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BRS 5: Musculoskeletal & Rheumatology > 5.3: Muscle microstructure and contraction > Flashcards

5.3: Muscle microstructure and contraction Flashcards

1
Q

3 muscle types

A

Smooth muscle
Cardiac muscle
Skeletal muscle

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2
Q

Control of smooth muscle

A

Under involuntary control from autonomic nervous system

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3
Q

Control of cardiac muscle

A

Can Contract autonomously
Under influence of autonomic nervous system and Circulating chemicals

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4
Q

Control of skeletal muscles

A

Under voluntary control, usually attached to bones and contract to bring about movement

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5
Q

Structure of skeletal muscles from macroscopic to microscopic

A

Bone
Tendon
Muscle
Fascicles
Myofibre
Myofibril
Myofilaments

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6
Q

Fascicles are

A

Bundles of muscle fibres (myofibres)

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7
Q

structure of myofibres

A

Covered by plasma membrane- sarcolemma
T-tubes tunnel into centre
Sarcoplasm - myoglobin and mitochondria present
Sarcoplasmic reticulum
Composed of myofibrils

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8
Q

What two proteins make up skeletal muscle myofibrils

A

Actin and myosin

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9
Q

Characteristics of myofilaments

A

Light and dark bands - striated appearance
Do not extend along length of myofibres
Overlap and arranged in sarcomeres

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10
Q

Structure of myofilaments

A

Dense protein Z discs separate sarcomeres
Dark band- A band (thick myosin)
Light band - I band (thick - actin)
Myosin and actin filaments overlap

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11
Q

Structure of myosin

A

Two globular heads
Single tail formed by two alpha helices
Tails of 100s molecules form one filament

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12
Q

Structure of actin

A

Twisted into helix
Each molecule has myosin binding site - exposed as Ca binds
Filaments contain tropomyosin and Troponin

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13
Q

Sliding filament theory

A

During contraction:
I band shortens
A band remains the same
H-zone narrows

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14
Q

Initiation of muscle contraction

A

1) Action potential opens voltage gated Ca2+ Channels
2) Ca enters pre-synaptic terminal
3) Ca triggers exocytosis of vesicles
4) ACH diffuses across synaptic cleft and binds to ACH receptors , inducing action potential in muscle
5) neurone depolarised, AP spreads along surface of muscle fibre membrane
6) ACH broken down by acetylcholine esterase - causes muscle fibre to cease

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15
Q

Activation of muscle contraction

A

1) AP spreads along muscle sarcolemma and into T-tubules
2) Dihydropyridine receptor in T-tubule membrane, senses voltage and changes shape of protein linked to ryanodine receptor
3) opens ryanodine receptor Ca2+ channel in SR, resleasing Ca2+ into space around filaments
4) Ca2+ binds to troponin and tropomyosin moves
5) actin myosin cross bridges form
5) Ca2+ actively transported into SR continuously, while AP continue . ATP driven pump

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16
Q

Excitation contraction coupling

A

1) in presence of Ca2+ troponin moves from tropomyosin chain
2) movement exposes myosin binding site on actin filament
3) charges myosin heads bind
4) power stroke : discharge of ADP causing myosin head to cock, pulling actin filament towards centre of sarcomeres
5) ATP binding releases myosin head from actin chain
6) ATP hydrolysis provides energy to recharge myosin head

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17
Q

What protein filament does the pulling during muscle contraction

A

Myosin

18
Q

Neural control of muscle contraction

A

Upper motor neurons in brain
Lower motor neurons in brain stem or spinal cord
Voluntary neural control from upper and lower motor neurons

19
Q

What is the name for a single motor neurone together with all muscle fibres that it innervates

A

Motor unit

20
Q

Stimulation of one motor unit causes

A

Contraction of all muscle fibres in that unit

21
Q

How many muscle fibres does each motor neuron supply

A

600

22
Q

How many motor neurones do humans have

A

420,000 motor neurones
250 million skeletal muscle fibres

23
Q

3 types of motor unit

A

Slow (S, type I)
Fast, fatigue resistant (FR, type IIA)
Fast fatiguable (FF, type IIB)

24
Q

Characteristics of Slow motor unit

A

Smallest diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity

25
Q

Characteristics of fast motor unit

A

Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity

26
Q

Distribution of muscle fibre types

A

Randomly distributed throughout muscle
Muscles have different proportions of slow and fast twitch muscles

27
Q

What are motor unit types classified by

A

Amount of tension generated, speed of contraction and fatiguability of motor unit

28
Q

Two mechanisms by which brain regulates force a single muscle can produce

A

Recruitment and rate coding

29
Q

In muscle recruitment

A

Motor units recruited in order
Governed by size principle, smaller first
More force required more units recruited, allowing fine control under which low force levels are required

30
Q

In muscle Rate cording

A

Motor unit can fire at range of frequencies, slow units fire at lower frequency
Firing rate increase force produced by unit increases
Summation occurs as units fire at frequency too fast to allow muscle to relax between arriving APs

31
Q

Neurotrophic factors are

A

Type of growth factor

32
Q

2 functions of neurotrophic factors

A

Prevent neuronal death
Promote growth of neurons after injury

33
Q

What are motor unit and fibre characteristics dependent on

A

Nerve that innervates them

34
Q

What occurs if a fast and slow twitch muscle are cross innervated

A

Slow muscle becomes fast and vice versa

35
Q

What does the motor neuron have some effect on

A

Properties of muscle fibres it innervates

36
Q

3 types of muscle contraction

A

Concentric
Eccentric
Isometric

37
Q

In isometric contraction

A

Muscle does not change length during contraction

38
Q

In concentric contraction

A

Contraction when muscle shortens

39
Q

In eccentric contraction

A

Contraction where muscle is longer

40
Q

What can change properties of muscle fibres

A

Training, type IIB and IIA most commonly

41
Q

What can change slow muscle into fast muscle

A

NOT training !
Unless severe reconditioning or spinal cord injury
Microgravity from space results in shift of muscle types

42
Q

What effect does aging have on muscle types

A

Loss of type I and II fibres
Preferential loss of type II fibres
Results in larger proportion of type I fibres in aged muscle - evidence from slower contraction times

BRS 5: Musculoskeletal & Rheumatology (6 decks)

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