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MBBS Year 2 - Physiology > MSK - Muscle Physiology > Flashcards

MSK - Muscle Physiology Flashcards

1
Q

Describe skeletal muscle.

A
  • Cylindrical
  • Under voluntary control
  • Multinucleated
  • Striated

Involved in bone movement

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2
Q

Describe smooth muscle.

A
  • Under involuntary control
  • Mononucleated
  • Non-striated
  • Found in hollow organs

Involved in substance movement

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3
Q

Describe cardiac muscle

A
  • Under involuntary control
  • Mono/binucleated
  • Striated
  • Branched

Involved in pumping of blood

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4
Q

How are skeletal muscle fibres arranged?

A
  • Muscle composed of groups of muscle fibres
  • Fibres bundled into fascicles
  • Within the fascicles are muscle fibres, within which are individual myofibrils
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5
Q

Describe muscle fibre cell structure. PART 1

A
  • Cells wrapped in sarcolemma
  • Invaginations in sarcolemma - T-Tubules descend into cells and wrap around myofibrils
  • Tubules increase SA for ion transfer between ECF and ICF
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6
Q

Describe muscle fibre cell structure. PART 2

A
  • SR wrapped around myofibrils - store of calcium ions
  • Terminal cisternae at ends of SR
  • Point where cisternae meets T-tubules is the triad
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7
Q

Describe myofibril structure.

A
  • Striated due to intraccellular cytoskeletal proteins
  • Made up of myosin and actin chains
  • Myosin is also known as the heavy chain, while actin is known as the light chain. Where these bands cross is where a band appears.
  • Functional units are sarcomeres
  • Tropomyosin wrapped around actin chains with troponin on its surface

Action potential generated in muscle cell to contract skeletal muscle

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8
Q

What is the relationship between muscle fibres and motor neurons?

A
  • Muscle fibre innervated by single motor neuron
  • Motor neurons innervate multiple fibres
  • In areas that require fine control, motor neuron will only innervate few muscle fibres.
  • OPPOSITE FOR AREAS THAT DON’T NEED FINE CONTROL
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9
Q

Outline what occurs at the neuromuscular junction. PART 1

A
  • Pre-synaptic terminals release ACh which bind to and cause opening of post-synaptic nAChRs
  • Ion channels activate causing sodium influx and cause action potential
  • Wave of depolarisation across sarcolemma due to action potential induced which reaches T-junction.
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10
Q

Outline what occurs at the neuromuscular junction. PART 2

A
  • Wave of depolarisation reaches T-tubules
  • Activates VGCCs allowing influx of extracellular calcium into cell
  • Channels connected to RyRs on wall of SR
  • When RyRs activated, conformational change in RyRs
  • The SR is a reservoir for intracellular calcium and when the RyR is activated, calcium flows into the cell.
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11
Q

Outline what occurs at the neuromuscular junction. PART 3

A
  • RyR activated and calcium flows into cell
  • Rise in intracellular calcium and myofibrils contract
  • Removal of calcium through SERCA2 (back into SR) and sodium-calcium exchanger (out of cell in exchange for sodium)
  • Myofibrils relax
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12
Q

What happens in the cross bridge cycle? PART 1

A
  • When relaxed, actin chains bound by troponin and tropomyosin. Prevent myosin binding
  • Myosin bound by ADP, are ‘cocked’
  • Calcium binds to troponin C following rise in intracellular calcium
  • Myosin binding sites are exposed as troponin released
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13
Q

What happens in the cross bridge cycle? PART 2

A
  • Myosin binds
  • Conformational change causing movement of myosin head
  • Causes movement of Z bands closer together
  • ADP released from myosin and ATP binds causing release of myosin head from actin.
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14
Q

What happens in the cross bridge cycle? PART 3

A
  • Reduction in intracellular calcium. Troponin reinstated and binding sites blocked
  • ATP hydrolysed back to ATP
  • Myosin head moved back into cocked position
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15
Q

Why does rigor mortis occur?

