Sarcomere Structure Flashcards
What are muscle fiber bundles called ?
Fasiculi
What layers of connective tissue are present in skeletal muscle and what do they cover ?
Epimysium - dense sheath on the surface
Perimysium - between fasiculi
EndoWmysium - between muscle fibers
What is the diameter of a skeletal muscle fiber ?
50-100 μ
What is a Sarcolemma ?
“cell membrane of a striated muscle fiber cell.”
What is the sarcoplasm ?
Intracellular fluid that fills spaces between myofibrils
What are myofibrils predominantly made of ?
Actin and myosin filaments
What is a sarcomere ?
The portion of a myofibril that lies between two successive Z disks
What is the diameter of a myofibril ?
1-2 μ
What is the main protein making up thick filaments ?
Myosin
What is the structure of thick filaments ?
Tail = 2 intertwined heavy chains
2 globular heads extending out forming cross-bridge =
Heavy and light chains (each head) + binding sites for actin and ATP (myosin-ATPase) (on each head)
What is the length of myosin in a sarcomere ?
1.6 μm
What is the length of actin in a sarcomere ?
1 μm
What are the main proteins making up thin filaments ?
Actin, tropomyosin and troponin (forming a complex with each other)
Which of the proteins making up the thin filaments are contractile ? Regulatory ?
Actin - contractile
Tropomyosin and troponin - regulatory
What is the structural specificity of actin in thin filaments ?
Each G actin has a binding site for myosin
What is the function of tropomyosin in the thin filament ?
Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state
What is the function of troponin in the thin filament ?
Binds Ca2+ reversibly and once bound changes conformation to pull tropomyosin away from the myosin interaction sites.
How does Ca2+ binding to troponin regulate skeletal muscle contraction ?
Because it moves the tropomyosin away and allows myosin to interact with the actin.
Where is nebulin found within the sarcomere ? What is its function ?
In the thin filament
Helps align actin
Where is titin found within the sarcomere ? What is its function ?
Connects the Z line to the M line.
Provides elasticity and stabilises myosin
What are all the structural proteins found in the sarcomere ?
Titin (connectin)
Z-line protein (alpha-actinin)
M-line proteins (myomesin, M-protein)
Nebulin
What are all the contractile proteins found in the sarcomere ?
Actin
Myosin
Tropomyosin
Troponin Complex
What is the A band made of ?
primarily myosin filaments in the region of overlap
What is the I band made of ?
Actin filaments bisected by Z-line
What is the H zone made of ?
Only myosin filaments
What happens to the sarcomere as a whole, the H zone, A band, and I band upon contraction ?
Sarcomere as a whole shortens
H zone and I band shorten
A band stays constant
Which feature of T tubules allows the fluid within to be continuous with external environment ?
Their walls are continuous with the sarcolemma
What structure contains intracellular structures and organelles ?
Sarcoplasmic reticulum
What is a terminal cisternae ? What is their function ?
“enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. Store calcium and release it when an action potential courses down the transverse tubules, eliciting muscle contraction.”
What is the function of the sarcoplasmic reticulum ?
Ca2+ storage and release following membrane excitation.
How are sarcoplasmic reticulum and T-tubules connected ?
With junctions
What is the structure of the junction connecting sarcoplasmic reticulum and T-Tubules ?
Two integral membrane proteins:
- One in the T-tubule membrane (modified voltage-sensitive Ca2+ channel called dihydropyridine (DHP) receptor, which acts as a voltage sensor)
- One in the membrane of the sarcoplasmic reticulum ( ryanodine receptor, which forms a Ca2+channel)
What is a triad ?
T tubule with terminal cisternae on both sides
