Sarcomere Structure

Question 1

What are the 5 defining characteristics of a skeletal muscle cell (fibre)?

Answer 1

1. Multinucleated
2. Contains many mitochondria
3. Has special structures called Transverse (T) tubules
4. Has myofibrils and sarcomeres
5. Has specific terms for some intracellular structures

Question 2

What is the specific skeletal muscle cell term for plasma membrane?

Answer 2

Sarcolemma

Question 3

What is the specific skeletal muscle cell term for cytoplasm?

Answer 3

Sarcoplasm

Question 4

What is the specific skeletal muscle cell term for smooth ER?

Answer 4

Sarcoplasmic reticulum

Question 5

What are muscle fibres arranged in?

Answer 5

Fasciculi

Question 6

What is the name of the structure that is a dense sheath that lies on the surface of the muscle?

Answer 6

Epimysium

Question 7

What structure lies between fasciculi?

Answer 7

Perimysium

Question 8

What structure separates the muscle fibres?

Answer 8

Endomysium

Question 9

What is the width of a muscle fibre (muscle cell)?

Answer 9

50 - 100 micrometers

Question 10

Define sarcolemma

Answer 10

True cell membrane and encloses the muscle fibre

Question 11

Define sarcoplasm

Answer 11

Intracellular fluid that fills spaces between myofibrils

Question 12

What are myofibrils predominantly composed of?

Answer 12

Actin and myosin filaments

Question 13

What is a sarcomere?

Answer 13

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

Question 14

What is the width of a sarcomere?

Answer 14

1 - 2 micrometers

Question 15

What are the different areas/sections of a sarcomere?

Answer 15

\+ A band
\+ M line
\+ I band
\+ H zone
\+ Z disk

Question 16

What is the THICK filament that myofibrils are predominantly composed of?

Answer 16

Myosin

Question 17

What is the THIN filament that myofibrils are predominantly composed of?

Answer 17

Actin

Question 18

What structures make up myosin?

Answer 18

+ Tail
+ Two globular heads
+ Chains (head and tail)

Question 19

What does the tail of myosin consist of?

Answer 19

Two intertwined heavy chains

Question 20

What are features of the globular heads of myosin?

Answer 20

+ Extend out forming cross-bridges

+ Contain heavy and light chains

+ Contain binding sites fro actin and ATP (myosin-ATPase)

Question 21

What size is myosin?

Answer 21

1.6 micrometers

Question 22

What size is actin?

Answer 22

1 micrometer

Question 23

What are features of actin?

Answer 23

+ Contractile protein

+ Each G actin has a binding site for myosin

Question 24

What are the proteins associated with actin?

Answer 24

Tropomyosin and troponin

Question 25

What are features of tropomyosin?

Answer 25

+ Regulatory protein + Overlaps binding sites on actin for myosin + Inhibits interaction when in the relaxed state

Question 26

What are features of troponin?

Answer 26

+ Regulatory protein + Forms a complex with the other proteins of the thin filament (actin and tropomyosin) + Binds Ca2+ reversibly

Question 27

What happens when troponin binds Ca2+?

Answer 27

Once bound, it changes conformation to pull tropomyosin away from the myosin interaction sites

Question 28

What is the effect of Ca2+ binding to troponin?

Answer 28

It regulates skeletal muscle contraction

Question 29

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

Answer 29

It moves tropomyosin away and allow myosin to interact with the actin

Question 30

What are the structural proteins in a sarcomere?

Answer 30

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

Question 31

What are the contractile proteins in a sarcomere?

Answer 31

+ Actin | + Myosin

Question 32

What properties does titin provide?

Answer 32

+ Elasticity | + Stabilises myosin

Question 33

What is the role of nebulin?

Answer 33

It helps align actin

Question 34

What are myofibrils?

Answer 34

The structures that give skeletal (and cardiac) muscle their characteristic striated appearance

Question 35

What are striations the result of?

Answer 35

The orderly arrangements of thick and thin filaments

Question 36

What type of filaments form I-bands in a cross section through a sarcomere

Answer 36

Thin filaments only

Question 37

What type of filaments form H-zones in a cross section through a sarcomere

Answer 37

Thick filaments only

Question 38

What type of filaments form M-lines in a cross section through a sarcomere

Answer 38

Thick filaments linked with accessory proteins

Question 39

What type of filaments form the outer edges of A-bands in a cross section through a sarcomere?

Answer 39

Thick and thin filament overlap

Question 40

What is the ultrastructural basis of sarcomere banding for A-bands?

Answer 40

Primarily myosin filaments in region of overlap

Question 41

What is the ultrastructural basis of sarcomere banding for I-bands?

Answer 41

Actin filaments bisected by Z-line

Question 42

What is the ultrastructural basis of sarcomere banding for H-zones?

Answer 42

Region of A-band which contains only myosin filaments

Question 43

What happens to the zones of a sarcomere when it shortens (muscle is contracted)?

Answer 43

+ I-bands and H-zones shorten | + A-band remains constant

Question 44

What are features of T-tubules?

Answer 44

+ Walls are continuous with the sarcolemma + Fluid within is continuous with external environment

Question 45

What is sarcoplasmic reticulum?

Answer 45

In muscle, it is homologous to the endoplasmic reticulum found in most cells - Ca2+ is stored and released following membrane excitation

Question 46

How are T-tubules and SR connected?

Answer 46

With junctions - they involve two integral membrane proteins, one in the T-tubule membrane and the other in the membrane of the sarcoplasmic reticulum

Question 47

What are features of the T-tubule protein?

Answer 47

It is a modified voltage-sensitive Ca2+ channel known as the dihydropyridine (DHP) receptor, which acts as a voltage sensor

Question 48

What is the protein embedded in the SR membrane known as?

Answer 48

Ryanodine receptor

Question 49

What does the ryanodine receptor form?

Answer 49

Ca2+ channels

Question 50

What does sarcoplasmic reticulum store?

Answer 50

Ca2+

Question 51

Where do T-tubules bring action potentials?

Answer 51

Into anterior of muscle fibre