A
  • Rise in intracellular calcium
  • Actin binding sites exposed
  • Myosin binds
  • No ATP available to remove myosin heads from actin binding sites
  • Continuous contraction
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16
Q

Describe Type 1 fibres

A
  • ‘Slow twitch’ - used when long-term sustained contractions requireed
  • ATP synthesised by oxidative metabolism
  • Require high oxygen supply so large amounts of myoglobin to facilitate this
  • Because they use oxidative metabolism, which in the presence of sufficient oxygen supply can supply enough energy to sustain contraction they are fatigue resistant (hence suited for long-term contractions)
17
Q

Describe Type IIa fibres.

A
  • Utilise oxidative and anaerobic metabolism
  • Anaerobic metabolism - fast but inefficient so contraction is rapid and provide more power than slow twitch fibres
  • Oxidative metabolism allows fibres to be slightly fatigue resistant
18
Q

Describe Type IIb fibres.

A
  • Fast twitch - reliant on anaerobic metabolism
  • Don’t have large amounts of myoglobin/mitochondria
  • Easily fatigable
  • Anaerobic respiration will provide energy quickly, but is much slower.
19
Q

Describe gap junctions in cardiac muscle

A
  • Muscle cells joined by gap junctions
  • Allows for spread of depolarisation to neighbouring cells (initiated from SAN)
20
Q

Describe electrical-contraction coupling in cardiac muscle.

A
  • Calcium entry into cell from extracellular spaces at T-tubules
  • Activates RyRs - causing calcium release from SR
  • Calcium interacts with myosin and actin to cause contraction
  • Removal by SERCA and NCX exchanger
21
Q

Describe multiunit smooth muscle cells.

A
  • Exist as separate entities stimulated by their own motor nerves
  • Behave independently
  • Have swellings called varicosities which are pre-synaptic terminals
22
Q

Describe unitary smooth muscle cells.

A
  • Most common type
  • SMCs joined by gap junctions - allowing for passage of depolarising ions from cell to cell
  • Only a few muscle cells need to be innervated by autonomic neurons
  • Portion of muscle that contracts are functional units
23
Q

Describe autonomic-mediated smooth muscle contraction

A
  • Activation of Gq linked GPCRs
  • Increase intracellular IP3
  • Binds to IP3 receptors on SR causing calcium release into cytosol
24
Q

What is another autonomic mediated method of muscle contraction?

A
  • Opening of calcium ion channels
  • Activates ryanodine receptors in SR - calcium released into cytosol
  • Calcium channels close once threshold met
  • Activity of neighbouring cells influenced by gap junctions
25
Q

What is the difference between skeletal/cardiac and smooth muscle contraction?

A
  • Smooth muscle contraction - dependent on calcium influx (not sodium)
  • Smooth muscle contraction slower - calcium channels open slower than sodium channels
26
Q

Describe muscle fibre recruitment

A

Muscle fibres are recruited in a step wise fashion that is dependent on the amount of force require. A small amount of force (e.g. picking up a pencil) requires recruitment only of type 1 fibres. Maximal force (e.g. running a 100m sprint) requires the recruitment of all fibre types

27
Q

Describe muscle fibre recruitment neurologically. PART 1

A
  • Recruitment achieved through the activation of alpha motor neurons of differing size
  • As the need for greater strength increases, so too do the signals originating from the CNS.
  • For muscle movements that require very little strength (i.e. picking up a pencil), the intensity of signals arising from the CNS is very low, and as such these preferentially activate alpha motor neurons with small cell bodies (as these reach threshold for action potential much easier).
  • These small alpha motor neurons typically innervate type 1 muscle fibres
  • RMP reaches threshold quicker. Slow conduction velocity
28
Q

Describe muscle fibre recruitment neurologically. PART 2

A

As greater strength is required, the signals arising from the CNS increase in intensity, leading to recruitment of larger alpha motor neurons which preferentially recruit fast twitch type 2 muscle fibres
- RMP reaches threshold slower
- Fast conduction velocity
- Innervate large number of Type II fibres
- Type IIa fibres recruited second
- Type IIb fibres recruited last

MBBS Year 2 - Physiology (22 decks)

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