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Semester 2 Year 1 > Force Generation, Energy Usage and Fibre Types > Flashcards

Force Generation, Energy Usage and Fibre Types Flashcards

1
Q

Give a brief review of sarcomere structure

A
  • Sarcomeres are highly organised within skeletal & cardiac muscle and give a striped appearance – striated muscle
  • A sarcomere is the portion of a myofibril that lies between two successive Z disks
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2
Q

What contractile proteins are found in sarcomeres?

A
  • Thin filaments (actin protein)

* Thick filaments (myosin protein)

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3
Q

What bands represent what contractile proteins in sarcomeres?

A
  • I band (isotropic) represents actin (thin) filaments

* A band (anisotropic) represents myosin (thick) filaments

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4
Q

Provide a brief review of cross bridge cycling?

A

• Ca2+ modulates contraction through regulatory proteins rather than direct interaction with contractile proteins
• Without the presence of Ca2+ these regulatory proteins act to inhibit actin-myosin interactions
• Troponin C is key Ca2+ sensitive regulator
– 2 high affinity binding sites (help in binding troponin C to thin filament)
– 2 low affinity binding sites (binding of Ca2+ to these sites brings about conformational change in troponin complex)
• When [Ca2+]i rises and Ca2+ binds to TnC a conformational change of proteins occurs
• Troponin I moves away from actin filament
• Troponin T pushes tropomyosin away from actin binding site
• As long as Ca2+ is present, multiple cross-bridges cycles can occur and muscle shortens
• When [Ca2+]i falls, Ca2+ dissociates from TnC leading to a reversal of the conformational change

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5
Q

What are the roles of ATP in skeletal muscle contractions?

A
  • Hydrolysis of ATP by the Na+/K+-ATPase in sarcolemma maintains Na+ and K+ gradients
  • Hydrolysis of ATP by the Ca2+-ATPase in the sarcoplasmic reticulum provides the energy for the active transport of calcium ions into the reticulum
  • Hydrolysis of ATP by myosin-ATPase energises the cross-bridge formation providing energy of force generation
  • Binding of ATP to myosin dissociates cross-bridges bound to actin
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6
Q

What does the hydrolysis of ATP by the Na+/K+-ATPase in sarcolemma maintaining Na+ and K+ gradients allow?

A

production and propagation of action potential

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7
Q

What does hydrolysis of ATP by the Ca2+-ATPase in the sarcoplasmic reticulum providing the energy for the active transport of calcium ions into the reticulum, so lowering Ca2+ allow for?

A

Allows for relaxation to occur as calcium ions are removed from the sarcomere and sequestered in the sarcoplasmic reticulum
*

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8
Q

What does the binding of ATP to myosin dissociating the cross-bridges bound to actin allow for?

A

Allows for bridges to repeat cycle activity

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9
Q

What are the 5 stages of the cross-bridge cycle in skeletal muscle?

A
  • Step 1 – ATP binding
  • Step 2 – ATP hydrolysis
  • Step 3 – Cross-bridge formation
  • Step 4 – Release of Pi from myosin
  • Step 5 – ADP release
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10
Q

What is the initial position of the myosin head in the cross-bridge cycle of skeletal muscle?

A

Initially myosin head attached to actin filament after “power stroke” from previous cycle – can remain in this state for an indefinitely long period, as occurs in rigor mortis

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11
Q

Describe the relationship between ADP-free myosin complex and ATP in the regulation of cross bridge cycle in skeletal muscle

A
  • ADP-free myosin complex (attached state) would quickly bind ATP at the concentrations found in cells
  • Muscle cells do not regulate cross- bridge formation by regulating [ATP]i
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12
Q

Describe the regulation of cross bridge cycle in skeletal muscle

A
  • Muscle cells control cycle by regulating step 3 i.e. preventing cross-bridge cycling until tropomyosin moves out of way as a result of es in [Ca2+]I
  • muscle cells need to resynthesize ATP from ADP
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13
Q

Describe the skeletal muscle energy metabolism (glycolysis and oxidative phosphorylation)

A
  1. Gives very rapid ATP formation at the onset of muscle contraction
  2. Oxidative phosphorylation supplies most ATP in “moderate” levels of activity (occurs in mitochondria)
  3. At higher intensity exercise, glycolysis dominates (occurs in cytosol)
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14
Q

Describe the different sources of ATP in “moderate” exercise over time

A
  1. From muscle glycogen (5-40min exercise)
  2. Blood glucose, and fatty acids (next 30 mins of exercise)
  3. Then fatty acids predominate
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15
Q

Expand on the gross action of glycolysis at higher intensity exercise where glycolysis dominates (occurs in cytosol)

A
  1. Small quantities of ATP are produced BUT at a higher rate

2. The source is either glucose or glycogen

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16
Q

How is muscle force determined in skeletal muscle?

