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Musculoskeletal > Muscle physiology > Flashcards

Muscle physiology Flashcards

1
Q

Motor unit

A

Alpha motor neuron + all the muscle fibres it innervates

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2
Q

Type I muscle fibres are:

A

Slow

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3
Q

Type II muscle fibres are:

A

Fast

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4
Q

Size principle

A

Small oxidative units recruited first because of their lower threshold, large glycolytic motor units last

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5
Q

Neuromuscular junction

A

Motor axon synapses on the motor end plate of the motor neuron. The axon loses its myelin sheath and splits into multiple branches.

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6
Q

2 distinct domains of the postsynaptic folds

A

Crests and depths of the folds

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7
Q

The crests of the postsynaptic folds have:

A

High concentration of AChRs, rapsyn and utrophin

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8
Q

Rapsyn

A

AChR clustering proteins

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9
Q

Utrophin

A

Ubiquitous dystrophin

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10
Q

The depths of the postsynaptic folds have:

A

High concentration of voltage gated sodium channels

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11
Q

Presynaptic events at the neuromuscular junction

A

1) AP reaches nerve terminal
2) Depolarisation opens VGCCs
3) Ca+2 influx
4) Increased Ca+2 triggers vesicle exocytosis and ACh release

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12
Q

Vesicle cycling and release

A

1) Vesicles filled
2) Vesicles form vesicle cluster
3) Filled vesicles dock at active zone
4) Vesciles are primed
5) Ca+2 triggered fusion-pore opening
6) Vesicles undergo exocytosis
7) Recycling of vesicles

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13
Q

3 ways vesicles can be recycled

A

Local reuse
Fast recycling
Clathrin mediated endocytosis

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14
Q

Postsynaptic events at the neuromuscular junction

A

1) ACh binds transmitter gated channels
2) Channels open
3) Na+ inflow, K+ outflow
4) Depolarisation of motor endplate
5) VGNCs open
6) Na+ inflow
7) Depolarisation
8) Propagated muscle AP

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15
Q

AChE

A

Acetylcholinesterase

Anchored to collagen fibrils of basement membrane

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16
Q

AChE works by:

A

Rapidly hydrolysing ACh with water to form choline and acetate
Choline diffuses back to presynaptic terminal and is reabsorbed

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17
Q

3 presynaptic examples of abnormal neuromuscular transmission

A

Lambert-Eaton syndrom
Diabetes
Botulinum and tetanus toxins

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18
Q

2 postsynaptic examples of abnormal neuromuscular transmission

A

Myasthenia Gravis

Alpha toxins

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19
Q

Myasthenia Gravis key points

A

Autoimmune - antibodies attack AChRs
Reduces number of functional receptors and inhibits AP initiation
Treated with anti-AChEs and immunosuppressants

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20
Q

Botulism key points

A

Botulinum toxin released by Clostridium botulinum
Toxins bind presynaptic terminal, are internalised and catalyse cleavage and inactivation of vesicle release system
Blocks depolarisation induced quantal release
Recovery only occurs when nerve terminals grow new sprouts to escape toxins and form new contacts with the muscle fibre

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21
Q

4 characteristics of skeletal muscle

A

Excitable
Contractile
Extensible
Elastic

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22
Q

Epimysium

A

Surrounds entire muscle

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23
Q

Perimysium

A

Surrounds entire fascicle

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24
Q

Endomysium

A

Surrounds each muscle fibre

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25
Q

Sarcolemma

A

Cell membrane of the muscle fibre

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26
Q

Na+ concentrations inside and outside cell

A

Inside: 10 mM
Outside: 145 mM

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27
Q

K+ concentrations inside and outside cell

A

Inside: 145 mM
Outside: 4 mM

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28
Q

Ca+2 concentrations inside and outside cell

A

Inside: 0.1 microM
Outside: 1.5 mM

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29
Q

Transverse tubular membrane system

A

Deep invaginations of sarcolemma into myocyte

Conduct propagated APs and result in localised contracture of filaments

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30
Q

T-tubule location

A

Either side of myosin strip at junction of overlap between A and I bands

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31
Q

Triad

A

T-tubule + 2 terminal cisternae of the sarcoplasmic reticulum
Essential for synchronised excitation-contraction coupling

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32
Q

Sarcoplasmic reticulum function

A

Stores Ca+2

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33
Q

3 major classes of SR calcium-regulatory proteins

A

Luminal calcium binding proteins
SR calcium release channels
Sarcoplasmic reticulum Ca+2 ATPase pumps (SERCA)

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34
Q

Basic unit of contraction

A

Sarcomere

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35
Q

A band

A

Both thick and thin filaments

Anisotropic

36
Q

I band

A

Only thin filaments

37
Q

Z line

A

Electron dense region in the middle of the I band

38
Q

H band

A

Only thick filaments

39
Q

M line

A

Electron dense region in the middle of the H band

40
Q

Actin filaments attach:

A

At the Z line

41
Q

Myosin filaments attach:

A

At the M line

42
Q

Thick filament composition

A

Myosin pairs oriented in opposite directions, staggered around fibre

43
Q

Thin filament compostion

A

Two strands of F-actin twisted together with tropomyosin molecule lying along the helix in a groove

44
Q

Titin

A

Acts as a spring connecting myosin to Z line

45
Q

Heads of myosin are present along the sarcomere except in:

