Skeletal Muscle Physiology

Question 1

What transcription factors determine if cells become myogenic?

Answer 1

MyoD and myf-5 early

Myogenin and MRF-4/herculin/myf-6 later

Overlapping functions

Question 2

How are myotubes formed?

Answer 2

Single cell myoblasts fuse to form multinucleated myotubes

Question 3

What is the bHLH region of the muscle transcription factors?

Answer 3

Basic helix-loop-helix region that is responsible for binding DNA

Question 4

What is the E-box?

Answer 4

Region on DNA that binds to these transcription factors

Need at least two to bind the TFs

Question 5

How are these TFs most effective?

Answer 5

When they form heterodimers with E2 family TFs

The HLH region is required for dimerization

Question 6

What is myostatin?

Answer 6

Member of TGFB family of signaling proteins

Negative regulator of muscle growth (prevents it)

Inhibits myoblast proliferation and progression from G1 to S

Question 7

What is the result of GoF and LoF of myostatin?

Answer 7

GoF = fewer muscle cells (hypotrophy)

LoF = more muscle cells (hypertrophy)

Question 8

What is the function of satellite cells?

Answer 8

Located on surface of muscle fiber, beneath basal lamina

Recruited to supply myoblasts for repair and regeneration

Question 9

How are satellite cells activated?

Answer 9

Immune response to injury

Start dividing, form new muscle fibers, reconstitute pool of SCs

Migrate to site of injury

Question 10

How do Satellite cells increase muscle mass after exercise?

Answer 10

Repair damage done, which increases thickness and size of muscle fiber

GFs and hormones increase protein synthesis and glucose uptake in satellite cells (e.g. effect of testosterone)

Question 11

What are the components of muscle sarcomere?

Answer 11

Actin and mysoin

Question 12

What is the structure of myosin?

Answer 12

2 heavy chains with a globular head and filamentous tail

2 light chains which are essential and regulatory

Question 13

What are cross bridges?

Answer 13

Composed of the head filament and project laterally

Serve as the link between thick and thin myofilaments

Acts as ATPase, binds to actin

Question 14

What is tropomyosin?

Answer 14

Lies in the groove between actin strands

Regulates binding sites of actin and provides structural rigidity

Question 15

What is troponin?

Answer 15

Complex of three proteins: TT, TC, TI

TC - contains two high and two low affinity sites for Ca

TT - binds troponin to tropomyosin

TI - inhibits interaction between actin and myosin

Question 16

What is the function of titin?

Answer 16

Acts as a spring to keep myosin filaments centered in sarcomere and maintain resting tension

Question 17

What is the function of nebulin?

Answer 17

Associated with actin

Regulates assembly of actin filaments

Question 18

What is the role of Ca in muscle contraction?

Answer 18

Binds to troponin C to change conformation of complex

Increase intracellular concentration when stimulated

Dissociates from regulatory proteins when concentration decreases

Question 19

What is the function of the T-tubule system of the sarcolemma?

Answer 19

Contains voltage-gated Ca channels

When depolarized due to stimulation from motor nerve, causes release of Ca

Question 20

What is the function of dihydropyridine calcium channels?

Answer 20

Act as a voltage sensor in the T-tubules, causes the opening of ryanodine receptor calcium channels

Ca does not travel through these

Question 21

What is the function of RyR1 calcium channels?

Answer 21

Release calcium in response to conformational changes in DHP channel proteins

Question 22

What is SERCA?

Answer 22

Sarcoendoplasmic reticulum calcium ATPase

Pumps calcium from cytoplasm back to the SR

Question 23

Describe the sliding filament model.

Answer 23

Describe in four steps, study separately

Question 24

What is the function of chloride channel CIC-1?

Answer 24

Creates a large resting chloride conductance of skeletal muscle

Stabilizes resting membrane potential to prevent false action potentials

Also important in repolarizing membrane

Question 25

What is a twitch contraction?

Answer 25

Single AP causes release of enough Ca to fully initiate a contraction

Ca removed so rapidly that muscle cell does not have enough time to develop maximum force

Question 26

What is a summation contraction?

Answer 26

Second stimulus is applied to muscle cell before relaxation is complete, causes release of additional Ca

Results in an increase in the amount of tension developed by muscle fiber because more crossbridges are formed

Question 27

What is a tetanus contraction?

Answer 27

When rate of stimulation of motor nerve increases to point where little or no relaxation is evident, level of tension reaches a plateau

Question 28

What is the state of myosin during relaxation?

Answer 28

Remains in high energy conformational state, but not bound to actin

(Bound to ATP - no hydrolysis)

Question 29

What is the function of calsequestrin?

Answer 29

Protein in the SR that is responsible for binding Ca ions and reducing the SR concentration of free Ca

Question 30

What is the first source used to generate ATP in muscle?

Answer 30

Cleavage of creatine phosphate and phosphorylation of ADP

Creatine phosphokinase

Question 31

What is the second source of ATP in muscle?

Answer 31

Glycolysis via glycogen breakdown during muscel contraction

Question 32

What is the third source of ATP in muscle?

Answer 32

Ox-phos

Also the most abundant