Lecture 30 - Muscle Structure, Function, Neuromuscular Transmission

Question 1

Difference between myopathies and dystrophies

Answer 1

Myopathies are disorders of muscle contractile apparatus. Are normally static (don’t progress)

Dystrophies are disorders of supporting structures. Often progressive.

Question 2

What leads to variable muscle colour?

Answer 2

Myoglobin content (more myoglobin –> deeper red colour)

Question 3

Feature of skeletal muscle cells

Answer 3

Multinucleated

Question 4

Skeletal muscle structure
1)
2)
3)
4)
5)
6)

Answer 4

1) Muscle made up of bundles of muscle fibres called fasciculi.
2) Fasciculi are wrapped in perimysium
3) Fasciculi are made up of muscle fibers wrapped in endomysium
4) Each fibre made up of myofibrils wrapped in sarcolemma
5) Myofibrils and sarcolemma are surrounded by sarcoplasm
6) Each myofibril is made up of sarcomeres, which are the smallest contractile unit in muscle

Question 5

What is wrapped around fasciculi?

Answer 5

Perimysium

Question 6

What is wrapped around muscle fibers?

Answer 6

Endomysium

Question 7

What makes up muscle fibres?

Answer 7

Myofibrils wrapped in sarcolemma

Question 8

In striated muscle, which band is light?

Answer 8

I band (actin)

Question 9

In striated muscle, which band is dark?

Answer 9

A band (actin and myosin)

Question 10

Thin filament

Answer 10

Actin

Question 11

Thick filament

Answer 11

Myosin

Question 12

Sarcoplasm
1)
2)

Answer 12

1) Liquid surrounding myofibrils

2) Contains glycogen, fat particles, enzymes, mitochondria

Question 13

Which filament has globular heads that crosslink?

Answer 13

Myosin

Question 14

Sarcomere structure
1)
2)
3)
4)
5)

Answer 14

1) Surrounded by Z discs. Where actin attaches
2) I band - Aligned actin filaments
3) A band - Aligned actin and myosin filaments
4) H zone - Aligned myosin filaments
5) M line - Centre of sarcomere. Where myosin anchor

Question 15

Fibre present in I band

Answer 15

Only actin

Question 16

Fibre present in H zone

Answer 16

Only myosin

Question 17

Part of sarcomere that only contains myosin

Answer 17

H zone

Question 18

Part of sarcomere that only contains actin

Answer 18

I band

Question 19

Sliding filament muscle contraction
1)
2)
3)
4)

Answer 19

1) At rest, tropomyosin covers myosin binding sites on actin
2) Action potential depolarises T-tubule membrane, causes Ca2+ release into sarcoplasm
3) Ca2+ binds tropomyosin, makes it unbind actin, revealing myosin binding site
4) Myosin head binds actin, begins crossbridging

Question 20

Is crossbridge formation synchronous?

Answer 20

No.

If it were, all would detach at the same time, and the muscle would return to uncontracted state

Question 21

Crossbridge formation 
1)
2)
3)
4)

Answer 21

1) ATP binds myosin head, myosin detaches from actin
2) ATP hydrolysed to ADP + Pi, myosin head is cocked
3) Myosin head attaches to actin
4) ADP and Pi detach from myosin head. This leads to a power stroke (pulls Z discs together)

Question 22

What causes muscle contraction?
1)
2)
3)

Answer 22

1) Motor neurons release ACh into neuromuscular junction.
2) ACh binds nicotinic ACh receptors, influx of Na+ into cell –> depolarisation of sarcolemma
3) Depolarisation of T-tubule membrane leads to Ca2+ influx into sarcoplasm

Question 23

T tubules

Answer 23

Transverse tubules

Invaginations in sarcolemma, aligned with I and A bands

Question 24

Supporting proteins of muscle fibers
1)
2)
3)
4)

Answer 24

1) Dystrophin-associated protein complex (in sarcolemma membrane)
2) Sarcoglycans (a complex) (in sarcolemms)
3) Dystrophin
4) Proteins in the nuclear envelope - emerin and lamin a

Question 25

ATP sources for muscle
1)
2)
3)
4)

Answer 25

1) Within the muscle fibre
2) Creatine phosphate
3) Glucose stored in the cell as glycogen
4) Glucose and fatty acids obtained from the bloodstream

Question 26

How long can ATP stored within a muscle maintain contraction?

Answer 26

A few seconds

Question 27

Creatine phosphate

Answer 27

Molecule in muscles that transfers a high-energy Pi to ADP to form ATP

Question 28

How long can creatine phosphate maintain muscle contraction?

Answer 28

About 15 seconds

Question 29

Where do the fatty acids and glucose taken from the bloodstream by muscles cone from?
1)
2)

Answer 29

1) Glucose from glycogen broken down in the liver

2) Fatty acids from adipose cells and the liver

Question 30

Advantages of anaerobic respiraiton

Answer 30

Fast

Doesn’t require oxygen

Question 31

Disadvantages of anaerobic respiration

Answer 31

Only generates 2 ATP

Generates lactic acid

Question 32

Number of ATP produced by aerobic respiration

Answer 32

36 (including 2 from glycolysis)

Question 33

Advantages of aerobic respiration

Answer 33

Produces a lot of ATP (36)

Question 34

Disadvantages of aerobic respiration

Answer 34

Relatively slow

Requires oxygen

Question 35

Timeline of cellular respiration in muscle
1)
2)
3)
4)

Answer 35

1) 0-30 seconds - ATP/Creatine phosphate used
2) 20-120 seconds - Anaerobic respiration
3) 110 seconds - Aerobic respiration, if contraction continues
4) Depletion of resources (ATP, oxygen, glycogen), lactic acid buildup, lead to cessation of contraction

Question 36

How long can anaerobic respiration sustain muscle contraction at maximum capacity?

Answer 36

Around 30 seconds

Question 37

Type I fibres
1)
2)
3)

Answer 37

1) Slow-twitch
2) Slow oxidative
3) Fatigue-resistant

Question 38

Type I fibre features
1)
2)
3)
4)

Answer 38

1) Myoglobin-, mitochondria-rich
2) Many capillaries
3) Generate ATP aerobically
4) Split ATP at a slow rate –> slow contraction velocity

Question 39

Where are many type I fibres found?

Answer 39

In the postural muscles of the trunk

Used in long-distance running

Question 40

Type IIA fibres
1)
2)
3)
4)

Answer 40

1) Fast-oxidative fibres
2) Myoglobin-, mitochondria-rich
3) Fatigue resistant, but not as much as type I
4) Split ATP at a rapid rate –> rapid contraction velocity
5) Rich in capillaries

Question 41

Type IIB fibres
1)
2)
3)
4)
5)
6)
7)

Answer 41

1) White fibres
2) Fast glycolytic fibres
3) Low in myoglobin, mitochondria
4) Few capillaries
5) Large amount of glycogen
6) Split ATP very quickly
7) Fatigue easily

Question 42

Another name for type IIB fibres

Answer 42

Type X fibres

Question 43

Age and sex differences in muscle fibre type distribution

Answer 43

None