isoforms forms result in?
Different elasticity
What can Titin be compacted to?
A spring
What is Nebulin?
Actin-binding protein
Large protein
Where is nebulin localised to?
I-band
Why is nebulin important?
Regulates thin filament length during sarcomere assembly - a ruler
What’s Dystrophin?
Large protein
Cross sections: I band
Thin filaments only
Cross sections: H zone
Tick filaments only
Cross sections: M line
Thick filaments linked with accessory proteins
Cross sections: Outer edge of A band
Thick and thin filaments overlap
What proteins do thin filaments consist of?
Actin
Tropomyosin
Troponin
What is the backbone of a thin filament like?
Double stranded a-helical polymer of actin molecules
What is the polymer of actin molecules in the thin filament also called?
F-actin
What does tropomyosin consist of?
2 identical a-helices
Coil around groove formed by actin
What type of protein is tropomyosin?
Regulatory protein
What does tropomyosin allow?
Overlap of binding sites on actin for myosin
What does tropomyosin do in relaxed state?
Inhibit interaction of actin and myosin
What does TnT do in troponin complex?
Binds to tropomyosin
What does TnI do in troponin complex?
Binds to actin
Inhibits contraction
What does TnC do in troponin complex?
Binds calcium
Why does calcium bind to troponin?
To regulate skeletal muscle contraction
How does calcium binding to troponin regulate skeletal muscle contraction?
Moves tropomyosin away + allows myosin to interact with actin
What are thick filaments?
Bipolar assemblies of multiple myosin-II molecules
What does each myosin-II molecule consist of?
2 intertwined heavy chains
2 regulatory light chains
2 essential light chains
What are the 2 regulatory light chains called in myosin-II molecule?
MLC-2
What does MLC-2 do?
Regulates ATPase activity of myosin
What are the 2 essential light chains called in myosin-II molecule?
MLC-1
What does MLC-1 do?
Stabilises myosin head
Where are MLC-1 and MLC-2 found in heavy chain?
Hinge region
What does the tail of thick filaments consist of?
Two intertwined a-helical heavy chains
What happens if calcium isn’t present?
Regulatory proteins inhibit actin-myosin interactions
What’s necessary in cross-bridge cycle?
Calcium
What is the key calcium sensitive regulator?
Troponin C
TnC
What binding sites does troponin C have?
2 high affinity and 2 low affinity
What do the 2 high affinity binding sites on TnC do?
Help in binding TnC to thin filament
What do the 2 low affinity binding sites on TnC do?
Binds calcium
What happens to Troponin complex when calcium binds to it?
Conformational change
What does the conformational change in troponin do?
Troponin I (TnI) moves away from actin filament
Troponin T (TnT) pushes tropomyosin away from actin binding site
What troponin binds to calcium?
TnC
What troponin binds to actin?
TnI
What troponin binds to tropomyosin?
TnT
WHat happens when calcium concentration falls?
Calcium disassociates from TnC - reverses conformational change
What happens when actin binding site exposed?
Myosin head can now bind to actin
What must happen before myosin binds to actin?
Must hydrolyse ATP to ADP and Pi
Changes to 90 degree angle -> thick and thin filament bind together
What happens after myosin is bound to actin?
ATP and Pi are released from myosin head
Myosin head changes to 45 degree angle
Why is myosin head changed back to 45 degree angle after bound to actin?
To move the thin filaments towards eachother
What is power stroke?
When the thin filaments are moved towards eachother
What happens after thin filaments are moved towards eachother?
ATP binds to myosin head
Thin filament is released from thick filament
Calcium also released
What does the removal of calcium from TnC cause?
Tropomyosin closes binding site to myosin
What happens to the bands in sacromere shortening?
I band and H zone shorten
Z lines move closer together
A band constant
What’s the neuromuscular junction also known as?
Motor endplate
What are contractions of skeletal muscle initiated by?
Cerebral cortex
What does the cerebral cortex act by in skeletal muscle contraction?
a-motor neurons
What forms the neuromuscular junction?
Motor neurons flattened to form motor endplate
What happens after acetylcholine is released into synaptic cleft?
Attached to Nicotinic Acetylcholine receptors
What breaks down acetylcholine in synaptic cleft?
Acetylcholinesterase
How are transverse tubules formed?
When plasma membrane of muscle cells invaginate into muscle cell
Where to T tubules penetrate muscle?
Junctions of I and A bands in each sarcomere
What’s along each T-tubule?
2 cisternae of sarcoplasmic reticulum
Where is calcium stored in T-tubules?
Sarcoplasmic reticulum
What’s a triad?
T tubule and 2 cisternae
What happens to action potential after reaching motor end plate?
Propagates along skeletal muscle membrane and down T tubules
What happens after AP goes down T-tubules?
Depolarisation
Opens voltage gated L-type calcium channels
How are voltage gated L-type calcium channels organised in T-tubules?
In tetras - groups of 4
Another name for L-type calcium channel:
Dihydropyridine (DHP) receptor
What does AP do to DHP receptor?
Conformational change
Activates Calcium-release channel
ANother name for calcium-release channel:
Ryanodine receptor
WHat does an increase in calcium result in?
Activation of TnC and cross-bridge cycling
What must happen for contraction to cease?
Remove calcium
Minor way for termination of contraction:
Sodium-calcium exchanger
Calcium pump at plasma membrane
Both remove calcium from cell
Major way for termination of contraction:
Calcium reuptake into sarcoplasmic reticulum by SERCA-type calcium pump
What happens when SERCA-type calcium pump reuptake calcium?
Calcium binds to calsequestrin and calreticulin proteins in skeletal muscle
