Exercise Phys (11) : Structure and regulation of skeletal muscle

Question 1

Structural components of skeletal muscle spindle?

Answer 1

Made from sarcomere functional unit of striated muscle

A band

• myosin filaments
• M band (myosin filaments attach here)

I band
- actin thin filaments

Z line/disk
(thin filaments attach here)

Question 2

Thick filaments are formed from what?

Answer 2

Myosin II molecules

regulatory light chain, alkali light chain, heads of myosin heavy chain, alpha helix tail region

Question 3

Thin filaments are formed from what?

Answer 3

Actin

Troponin complex
Tropomyosin
Myosin binding site

Question 4

What is the function of titin and where is it found?

Answer 4

Titin forms arrays of fine filaments that stabilise the myosin filament positions - extends from Z disk to M line

Very large molecule

Provides increased force when muscle is stretched, resists over-stretching

Question 5

Describe the pathway of muscle contraction as an action potential arrives at a muscle cell

Answer 5

Action potential arrives at muscle cell, depolarises sarcolemma -> depolarises T-tubules, open voltage-gated DHPR Ca2+ channels

• > mechanical coupling of DHPR to RYR triggers the release of Ca2+ from storage in sarcoplasmic reticulum via RYR ca2+ channels
• > Ca2+ binds to troponin, causes conformational change, tropomyosin pulled away from myosin binding sites
• > myosin heads binds and initiates cross bridge cycling
• > As long as Ca++ ions remain in the sarcoplasm, and as long as ATP is available to drive the cross-bridge cycling + pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit

Question 6

How is cross bridge cycling terminated?

Answer 6

Ca2+ is pumped back into SR via SERCA

Question 7

Types of muscle fibre? Uses?

Answer 7

Type I (slow twitch) ~ 50% muscle fibres in average musc
- peak tension in 110ms

Type II

• IIa (fatigue resistant) ~25%
• IIx (fast fatigue) ~25%
• peak tension 50ms

Question 8

Main differences between type I and type II fibres?

Answer 8

Speed of myosin ATPase
- faster myosin ATPase in type II -> faster contraction

SR
- type II have more developed SR -> 3-5 times faster calcium release

Motor units
- type II larger motor units (>300 fibres)

Question 9

Metabolic activity of each type of fibre?

Answer 9

Type I
- slow oxidative

Type IIa
- fast oxidative/glycolytic

Type IIx
- glycolytic

Question 10

Muscle fibre roles during exercise?

Answer 10

I -> high aerobic endurance, used in low intensity aerobic exercise, daily activities

II
a -> fatigue resistant, used in ~ 1600m
x -> fast fatigue, used in ~100m