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BRS Musculoskeletal System > Muscle Microstructure > Flashcards

Muscle Microstructure Flashcards

1
Q

What are the 3 different muscle types?

A

Smooth, cardiac and skeletal muscle.

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2
Q

Describe the nucleation and innervation of each type of muscle.

A

Smooth - Mononucleated; under involuntary control from ANS.

Cardiac - Mononucleated under involuntary control from ANS.

Skeletal - Multinucleated; under voluntary control from SNS.

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3
Q

Where is smooth muscle found?

A

In the wall of the airways.

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4
Q

What are skeletal muscles usually attached to?

A

Bones via tendons

They contract to bring about movement.

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5
Q

What are individual muscles wrapped by?

A

Wrapped in sheath of connective tissue (Epimysium)

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6
Q

What does epimysium enable muscle to do?

A

Contract and move powerfully while maintaining structural integrity.

Epimysium separates muscle from other tissues.

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7
Q

What are muscle fibres arranged in?

A

Bundles known as fascicles.

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8
Q

What are muscle fascicles surrounded by?

A

Perimysium

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9
Q

What does fascicular arrangement enable?

A

Fascicular arrangement enables system to trigger specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle.

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10
Q

What is each myofibre (muscle fibre) surrounded by?

A

Endomysium - thin layer of collagen and reticular fibres.

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11
Q

What is sarcolemma?

A

Plasma membrane of myofibres

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12
Q

What is cytoplasm of myofibres known as and what is present in it?

A

Sarcoplasm

Myoglobin and MC present. Also glycogen granules.

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13
Q

What is the SR?

A

Network of fluid filled tubules, constitutes the main intracellular calcium store in striated muscle, cardinal role in the regulation of excitation-contraction coupling.

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14
Q

What are myofibres composed of?

A

Myofibrils

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15
Q

What is the smallest functional unit of skeletal muscle fibres?

A

Sarcomere

Shortening of individual sarcomeres leads to contraction of individual myofibres.

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16
Q

What 2 main proteins are myofibrils composed of?

A

Actin and myosin

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17
Q

What are T-tubules?

A

Extensions of the sarcolemma that penetrate into the centre of skeletal muscle cells, a conduit of APs.

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18
Q

What is the diameter of myofibrils?

A

1-2 micrometres

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19
Q

What are the ends of sarcomeres known as?

A

Z-lines

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20
Q

Describe the A-band.

A

Composed of thick filaments containing myosin, span the centre of the sarcomere extending towards the Z-discs.

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21
Q

Describe M-line.

A

Thick filaments are anchored at the middle of the sarcomere by myomesin.

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22
Q

Describe I-band.

A

Lighter regions contain thin actin filaments anchored at the Z-discs by alpha-actinin. Thin filaments extend into the A-band towards the M-line, overlapping with regions of the thick filament.

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23
Q

Why is A-band darker than I-band?

A

A band is dark due to thicker composition of myosin filaments, in addition to overlapping actin filaments.

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24
Q

What is the H-zone?

A

Middle of the A-band, thin filaments don’t extend into this region (No actin present).

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25
Q

What does a single sarcomere contain?

A

A single sarcomere contains a single dark A band, with half of the I band on either end.

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26
Q

Describe actin and myosin.

A

Actin - thin filaments

Myosin - thick filaments

27
Q

What happens during contraction?

A

During contractions, the length of myofilaments aren’t altered, however slide across each other, to the extent that the distance between the Z-discs shorten.

28
Q

Describe the structure of myosin.

A

2 globular heads.
Single tail by 2-alpha helices.
Tails of several hundred molecules from one filament.

29
Q

Describe the structure of actin.

A

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

30
Q

Explain the troponin protein complex.

A

TnI binds to actin, TnT binds to tropomyosin and TnC binds Ca2+ ions. Troponin and tropomyosin traverse along the actin filaments and control when the actin binding sites are available to bind with myosin.

31
Q

What happens to bands during muscle contraction?

A

I-band become shorter.
A-band remained same length.
H-zone narrowed or disappeared.

32
Q

Outline the initiation of muscle contraction.

A
  1. AP stimulates Ca2+ voltage-gated ion channels to open.
  2. Influx of Ca2+ into pre-synaptic knob, results in vesicles fusing with the pre-synaptic membrane and releasing Ach into synaptic cleft by exocytosis.
  3. ACh diffuses across synaptic cleft, binding ACh receptors on the post-synaptic membrane within the motor-end plate of the sarcolemma. Inducing an AP, voltage-gated Na+ channels open to depolarise membrane.
  4. Sodium ions enter the muscle fibre, AP is triggered along the membrane, initiating excitation-contraction coupling.
  5. ACh is hydrolysed in synaptic cleft by acetylcholinesterase, thereby preventing continued triggering of AP.
33
Q

Outline the activation of muscle contraction.

A
  1. Propagation of AP along sarcolemma into T-tubules.
  2. Voltage-sensitive DHPR in T-tubule mechanically linked to RyR in adjacent SR. AP stimulates conformational change of DHPR (Detects potential difference).
  3. Triggers opening of RyR, enabling Ca2+ efflux from the SR into the sarcoplasm.
  4. Ca2+ binds to TnC (Troponin complexes) causing tropomyosin to expose the myosin binding sites.
  5. Myosin globular heads forms cross-bridge with actin-binding site via the myosin binding site.
  6. Ca2+ actively transported into the SR continuously while AP continue. ATP-driven pump.
  7. Muscle contraction stops when there is a signal termination from the motor neurone, re-polarisation of sarcolemma and T-tubules, RyR close, and tropomyosin occupy the myosin binding sites on the actin filaments.
34
Q

Outline excitation-contraction coupling.

