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MSS- Musculoskeletal System > S1: Microstructure and Function of Human Muscle Systems > Flashcards

S1: Microstructure and Function of Human Muscle Systems Flashcards

1
Q

What is muscle?

A

It is any contractile tissue

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2
Q

What is the function of muscle?

A

To generate forces through contraction

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3
Q

What are contractile forces affected by?

A

Interaction of the intracellular proteins actin and myosin which are contractile proteins

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4
Q

What are the three muscle types?

A

2 striated muscle (skeletal and cardiac muscle)

1 non striated muscle (smooth muscle)

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5
Q

What other muscles apart from smooth are non striated?

A 
Myoepithelial cells (make glands contract and iris of eye)
Myofibroblasts (in healing wounds)
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6
Q

What are the different functions of skeletal, cardiac and smooth muscle?

A

Skeletal:

  • Movement
  • Rapid contraction but subject to fatigue
  • Voluntary control

Cardiac:

  • In the heart
  • Quite rapid and resists fatigue
  • Contract regularly for a very long period of time
  • Involuntary control

Smooth:

  • Slower contraction but very powerful
  • Energy efficient and little fatigue
  • Involuntary control with one exception which is the bladder
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7
Q

What muscle is associated with the skeleton, body wall, diaphragm and limbs?

A

Skeletal muscle

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8
Q

Skeletal muscle cells (muscle fibres) are syncytial.

What does this mean?

A

This means they have many nuclei, due to being formed by fusion of embryonic muscle precursor cells (myoblasts) with one nucleus each.

Each mature skeletal muscle fibre contains thousands of nuclei in peripheral rows

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9
Q

Explain the structure of skeletal muscle in bundles

A
  • The whole muscle is surrounded by connective tissue called epimysium
  • The muscle itself is made of a bundle of fascicles and each of these are surrounded by loose connective tissue called perimysium.
  • A fascicle is a bundle of muscle fibres and each one is surrounded by a plasma membrane called the sarcolemma
  • Endomysium is between fibres
  • Sarcolemma surrounds fibres.
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10
Q

Describe the structure of a muscle fibre

A

A muscle fibre contains bundles of myofibrils each surrounded by a sarcosplasmic reticulum and T tubules.
The cell membrane of the muscle fibre is called sarcolemma.
There is fine connective tissue around each fibre is called endomysium.

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11
Q

Describe the structure of a microfibril

A

Bundle of thick protein filaments (myosin) and thin protein filaments (actin) surrounded by each other.
It is in hexagonal array.

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12
Q

What allows independent contraction of different fibres and fascicles?

A

The connective tissue sheaths carry the blood and nerve supply which provides mechanical support and allows independence of contraction of different fibres and fascicles.

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13
Q

Summarise the structure of skeletal muscle

A

Muscle > Fascicle > Muscle Fibre ( a cell) > Myofibrils (in cell)

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14
Q

What is a motor unit?

A

Large motor nerves which supply fascicles and control skeletal muscle contraction

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15
Q

Why is skeletal muscle called voluntary muscle?

A

It is under conscious control

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16
Q

Describe the shape of a skeletal muscle fibre

A

They are like very long spindles. Up to 4 cm in length and 40-100 micrometres wide

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17
Q

Are skeletal myofibrils branched?

A

No

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18
Q

Describe the ‘Sliding filament model’

A

Actin slides over myosin during contraction
A stays the same
I band gets narrower

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19
Q

What happens at motor end plates?

A

Skeletal muscle contracts when it encounters neural stimulation via motor end plates at the end of nerves.

The SR carries the signal to all parts of this very large cell (fibre) because diffusion is not fast enough

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20
Q

What are type 1 fibres?

A

They are slow twitch fibres and depend mainly on aerobic metabolism and contain lots of mitochondria and myoglobin.

21
Q

What are type 2 fibres?

A

They are fast twitch fibres and depend on anaerobic metabolism and contain less mitochondria and myoglobin but rich in glycogen and glycolytic enzymes.

22
Q

How can type 1 and type 2 fibres be distinguished histologically?

A

There different staining with myosin, ATPase, succinate dehydrogenase, NADPH, PH and glycogen.

Muscle groups contain a mix of fast/slow twitch fibres and the relative amount depends on function of the main muscle group.

23
Q

What causes dark and light bands in skeletal muscle?

A

They are caused by the A and I bands standing for anisotropic and isotropic (appearences with polarised light)

A- Apparent (dark bands)
I- Invisible (light bands)

24
Q

What does the A band contain?

A

Both thick and thin filaments overlapping

25
Q

What does the I band contain?

A

Contains mainly thin filaments.

26
Q

What is thick and thin filaments mostly made of?

A

Thick filaments contain mostly the protein myosin, while thin filaments mostly contain the protein actin.

27
Q

What is stimulus-contraction coupling?

