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Medicine Year 1 - Semester 2 - Part 1 > Skeletal Muscle Force Generation, Energy Usage & Fibre Type > Flashcards

Skeletal Muscle Force Generation, Energy Usage & Fibre Type Flashcards

1
Q

What are sarcomeres ?

A

Highly organised, functional units of skeletal and cardiac muscle.

They have a striated appearance.

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2
Q

State the contractile proteins

A

Actin (thin filaments)
Myosin (thick filaments)

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3
Q

Z disc

A

Defines the boundary of each sarcomere - thousands of sarcomeres can make up a single muscle myofibre

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4
Q

I band

A

Mainly actin filaments

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5
Q

A band

A

Mainly myosin and overlapping actin filaments

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6
Q

Describe sarcomere shortening

A

A band remains constant
H zone and I band both shorten

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7
Q

State the role of ATP in muscle contraction

A

Membrane potential
Ca2+ gradient
Power stroke
Cross bridge dissociation

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8
Q

State the role of ATP in Membrane potential

A

Sodium/Potassium ATPase in sacrolemma maintains Na+ and K+ gradients, allowing production and propagation of action potentials.

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9
Q

State the role of ATP in Ca2+ gradient

A

Active transport of calcium ions into the sarcoplasmic reticulum - lowering [Ca2+]i

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10
Q

State the role of ATP in Power Stroke

A

Hydrolysis of ATP by myosin - ATPase energises the cross-bridge formation, enabling sarcomere shortening and contraction.

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11
Q

State the role of ATP in Cross bridge dissociation

A

Binding of ATP to myosin dissociates cross-bridges bound to actin.

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12
Q

How much ATP do muscles contain ?

A

4mM of ATP
which is enough for 2 seconds of contraction

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13
Q

State an energy source available for contraction

A

Stores of Creatine Phosphate in muscle provide enough energy for around 8 seconds of contraction.

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14
Q

Myosin + ATPase

A

Contraction

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15
Q

Ca2+ ATPase

A

Relaxation

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16
Q

Describe events during cross-bridge formation to generate sarcomere shortening

A

6 STEPS

  1. ATP binding
  2. ATP hydrolysis
  3. Cross-bridge formation
  4. Release of Pi from myosin
  5. Power stroke
  6. ADP release
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17
Q

How is cross bridge formation regulated ?

A

Regulation of cross-bridge formation is due to availability of myosin binding sites on actin, via [Ca2+]i and tropomyosin.

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18
Q

What is rigor mortis ?

A

Muscular stiffness that occurs after death - post mortem rigidity.

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19
Q

When does rigor mortis occur ?

A

Can begin around 4 hours after death, peaks at about 13 hours and lasts around 50 hours.

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20
Q

Describe how rigor mortis happens

A

Death

Loss of muscle cell integrity

Ca2+ leaks into the cytosol from the SR

Ca2+ binds to tropomyosin

Myosin binds to actin

Metabolism and ATP production ceases

No ATP present to break cross-bridge

Muscles become stiff

Proteolytic enzymes work within a few days

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21
Q

State the main different types of skeletal muscle fibre

A

Type IIB/IIX - Fast

Type IIA - Intermediate

Type I - Slow

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22
Q

State some properties of Type IIB/IIX skeletal muscle fibre

A

Fast
Glycolytic Anaerobic

Low in myoglobin oxygen

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23
Q

State some properties of Type IIA skeletal muscle fibre

A

Intermediate
Mixture of fast oxidative and glycolytic

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24
Q

State some properties of Type I skeletal muscle fibre

A

Slow
Oxidative Aerobic

High in Myoglobin oxygen

25
Q

How can skeletal muscle fibres be classified ?

A

Based on mechanical and metabolic characteristics :

  • Maximal velocities of contraction (fast/slow)
  • Major pathway for generation of ATP (oxidative or glycolytic)
26
Q

State the different physiological properties of skeletal muscle fibre types

A

Number of mitochondria
Amount of myoglobin
Blood vessels/capillaries

Stores of glycogen / glycolytic enzymes/ creatine phosphate
Size

27
Q

Which physiological properties are greater in slow oxidative fibres ?

