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MSK > Muscle microstructure and contraction > Flashcards

Muscle microstructure and contraction Flashcards

1
Q

What are the three main types of muscle in the body?

A

Smooth
Cardiac
Skeletal

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2
Q

How is smooth muscle controlled?

A

Under involuntary control for the autonomic nervous system

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3
Q

How is cardiac muscle controlled?

A

Can contract autonomously, but is under the influence of the autonomic nervous system and circulating chemicals

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4
Q

How are skeletal muscles controlled?

A

Are under voluntary control

From the somatic nervous system

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5
Q

Where do you typically find skeletal muscle?

A

Attached to bones

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6
Q

What is the main function of skeletal muscle?

A

Contract to bring about movement

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7
Q

What do tendons do?

A

Attach muscles to bones

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8
Q

What are fascicles?

A

Bundles of muscle fibres

Surrounded by connective tissues

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9
Q

What are muscle fibres known as?

A

Myofibres

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10
Q

What comprise myofibrils?

A

Myofibrils

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11
Q

What comprises myofibrils?

A

Myofilaments

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12
Q

What is the connective tissue surrounding the muscle called?

A

Epimysium

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13
Q

What is the connective tissue surrounding the fassicle called?

A

Perimysium

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14
Q

What is the connective tissue surrounding the muscle fibre called?

A

Endomysium

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15
Q

What is the muscle plasma membrane called?

A

Sarcolemma

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16
Q

Why do muscles have a very good vascular supply?

A

Muscles require lots of energy

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17
Q

Where are the t-tubules found?

A

Tunnels into the centre of the myofibres

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18
Q

What is the cytoplasm called?

A

Sarcoplasm

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19
Q

How big are myofibrils?

A

1-2 micrometers in diameter

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20
Q

What are the repeated bands of muscle know as?

A

Sarcomeres

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21
Q

What comprises a sarcomere?

A

Z disc
H zone
Z disc

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22
Q

What are myofibrils composed of?

A

Actin

Myosin

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23
Q

What gives the muscle striated appearance?

A

Light and dark bands

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24
Q

Where is the M-line?

A

Half way between adjacent Z discs

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25
Q

What separate sarcomeres?

A

Z-discs

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26
Q

What are the dark bands?

A

A band

Thick myosin

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27
Q

What are the light bands?

A

I bands

Thin actin

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28
Q

Describe the structure of myosin

A

Two globular heads
Single tail formed by two alpha helices
Tails of several hundred molecules form one filament

29
Q

Describe the structure of actin

A

Actin molecules twisted into helix
Each molecule has a myosin binding site
Filaments also contain troponin and tropomyosin

30
Q

What do the troponin complex cover?

A

Covering the binding sites for myosin

31
Q

Summarise the sidling filament theory?

A

I band becomes shorter
A band reminded the same length
H-zone narrowed or disappeared

32
Q

How muscle contraction initiated?

A

Action potential opens voltage gated Ca2+ channels

Ca2+ enters pre-synaptic terminal

Ca2+ triggers exocytosis of vesicles

Acetylcholine diffuses across cleft

Binds to acetylcholine receptors and induces action potentials (AP) in muscle

Local currents flow from depolarized region and adjacent region

33
Q

How is contraction stopped?

A

Acetylcholine broken down by acetylcholine esterase. Muscle fibre response to that molecule of acetylcholine ceases.

Action potential stop coming down the neurones

34
Q

How is muscle contraction activated?

A

Action potential propagates along surface membrane and into T-tubules

35
Q

What is the second step of muscle activation?

A

Dihydropyridine (DHP) receptor in T-tubule membrane: senses deltaV & changes shape of the protein linked to ryanodine receptor

Opens the ryanodine receptor Ca2+ channel in the sarcoplasmic reticulum (SR)

Ca2+ released from SR into space around the filaments

36
Q

What is the third step of muscle activation?

A

Ca2+ binds to troponin & tropomyosin moves

Movement exposes myosin binding site on surface of actin chain

37
Q

What is the fourth step of muscle activation?

A

‘Charged’ myosin heads bind to exposed site on actin filament

38
Q

What is the fifth step of muscle activation?

A

Ca2+ is actively transported into the SR continuously while action potentials continue. ATP- driven pump (uptake rate < or = release rate).

This binding & discharge of ADP causes myosin head to pivot (the ‘power stroke’)  pulling actin filament towards centre of sarcomere

39
Q

How is myosin ‘recharged’?

A

ATP binding - releases myosin head from actin chain

ATP hydrolysis - provides energy to ‘recharge’ the myosin head

40
Q

How do neurones control muscle contraction?

A

Upper neurones in the primary motor cortex
Lower motor neurones in brainstem or spinal cord
Voluntary neural control

41
Q

What is the motor unit?

A

Name given to a single motor neurone together with all the muscle fibres that it innervates

42
Q

How many muscle fibres do each motor neurone supply?

A

On average 600

43
Q

What does stimulation of one motor unit cause?

A

Contraction of all the muscle fibres in that unit

44
Q

Can multiple nerves innervate the same muscle fibre?

A

No

45
Q

What causes variability in control of muscles?

A

Different number of nerves innervate them

Fewer nerves = more control

46
Q

What are the three types of motor unit?

A

Slow
Fast, fatigue resistant
Fast, fatiguable

47
Q

What are the main features of type I units?

A 
Slow 
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
Red
High myoglobin content
High aerobic capacity
48
Q

What are the main features of type IIA units?

A 
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
Pink
High myoglobin content 
Moderate aerobic capacity
High anaerobic capacity
49
Q

What are the main features of type IIB units?

A 
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
White
Low aerobic capacity
High anaerobic capacity 
Low myoglobin content
50
Q

How are the different muscle fibres distributes?

A

Randomly

Muscles have different proportions of slow and fast twitch muscles

51
Q

How are motor unit types classified?

A

Amount of tension generated
Speed of contraction
Fatiguability

52
Q

Classify type I

A

Slow twitch
Low force
Fatigue resistant

53
Q

Classify type IIa?

A

Fast twitch
Moderate force
Fatigue resistant

54
Q

Classify type IIb?

A

Fast twitch
High force
High fatigue

55
Q

What are the two mechanisms or regulation of muscle force?

A

Recruitment

Rate coding

56
Q

What are the main features of recruitment?

A

Motor units are not randomly recruited, there is an order.

Slow–> Fast

57
Q

How does the brain govern recruitment?

A

Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).

58
Q

What happens during recruitment when more force is required?

A

As more force is required, more units are recruited.

This allows fine control (e.g. when writing), under which low force levels are required.

59
Q

What are the main features of rate coding?

A

A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency.

As the firing rate increases, the force produced by the unit increases.

60
Q

What causes summation?

A

When units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

61
Q

How did scientist investigate neurotrophic factors?

A

Motor unit and fibre characteristics are dependent on the nerve which innervates them

If a fast and slow twitch muscle are cross innervated, the slow one becomes fast and vice versa

62
Q

What are the three main types of muscle contraction?

A

Isometric
Concentric
Eccentric

63
Q

What is isocentric contraction?

A

The muscle stays the same length but you generate force

64
Q

What is an eccentric contraction?

A

The muscle elongates but you generate force?

65
Q

What can happen under different conditions to fibre type?

A

Fibre types can change properties under many different conditions.

66
Q

What happens following training?

A

Type IIB to IIA most common following training

67
Q

What can cause type I to II?

A

II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types

68
Q

What is ageing associated with?

A

with loss of type I and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times)

MSK (6 decks)

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