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Z YEAR 1 BRS Bones > Muscle Microstructure and Contraction > Flashcards

Muscle Microstructure and Contraction Flashcards

1
Q

What is smooth muscle?

A

-under involuntary control from the autonomic nervous system

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2
Q

What is cardiac muscle?

A

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

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3
Q

What is skeletal muscle?

A
  • under voluntary control, usually attached to bones and contract to bring about movement
  • Skeletal muscles are under voluntary control from the somatic nervous system
  • They are usually attached to bones
  • They contract to bring about movement
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4
Q

What is the structure of myofibres of skeletal muscles?

A
  1. Covered by plasma membrane – sarcolemma
  2. T-tubules tunnel into centre
  3. Cytoplasm called sarcoplasm – myoglobin and mitochondria present
  4. Network of fluid filled tubules – sarcoplasmic reticulum
  5. Composed of myofibrils
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5
Q

What is the structure of myofibrils of skeletal muscle?

A
  • 1-2μm in diameter
  • Extend along entire length of myofibres
  • Composed of two main types of protein – actin and myosin
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6
Q

What is the structure of myofilamenta of skeletal muscle?

A
  • Light and dark bands give muscle striated (striped) appearance
  • Do not extend along length of myofibres
  • Overlap and are arranged in compartments called sarcomeres
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7
Q

What are the different types of band?

A
  • Dense protein Z-discs separate sarcomeres
  • Dark bands – A band (thick - myosin)
  • Light bands – I band (thin - actin)
  • Myosin and actin filaments overlap
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8
Q

What is myosin?

A
  • Two globular heads
  • Single tail formed by two α-helices
  • Tails of several hundred molecules form one filament
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9
Q

What is actin?

A
  • Twisted into helix
  • Each molecule has myosin bonding site
  • Filaments contain troponin and tropomyosin
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10
Q

What happens in muscle contraction (sliding filament theory)?

A
  1. During contraction I band became shorter
  2. A-band remained same length
  3. H-zone narrowed or disappeared
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11
Q

What is the process of muscle contraction?

A
  1. Action potential opens voltage gated Ca2+ channels
  2. Ca2+ enters pre-synaptic terminal
  3. Ca2+ triggers exocytosis of vesicles
  4. Acetylcholine diffuses across cleft
  5. Binds to acetylcholine receptors and induces action potentials (AP) in muscle
  6. Local currents flow from depolarized region and adjacent region
    AP spreads along surface of muscle fibre membrane
  7. Acetylcholine broken down by acetylcholine esterase. Muscle fibre response to that molecule of acetylcholine ceases.
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12
Q

What is initiation of muscle contraction?

A
  1. Action potential propagates along surface membrane and into T-tubules
  2. Dihydropyridine (DHP) receptor in T-tubule membrane: senses DV & 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
  3. Ca2+ binds to troponin & tropomyosin moves allowing..
  4. Crossbridges to attach to actin
  5. Ca2+ is actively transported into the SR continuously while action potentials continue. ATP- driven pump (uptake rate < or = release rate).
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13
Q

What is the process of excitation contraction coupling?

A
  1. In the presence of Ca2+ -> movement of troponin from tropomyosin chain
  2. Movement exposes myosin binding site on surface of actin chain
  3. ‘Charged’ myosin heads bind to exposed site on actin filament
  4. This binding & discharge of ADP causes myosin head to pivot (the ‘power stroke’) -> pulling actin filament towards centre of sarcomere
  5. ATP binding -> releases myosin head from actin chain
  6. ATP hydrolysis -> provides energy to ‘recharge’ the myosin head
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14
Q

What is the neural control of muscle control?

A
  1. Upper motor neurones in brain
  2. Lower motor mourns in brainstem or spinal cord
  3. Voluntary neural control from upper and lower. motor neurons
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15
Q

Describe motor units

A
  • Name given to a single motor neuron together with all the muscle fibres that it innervates.
  • Humans have approximately 420,000 motor neurons and 250 million skeletal muscle fibres.
  • On average each motor neuron supplies about 600 muscle fibres.
  • Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
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16
Q

What is the slow types of motor unit?

A 
-Type I
•smallest diameter cell bodies
•small dendritic trees
•thinnest axons
•slowest conduction velocity
17
Q

What is the fast types IIA of motor unit?

A 
Fast, fatigue resistant (FR, type IIA)
•larger diameter cell bodies
•larger dendritic trees
•thicker axons
•faster conduction velocity
18
Q

What is the fast types of IIB of motor unit?

A 
Fast, fatiguable (FF, type IIB)
•larger diameter cell bodies
•larger dendritic trees
•thicker axons
•faster conduction velocity
19
Q

How are muscle fibre types distributed?

A
  • Randomly distributed throughout the muscle

- Muscles have different proportions of slow and fast twitch muscles

20
Q

How are motor units classified? Give examples

A

Motor unit types are classified by the amount of tension generated, speed of contraction and fatiguability of the motor unit.

  1. Type IIb: fast twitch, high force, high fatigue
  2. Type IIa: fast twitch moderate force, fatigue resistant
  3. Type I: slow twitch, low force, fatigue resistant
21
Q

What is recruitment mechanism?

A

-Motor units are not randomly recruited, there is an order.
-Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).
-As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.

22
Q

What is rate coding mechanism?

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.
  • Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
23
Q

How is muscle force regulated?

A

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

24
Q

What muscles are always on?

A

posturale (slow)

25
Q

What are characteristic of slow motor unit?

A

Myoglobin content: high
Colour: red
Aerobic capacity: high
Anaerobic capacity: low

26
Q

What are characteristic of IIA fibre?

A

Myoglobin content: high
Colour: pink
Aerobic capacity: moderate
Anaerobic capacity: high

27
Q

What are characteristic of IIB fibre?

A

Myoglobin content: low
Colour: white
Aerobic capacity: low
Anaerobic capacity: high

28
Q

What are the effect of neurotrophic factors?

A

-Greek trophē, food
-Are a type of growth factor
Prevent neuronal death
Promote growth of neurons after injury
-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
-The motor neuron has some effect on the properties of the muscle fibres it innervates

29
Q

What are the three types of muscle contraction?

A

Concentric
Eccentric
Isometric

30
Q

What are the plasciticty of motor units/muscle fibres?

A
  • Fibre types can change properties under many different conditions.
    1. Type IIB to IIA most common following training
    2. Type I to II possible in cases of severe deconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types
    3. Ageing associated 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).

Z YEAR 1 BRS Bones (5 decks)

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