A

Number of mitochondria
Amount of myoglobin
Blood vessels/capillaries

28
Q

Which physiological properties are greater in fast glycolytic fibres ?

A

Stores of glycogen / glycolytic enzymes/ creatine phosphate
Size

29
Q

Describe Slow oxidative fibres (Type I)

A

Combine slow myosin-ATPase activity with high oxidative capacity

30
Q

Describe Fast oxidative-glycolytic fibres (Type IIa)

A

Combine fast myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity.

31
Q

Describe Fast glycolytic fibres (Type IIb or Iix in human)

A

Combines fast myosin-ATPase activity with high glycolytic capacity

32
Q

Where do skeletal muscles receive stimuli from ?

A

From a motor neurone

Each myofibre/muscle cell has its own neuromuscular junction

33
Q

What does duration of contraction depend on ?

A

Fibre type composition of individual muscles

34
Q

Ocular muscle

A

Extremely rapid contraction velocity

35
Q

Gastrocnemius muscle

A

Moderately rapid contraction velocity

36
Q

Soleus muscle

A

Relatively slow contraction velocity

37
Q

How does force change ?

A

Force changes depending on recruitment of muscle fibres.

38
Q

What is a motor unit ?

A

All fibres innervated by a single neurone are called a motor unit.

A single motor neurone innervates multiple muscle fibres.

39
Q

Describe the muscle fibres of small muscles

A

Small muscles with fine control have fewer muscle fibres per neurone.

e.g. laryngeal muscles

40
Q

Describe the muscle fibres of large muscles

A

Large muscles may have hundreds of fibres in a motor unit.

e.g. soleus

41
Q

What does the force of contraction depend on ?

A

Number of motor neurons recruited
Frequency of action potentials

42
Q

How are motor units recruited ?

A

In a progressive way from smallest (weakest) to large (strongest)

43
Q

Describe small motor units

A

More excitable
Conduct action potentials more slowly
Typically Type I (slow) fibres

44
Q

Describe large motor units

A

Less excitable
Conduct action potentials more rapidly
Typically Type II (fast) fibres

45
Q

What is muscle tension ?

A

The force exerted by a contracting muscle

46
Q

What is load ?

A

The force exerted by an object to be moved

47
Q

What must muscle do in order to shorten ?

A

Overcome the force exerted by an object to be moved (the load).

Greater force required –> More motor units recruited

48
Q

What is summation ?

A

As muscle twitch far exceeds duration of AP, it is possible to initiate a second AP before 1st contraction has subsided.

The 2nd twitch is stronger than the 1st due to higher [Ca2+]i

49
Q

What is frequency summation ?

A

Multiple action potentials occurring close together

50
Q

What is tetanus ?

A

Stimulation frequency is so high that individual contractions fuse.

51
Q

What is isotonic contraction ?

A

Muscle shortening will occur of peak tension is greater than the load force.

52
Q

What is isometric contraction ?

A

Muscle stimulation will increase tension, but no shortening will occur if load force is greater than muscle peak force.

53
Q

Define the length tension relationship

A

Length tension is directly related to the overlap between actin and myosin within the sarcomere.

54
Q

When is tension high ?

A

When actin and myosin overlap within the sarcomere

55
Q

Zero tension

A

No actin/myosin overlap

56
Q

Describe the load-velocity relationship

A

When loads are applied, contraction velocity decreases with increasing load.

57
Q

Describe the load-velocity relationship of muscle contraction

A

When the load equals the maximum force or tension that a muscle can exert, velocity of contraction is zero (isometric contraction)

58
Q

Describe full muscle contraction with no load

A

Rapid
around 100ms

Medicine Year 1 - Semester 2 - Part 1 (51 decks)

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