A

The H zone

46
Q

Nebulin

A

Helps align thin filaments

47
Q

3 parts of troponin complex

A

TnT
TnC
TnI

48
Q

TnT

A

Troponin tropomyosin

Positions complex on tropomyosin molecule

49
Q

TnC

A

Troponin calcium

Contains Ca+2 binding sites

50
Q

TnI

A

Troponin inhibitor

Binds actin and inhibits myosin head from binding to the actin binding site in the process

51
Q

Troponin complex + tropomyosin =

A

Ca+2 sensitive switch

52
Q

Roles of ATP in cross-bridge cycle

A

ATP + myosin binding breaks link formed between actin and myosin
ATP hydrolysis provides energy for cross-bridge movement

53
Q

6 steps of cross-bridge cycle

A

1) Myosin bound to actin
2) Myosin dissociation
3) ATP hydrolysis
4) Conformational change
5) Power stroke
6) Myosin binds actin

54
Q

Describe Step 1 - Rigor state of the cross-bridge cycle

A

Myosin is tightly bound to actin at the actin binding site. The myosin head is at 45° relative to the filaments

55
Q

Describe Step 2 - Myosin dissociation of the cross-bridge cycle

A

ATP binds nucleotide binding site on the myosin which changes the configuration of the myosin head and allows it to dissociate from the actin binding site

56
Q

Describe Step 3 - ATP hydrolysis of the cross-bridge cycle

A

ATPase activity of myosin hydrolyses ATP into ADP + P. At this stage, both products are still bound to myosin which is unbound from actin.

57
Q

Describe Step 4 - Relaxed state of the cross-bridge cycle

A

Myosin head swings over and binds weakly to a new actin molecule, changing the angle from 45° to 90° relative to the filament. ADP and P are still bound to myosin.

58
Q

Describe Step 5 - Power stroke of the cross-bridge cycle

A

P dissociates from the myosin head which causes the head to rotate on its hinge back to 45°. It is still attached to the same position on the thin filament and therefore pushes the thin filament back with it - therefore power stroke.

59
Q

Describe Step 6 - Repositioning of the cross-bridge cycle

A

After power stroke the ADP dissociates from the myosin head. With this conformational change the myosin forms the rigor state attached to the actin once again.

60
Q

At rest, tropomyosin prevents:

A

Interaction between actin and myosin

61
Q

Troponin C has ____ Ca+2 binding sites

A

4

2 high affinity and 2 low affinity

62
Q

The binding of the 2 extra Ca+2 to troponin C causes:

A

The conformational change in the troponin complex which allows tropomyosin to shift in respect to the actin filament

63
Q

The mechanical coupling hypothesis

A

High density of dihydropyridine receptors in tetrads opposite 4 ryanodine receptors in SR terminal cisternae. Depolarisation of TT membrane flips DHPR, inducing conformational change in RyR which allows them to become open Ca+2 channels. Therefore DHPRs are essential for excitation-contraction coupling.

64
Q

4 benefits of voltage dependent excitation contraction coupling

A

1) Rapid kinetics
2) No dependence on current flow
3) No reliance of diffusion of substances from sarcolemma
4) Activation can occur in absence of extracellular Ca+2

65
Q

Isometric contraction

A

No external shortening takes place
Same length
Force of weight = force developed by muscle

66
Q

Isotonic contraction

A

Movement takes place

Same force

67
Q

2 types of isotonic contraction

A

Concentric

Eccentric

68
Q

Concentric contraction

A

Force of weight is less than force developed by muscle

Muscle shortens

69
Q

Eccentric contraction

A

Force of weight is more than force developed by muscle

Muscle lengthens

70
Q

Force velocity relationship

A

Load opposing contraction increases so velocity of shortening decreases

71
Q

Force = ?

A

Mass x Acceleration

72
Q

Work = ?

A

Force x Distance

73
Q

Power = ?

A

Work / Time

74
Q

Type 1 slow twitch fibres

A

Red due to myoglobin
Lots of mitochondria
Resistant to fatigue
Abundant in postural muscles and endurance athletes

75
Q

Type 2a fibres

A 
Fast oxidative
Hybrid of type I and II fibres
Red, lots of mitochondria
Anaerobic and aerobic
More prone to fatigue than type I
76
Q

Type 2b fibres

A 
Fast glycolytic
White
Anaerobic
Fatigue rapidly
Lots of power
77
Q

3 causes of muscle weakness

A

Muscle fatigue
Muscular dystrophy
Sarcopenia

78
Q

Muscle fatigue

A

Failure to maintain required or expected power output

Reduced muscle performance

79
Q

Central fatigue

A

Muscle fatigue resulting from decreased activation from CNS and decreased number of motor units recruited

80
Q

Peripheral fatigue

A

Muscle fatigue resulting from affected cellular mechanisms that control force such as smaller Ca+2 transient, reduced Ca+2 sensitivity of myofilamentsand slower crossbridge cycling

81
Q

Proposed causes of fatigue

A

Accumulation of metabolites

Depletion of muscle energy supplies

82
Q

4 key metabolite products that could accumulate

A

Lactic acid
Extracellular K+
Inorganic phosphate
ROS

83
Q

4 key products that could be depleted

A

Glucose
Creatine phosphate
ATP
Oxygen

84
Q

Duchennes muscular dystrophy

A

Mutation in dystrophin gene causes loss of dystrophin
Increased membrane permeability leads to skeletal muscle weakness and degeneration
Respiratory failure common around age 20

85
Q

Sarcopenia

A

Age related loss of muscle function

Muscle mass/body mass ratio decreases leading to significant loss of strength

Musculoskeletal (9 decks)

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