A
  1. In the presence of Ca2+ → Movement of troponin from tropomyosin chain.
  2. Movement exposes the myosin-binding site, on surface of actin chain.
  3. Charged myosin heads bind to actin forming the cross-bridge. Cross-bridge formation occurs when myosin-head attaches to actin, ADP and Pi bound to myosin.
  4. ADP and Pi released resulting in myosin to form a stronger attachment to the actin, myosin head retracts towards the M-line, consequently pulling the actin-filament into the A-band. Movement defined as ‘power stroke’ - movement of thin actin filament.
  5. ATP binding - Myosin head detaches from actin.
  6. ATP hydrolysed by the intrinsic ATPase activity of myosin into ADP and Pi. The energy released from ATP hydrolysis changes the angle of the myosin head into a cocked position, position for further movement.
35
Q

Outline neural control of muscle contraction.

A

Upper motor neurones in the brain - Primary motor cortex provides the descending corticospinal tract, decussating at the medullary pyramids within the medulla oblongata.

Terminates with lower motor neurones (Spinal cord or brainstem)

Voluntary neural control from upper and lower motor neurones.

36
Q

Define motor unit.

A

Single motor neurone in conjunction with corresponding muscle fibres that it innervates. Smallest functional unit with which to produce force.

37
Q

What do a collection of motor neurones of a muscle collectively coordinate?

A

Contractions of a single muscle, all of the motor units in a muscle are considered motor pool.

38
Q

What does stimulation of one motor unit cause?

A

Contraction of all the muscle fibres

39
Q

What is the innervation ration?

A

It defines the number of muscle fibres innervated by a single motor neurone. The innervation ratio is inversely proportional to the level of control.

40
Q

Describe the innervation required for fine control/delicate movements.

A

Innervation ratio will tend to be small, enabling nuances of movement of the entire movement.

41
Q

Describe the innervation required for coarse movements.

A

Motor units have high innervation ratio, no necessity for individual muscle fibres to undergo highly coordinated, differentiation contractions to produce a fine movement.

42
Q

What are the 3 motor fibre types?

A

Slow type I

Fast fatigue resistant (FR) - type IIA

Fast fatiguable (FF) - type IIB

43
Q

Explain the difference between type IIA and IIB.

A

IIA - ATP hydrolysis is faster, resulting in a faster cross-bridge cycling. Oxidative fibres is more resistant to fatigue considering that ATP is primarily produced through aerobic pathways.

IIB - Glycolytic fibres primarily generate ATP through anaerobic glycolysis, produces less ATP per cycle. Hence fatigue at a quicker rate, permitting them to be used for short periods - rapid, forceful contractions associated with quick, powerful movements.

44
Q

Structure of IIA, IIB and I?

A

IIA - Larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity.

IIB - exact same as IIA

I - Small soma diameter, small dendritic trees, thin axons, slowest conduction velocity.

45
Q

Colour of IIA, IIB and I?

A

IIA - Pink

IIB - White

I - Red

46
Q

Myoglobin content of IIA, IIB and I?

A

IIA - High

IIB - Low

I - High

47
Q

List the properties of IIA, IIB and I.

A

IIA - Fast twitch, moderate force, FR

IIB - Fast twitch, high force, high fatigue

I - Slow twitch, low force, FR

48
Q

What are motor units classified by?

A

Amount of tension generated, speed of contraction and fatiguability.

49
Q

What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?

A

Recruitment

Rate coding

50
Q

Are motor units randomly recruited?

A

Not randomly recruited.

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

51
Q

What happens to recruitment as 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.

52
Q

Explain rate coding.

A

A motor unit fires at a range of frequencies. Slow units fire at a lower frequency. As firing rate increases, the force produced by the unit increases.

53
Q

When does summation occur?

A

Units fire at frequency too fast to allow muscle to relax between arriving acton potentials.

54
Q

What can muscle force be regulated by?

A

Number of motor units recruited.

55
Q

What are neurotrophic factors?

A

Type of growth factor.

Prevent neuronal death and promote growth of neurones after injury.

56
Q

What are motor unit and fibre characteristics dependent on?

A

The nerve which innervates them.

57
Q

What happens if a fast and slow twitch muscle are cross innervated?

A

The slow one becomes fast and vice versa.

58
Q

What are the 3 types of muscle contraction and explain each one?

A

Concentric - Involves the muscles shortening to move a load.

Eccentric - Muscle tension diminishes, and a muscle lengthens.

Isometric - Tension is produced, however muscle length doesn’t change. Sarcomere shortening and increasing muscle tension, though the force produced can’t overcome resistance provided by load.

59
Q

Define muscle plasticity.

A

Ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it.

60
Q

In what conditions does conversion of type I to II occur?

A

Situations of severe de-conditioning or spinal cord injury.

61
Q

What does microgravity result in terms of muscle fibre types?

A

Slow to fast muscle fibre types.

62
Q

Explain the vestibular system.

A

Senses gravity.

Exposure to microgravity results in diminished load on the MSK system and hydrostatic pressure difference; contributes towards muscular atrophy.

63
Q

What is aging associated with in terms of muscle fibres?

A

Associated with loss of type I and II fibres, preferential loss fo type II fibres. Results in a larger proportion of type I fibres in aged muscle (Evidence from slower contraction times)

64
Q

What does training do to muscle fibres?

A

Conversion from IIB → IIA

BRS Musculoskeletal System (6 decks)

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