Muscle Structure Flashcards
How many types of muscle are there?
Skeletal, Cardiac and smooth
Which types of muscle are striated?
skeletal and cardiac
What is most skeletal muscle attached to?
bone
what attaches skeletal muscle to bonds?
tendons
what are the 2 main functions of skeletal muscle?
- movement
- generation of heat
Is muscle contraction endothermic or exothermic?
exothermic
What are muscle cells called? (2 names)
- myocytes
- muscle fibres
What covers muscle fibres?
a layer of connective tissue called endomysium
is the endomysium part of the myocyte membrane or not?
not- it’s distinct
what is the name of muscle fibre membrane?
sarcolemma
are muscle cells multinucleate?
yes (each cell has several nuclei)
what makes muscle cells multinucleate?
cell fusion during development
How are muscle fibres grouped?
into bundles called Fascicles
what covers fascicles?
a sheath called the epimysium (forms the muscle)
what is an individual muscle fibre made up of?
bundles of protein filaments called myofibrils
what are myofibrils composed of?
bundles of protein filaments called myofilaments
what are the Tunnels in muscle fibres called?
T-tubules
where are the T-tubules?
in muscle fibres leading off from the sarcolemma into the interior of the muscle fibre
where is the sarcoplasmic reticulum found?
wrapped around the myofibrils
What forms a triad?
specialised parts of the sarcoplasmic reticulum called terminal cisternae and T-tubules
What is the function of a triad?
couples membrane excitation with contraction
What are the 2 main structural features of the sarcomere?
thin and thick filaments
what are thick filaments made up of? (2)
myosin and titin
what are thin filaments made up of?
actin and nebulin
what do both the thick and the thin filament connect to?
the Z-disk
what is the z-disk?
the part of the sarcomere which joins 2 sarcomeres together
what are only the thick filaments connected to?
proteins of the M-line
what is the M-line?
the centre of a sarcomere
what’s the name of the model suggesting how muscle contraction occurs?
sliding filament theory
what type of myosin is found in skeletal muscle?
Type II
What activity does a myosin head have?
ATPase
What controls the movement in sliding filament?
hydrolysis of ATP
how many forms does actin exist in?
2
what 2 forms does actin exist in?
- Globular (G)
- Filamentous (F)
which form of actin is found in proteins?
Filamentous (F)
Describe tropomyosin
runs along the actin chain and consists of 2 alpha helical chains that coil around each other. 1 tropomyosin interacts with 7 actin monomers
describe troponin
a complex of 3 proteins- troponin T, I and C
What does Troponin T do?
binds to tropomyosin
what does Troponin I do?
binds to actin and inhibits contraction
what does Tropnin C do?
binds calcium
In terms of just Actin and Myosin interaction, what happens during muscle contraction?
- Myosin is found to actin
- Myosin head binds ATP and the myosin-actin cross-bridge dissociates
- Myosin head has built-in ATPase activity- breaks down ATP to ADP+ Pi - changing myosin conformation (cocked state)
- new bridge forms between actin and myosin
- phosphate is released and power stroke occurs- myosin returns to uncocked state. ADP is released and system is back at starting point- but the actin and myosin have moved relative to eachother- contraction
What happens in terms of Troponin and Troppomyosin to trigger contraction?
- At rest, the troponin trimer sits so that TnI inhibits the formation of actin-myosin bridges (covers myosin binding site)
- When contraction is signaled, Ca2+ conc. increases, Ca2+ can now bind to troponin C- triggering a change in conformation of troponin and tropomyosin
- Tropomyosin moves deeper into the actin groove, whilst troponin complex shifts up, revealing the myosin binding sites for actin- cross bridges and contraction can occur
describe the chain of events at the NMJ?
- Motor neurons release ACh into synapse which bind to nAChR at the muscle membrane, which open, allowing Na+ ions to flood in to the muscle and depolarise the membrane
this depolarisation causes a calcium signal which can start the actin-myosin bridge formation
where in the muscle fibre is Calcium high?
the sarcoplasmic reticulum
How does calcium get into the sarcoplasmic reticulum in the first place?
SERCA (sarcoplasmic/endoplasmic reticulum calcium ATPase) actively pumps it in
describe the movement of calcium when muscle contraction occurs
An AP spreads down the T-tubule, cuasing a change in Dihydropyridine/ DHP/ L-type VSCC
this pulls open the ryanodine receptor, releasing Ca2+ from the sarcoplasmic reticulum into the cytoplasm
after contraction has taken place, SERCA pumps calcium ions back into SR, restoring low Ca2+ concentration in the cytoplasm
