Skeletal muscle contraction

Question 1

Prominent A and I bands can be found in what muscle types? What about muscle that lacks definite A and I bands?

Answer 1

Cardiac and skeletal muscle are prominent. Smooth muscle lacks definitive bands. (A appears dark in phase contrast I is light)

Question 2

What is the Z line, I band, dark A band, H zone and M line?

Answer 2

Z-boundary btwn two sarcomeres. I-contains only actin thin filaments. A-contain myosin thick filaments. H-there is no overlap between actin and myosin. M-at the center of the sarcomere, site of thick filament linkage to thick filament

Question 3

What is the organization of muscle?

Answer 3

Fasicles comprised of multinucleated muscle fibers composed of myofibrils which contain sarcomeres. (sarcomere site where sliding of actin and myosin filaments produce contraction)

Question 4

During contraction what happens to the different zones in a sarcomere?

Answer 4

Z-lines move closer together. Length of A band is constant. Length of I band shortens. Shortening sarcomere shortens muscle

Question 5

What comprises a myosin molecule?

Answer 5

6 polypeptide chains. Two globular heads and four light chains. Each myosin molecule has two globular heads and one double coiled helix tail

Question 6

What is the myosin heavy chain? What regions does it contain?

Answer 6

A single protein that contains the globular region that both binds actin and hydrolyzes ATP during muscle contraction. It also has the hinge region and extended tail. Two heavy chains are wound together.

Question 7

What modification is made to the regulatory light chain (RLC)? What enzyme does this?

Answer 7

It is phosphorylated in striated muscles by Ca++/calmodulin-dependent myosin light chain kinase.

Question 8

What function does the Essential Light chain (ELC) provide?

Answer 8

The fine tuning of the myosin motor function. It is regulated in a tissue-dependent matter

Question 9

What does each myosin head contain?

Answer 9

An ATPase catalytic site and an actin binding site

Question 10

Outline the excitation contraction coupling in skeletal muscle.

Answer 10

(pg 232) Motor axon AP-> end-plate depolarization-> muscle cell surface AP-> TTS transmission-> Coupling to SR-> Ca release from SR-> Activation of contractile proteins-> Ca accumulation by SR

Question 11

What makes up a triad?

Answer 11

2 cisternae + 1 T-tubule. The cisternae are the terminal portions of the SR. T-tubules are elaboration of the plasma membrane of the muscle fiber

Question 12

What is tropomyosin?

Answer 12

It is an alpha-helical rod protein that covers the myosin binding sitres on the thin filament in resting muscle. It blocks myosin from binding to the thin filament.

Question 13

What is troponin? What does each domain function to achieve?

Answer 13

It is a complex of three protein subunits. Troponin C binds calcium. Troponin I inhibits actomyosin ATPase and binds to actin. Troponin T binds to tropomyosin

Question 14

Discuss the crossbridge cycle in muscle (1-2)

Answer 14

1-ATP binds to myosin head, causing the dissociation of the actin-myosin complex. 2-ATP is hydrolyzed causing myosin heads to return to their resting conformation.

Question 15

Discuss the crossbridge cycle in muscle (3-5)

Answer 15

3-A cross-bridge forms and the myosin head binds to a new position on actin. 4-P is released. Myosin heads change conformation, resulting in the power stroke. ADP is released.

Question 16

What are the states of Myosin and actin binding in order?

Answer 16

Attached state-> released state-> cocked state-> cross-bridge state-> power-stroke state

Question 17

Binding of ATP causes myosin to do what?

Answer 17

Detatch from actin

Question 18

ATP hydrolysis causes myosin to do what?

Answer 18

It causes a cocked formation. ADP and Pi are not released here

Question 19

Release of phosphate from myosin has what effect?

Answer 19

It causes a conformational change resulting in the power stroke.

Question 20

What do you call the contraction generated by a single action potential?

Answer 20

A Twitch. (It is the basis of muscle contraction. the heart contracts by twitch only)

Question 21

What are some factors dictating different kinetics between isometric twitches?

Answer 21

The type of myosin heavy chain (different rates of ATP hydrolysis influences the speed of crossbridge cycle and contraction) expressed and the speed of sarcoplasmic Ca elevation and clearance.

Question 22

The heavy chain in muscle myosin is encoded by what multigene of the myosin family?

Answer 22

Myosin II

Question 23

Describe the properties of Slow twitch fibers

Answer 23

Type I. MYH7. Fatigue resistant. Red (myoglobin). Oxidative metabolism. High mitochondria. Low glycogen.

Question 24

Describe the properties of fast twitch fibers (type Iia)

Answer 24

Type IIa. MYH2. Fatigue resistant. Red (myoglobin). Oxidative metabolism. Higher mitochondria. Abundant glycogen

Question 25

Describe the properties of fast twitch fibers (type Iib)

Answer 25

Type Iib. MYH4. Fatigable. White (low myoglobin). Glycolytic metabolism. Fewer mitochondria. High glycogen

Question 26

What did isometric studies of muscles show?

Answer 26

There is an optimal muscle length for active tension generated

Question 27

What increases the tension in isometric experiments?

Answer 27

An increase in the degree of overlap of thick and thin filaments increases tension.

Question 28

What is a major mechanism for increasing muscle size? How is this accomplished?

Answer 28

Increasing muscle size or myofiber size accomplished by stallite cell fusion.