A

It is the physiological process of converting an electrical stimulus to an mechanical response

28
Q

What are T tubules?

A

They stand for ‘transverse tubules’. They are a honeycomb of intracellular tubules that are continuous with the sarcolemma and pass around each myofibril at each A-I junction.

29
Q

What stores Ca2+ ions and wraps around the microfibril?

A

The specialised network of smooth endoplasmic reticulum called Sarcoplasmic reticulum (SR)

30
Q

What is a triad?

A

Specialised enlarged cisternae (terminal cisternae) of the reticulum in close proximity to the T tubules.

31
Q

What is a triad?

A

Specialised enlarged cisternae (terminal cisternae) of the reticulum in close proximity to the T tubules in triplet structures.

32
Q

Where does the AP in T tubules travel to?

A

An AP in the T-tubules causes a signal through the triads to the sarcoplasmic reticulum, which causes muscle contraction.

33
Q

Why are there many mitochondria between myofibrils?

A

To provide energy for contraction and glycogen granules are also plentiful (as a store of energy)

34
Q

What is cardiac muscle called?

A

Myocardium

35
Q

What are the similarities between cardiac muscle and skeletal muscle?

A

Striations
Sarcomeres
Abundant mitochondria and glycogen
- T tubules and SR in heart are not as regularly organised as in smooth muscle

36
Q

What are the differences between cardiac muscle and skeletal muscle?

A
  • Cardiac fibres are much smaller with usually only one cenral (or two) nuclei
  • Myofibrils branch and are less discrete (seperate and distinct)
  • Fibres are joined end to end to end by huge junctional complexes called intercalated discs
  • Dermosomes present (attach adjacent cells)
  • Many gap junctions
  • Contraction initiated within special muscle cells (SAN) rather than nerves
  • Excitation spreads from cell to cell through gap junctions of intercalated discs
  • -> Relatively slow so distinct wave of contraction spreads through heart at each beat.
  • Cardiac muscle doesn’t contain a stem cell population therefore regeneration following damage is not possible.
37
Q

What happens at intercalated discs at cardiac muscle?

A

Here actin filaments are anchored into large, unusual, sheet like intermediate junctions each called fascia adherens.

38
Q

What are gap junctions (intercalated discs)?

A

Special junction that allows the passage of various molecules, ions and electrical impulses to directly pass through a regulated gate between the cytoplasm of two cells.

39
Q

List some functional aspects of cardiac muscle

A
  • Branching fibres provide extra strength and resistance to splitting by high blood pressure
  • Small diameter fibres allow rich blood supply and additional connective tissue (for strength)
  • Numerous mitochondria allow continuous energy supply so cardiac muscle is resistant to fatigue
  • Intercalated disks give very strong attachment between fibres (actin filaments attach to them)
40
Q

How is contraction initiated in cardiac muscle?

A
  • ‘Myogenic Stimulus’ - Action potential starts from the pacemaker region SAN in the right atrium and carries a wave of contraction across the heart assisted by ion diffusion through gap junctions in the intercalated disks
  • Larger purjinke (modifies muscle) fibres carry stimulus rapidly to ventricle
41
Q

What type of action is smooth muscle responsible for?

A

Smooth muscle is responsible for most involuntary actions mostly in visceral organs and blood vessels

42
Q

How fast is smooth muscle contraction?

A

Slower than striated muscle but can be sustained for longer and very powerful

43
Q

What surrounds each smooth muscle fibre?

A

Collagen

They are attached to each other by collagen

44
Q

Why might smooth muscle be considered a more primitive form of muscle?

A

It is less organised

45
Q

What are the similarities of smooth muscle to striated muscles?

A
  • Thick and thin filaments fill most of the cytoplasm (except in area around nucleus which contains mitochondria and glycogen)
  • Higher proportion of thin filaments than in skeletal or cardiac muscle so array doesn’t look hexagonal
  • SR present that contains and releases Ca2+ but less of it and less organised
  • Fibres linked by gap junctions but not in large numbers
46
Q

What are the differences of smooth muscle to striated muscles?

A
  • Contraction may be initiated by various signals including neural stimuli and diffusible or blood borne signals like histamine or NA.
  • Smooth muscle fibres are spindle shaped with one central nucleus
  • Smaller compared to other muscle fibre types but relatively large compared to other cells
  • No T tubules
  • Caveolae are numerous at cell membrane and probably have role in signal transduction
  • Fibres anchored to basement membrane instead of each other
  • Structures of smooth muscle allows force to be exerted at greater degree of stretch than for striated muscle - this is due to the relatively few myosin (thick) and more actin.
47
Q

Does smooth muscle have myofibrils and sacromeres?

A

No

48
Q

Why does smooth muscle have dense bodies?

A

Anchors for contraction

MSS- Musculoskeletal System (14 decks)

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