Sarcomere Structure

Question 1

What are muscle fiber bundles called ?

Answer 1

Fasiculi

Question 2

What layers of connective tissue are present in skeletal muscle and what do they cover ?

Answer 2

Epimysium - dense sheath on the surface
Perimysium - between fasiculi
EndoWmysium - between muscle fibers

Question 3

What is the diameter of a skeletal muscle fiber ?

Answer 3

50-100 μ

Question 4

What is a Sarcolemma ?

Answer 4

“cell membrane of a striated muscle fiber cell.”

Question 5

What is the sarcoplasm ?

Answer 5

Intracellular fluid that fills spaces between myofibrils

Question 6

What are myofibrils predominantly made of ?

Answer 6

Actin and myosin filaments

Question 7

What is a sarcomere ?

Answer 7

The portion of a myofibril that lies between two successive Z disks

Question 8

What is the diameter of a myofibril ?

Answer 8

1-2 μ

Question 9

What is the main protein making up thick filaments ?

Answer 9

Myosin

Question 10

What is the structure of thick filaments ?

Answer 10

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)

Question 11

What is the length of myosin in a sarcomere ?

Answer 11

1.6 μm

Question 12

What is the length of actin in a sarcomere ?

Answer 12

1 μm

Question 13

What are the main proteins making up thin filaments ?

Answer 13

Actin, tropomyosin and troponin (forming a complex with each other)

Question 14

Which of the proteins making up the thin filaments are contractile ? Regulatory ?

Answer 14

Actin - contractile

Tropomyosin and troponin - regulatory

Question 15

What is the structural specificity of actin in thin filaments ?

Answer 15

Each G actin has a binding site for myosin

Question 16

What is the function of tropomyosin in the thin filament ?

Answer 16

Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state

Question 17

What is the function of troponin in the thin filament ?

Answer 17

Binds Ca2+ reversibly and once bound changes conformation to pull tropomyosin away from the myosin interaction sites.

Question 18

How does Ca2+ binding to troponin regulate skeletal muscle contraction ?

Answer 18

Because it moves the tropomyosin away and allows myosin to interact with the actin.

Question 19

Where is nebulin found within the sarcomere ? What is its function ?

Answer 19

In the thin filament

Helps align actin

Question 20

Where is titin found within the sarcomere ? What is its function ?

Answer 20

Connects the Z line to the M line.

Provides elasticity and stabilises myosin

Question 21

What are all the structural proteins found in the sarcomere ?

Answer 21

Titin (connectin)
Z-line protein (alpha-actinin)
M-line proteins (myomesin, M-protein)
Nebulin

Question 22

What are all the contractile proteins found in the sarcomere ?

Answer 22

Actin
Myosin
Tropomyosin
Troponin Complex

Question 23

What is the A band made of ?

Answer 23

primarily myosin filaments in the region of overlap

Question 24

What is the I band made of ?

Answer 24

Actin filaments bisected by Z-line

Question 25

What is the H zone made of ?

Answer 25

Only myosin filaments

Question 26

What happens to the sarcomere as a whole, the H zone, A band, and I band upon contraction ?

Answer 26

Sarcomere as a whole shortens H zone and I band shorten A band stays constant

Question 27

Which feature of T tubules allows the fluid within to be continuous with external environment ?

Answer 27

Their walls are continuous with the sarcolemma

Question 28

What structure contains intracellular structures and organelles ?

Answer 28

Sarcoplasmic reticulum

Question 29

What is a terminal cisternae ? What is their function ?

Answer 29

"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."

Question 30

What is the function of the sarcoplasmic reticulum ?

Answer 30

Ca2+ storage and release following membrane excitation.

Question 31

How are sarcoplasmic reticulum and T-tubules connected ?

Answer 31

With junctions

Question 32

What is the structure of the junction connecting sarcoplasmic reticulum and T-Tubules ?

Answer 32

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)

Question 33

What is a triad ?

Answer 33

T tubule with terminal cisternae on both sides