A

• Muscle force can be determined by number of individual muscle fibres stimulated at a given time

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17
Q

Describe the process of muscle force generation

A
  • Multiple-fibre summation (or spatial summation)

* Small motor neuron cell bodies will be recruited first then larger and larger cell bodies (motor units)

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18
Q

What 5 things does the amount of force generated in whole muscle force generation in skeletal muscle depend on?

A 
– Number of active muscle fibres
– Cross-sectional area of muscle
– Initial resting length of muscle
– Rate at which muscle shortens
– Frequency of stimulation
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19
Q

In what order are motor units recruited in size principle in skeletal muscle?

A

• Motor units recruited in a progressive order from smallest unit (weakest) to largest unit (strongest)

20
Q

Compared to large motor units, small motor units are:

A

– more excitable
– conduct APs more slowly
– excite fewer fibres that tend to be
Type I

21
Q

Compared to small motor units, large motor units are:

A

– less excitable
– conduct APs more rapidly
– excite many fibres that tend to be
Type II

22
Q

Do skeletal muscle fibres all have the same mechanical and metabolic characteristics?

A

Naw

23
Q

How are skeletal muscle fibres classified?

A

They are classified based on:
– their maximal velocities of shortening (fast or slow)
– the major pathway they use to form ATP - oxidative or glycolytic

24
Q

How do fast and slow skeletal muscle fibres differ?

A

• Fast and slow fibres contain forms of myosin that differ in the maximal rates at which they use ATP

25
Q

How does the maximal rate of ATP usage in fast/slow skeletal muscle fibre affect the function of the muscle?

A

• This determines the maximal rate of cross-bridge cycling and thus the maximal shortening velocity

26
Q

What are oxidative skeletal muscle fibres?

A

Muscle fibres that contain numerous mitochondria and therefore have a high capacity for oxidative phosphorylation

27
Q

What are glycolytic skeletal muscle fibres?

A

Muscle fibres that have few mitochondria but possess a high concentration of glycolytic enzymes and a large store of glycogen

28
Q

What is the difference between glycolytic and oxidative skeletal muscle fibres?

A

Oxidative fibres generate more ATP through oxidative phosphorylation in normal conditions and have the organelles to support this
Glycolytic fibres generate more ATP through glycolysis in normal conditions and have the enzymes and glycogen stores to support this

29
Q

Based on the mechanical and metabolic classification of skeletal muscle fibre (slow/fast, oxidative/glycolytic) what are the three principal types of skeletal muscle fibre?

A
  • Slow-oxidative fibres (Type I)
  • Fast-oxidative-glycolytic fibres (Type IIa)
  • Fast-glycolytic fibres (Type IIb or IIx in human)
30
Q

What are slow-oxidative fibres (Type I)?

A

Skeletal muscle fibres that combine low myosin-ATPase activity with high oxidative capacity

31
Q

What are fast-oxidative-glycolytic fibres (Type IIa)?

A

Skeletal muscle fibres that combine high myosin-ATPase activity with high oxidative capacity and intermediate glycolytic capacity

32
Q

What are fast-glycolytic fibres (Type IIb or IIx in human)?

A

Skeletal muscle fibres that combine high myosin- ATPase activity with high glycolytic capacity

33
Q

How may type I and II skeletal muscle fibres be identified?

A

by the nature of their myosin ATPase and the amount of specific mitochondrial enzyme succinate dehydrogenase

This can be seen on a micrograph where type I fibres generally stain dark, type IIa stain lighter and type IIx stain poorly

34
Q

What is isometric muscle contraction?

A

Stimulation of a muscle that causes increase in tension but no shortening

35
Q

When does isometric muscle contraction occur?

A

When muscle length is fixed

36
Q

What is isotonic muscle contraction?

A

Stimulation of a muscle that causes muscle shortening, provided tension generated is greater than opposing load

37
Q

When does isotonic muscle contraction occur?

A

When muscle length is not fixed

38
Q

Describe the length tension relationship in reference to percent of maximum active tension and length as a percentage of optimal length

A

The relationship may be graphed as a kinda inverted sigmoid, with peaks at 100% and 145%

39
Q

What 3 types of tension can be measured to analyse length-tension relationship of skeletal muscle?

A

• Total Tension – tension measured at
various muscle lengths during contraction
• Passive Tension - tension measured at any fixed length before muscle contraction
• Active Tension – tension measured at any fixed length when muscle is contracted due to cross-bridge formation

40
Q

What is the length of time a single skeletal muscle twitch lasts and what causes it?

A

from 25 -200 msec,

caused by one action potential

41
Q

What phenomena may be a result of the fact that a single muscle twitch outlasts a single action potential?

A

it is possible to initiate a second AP before 1st contraction has subsided,
This is known as temporal summation (when neurotransmitters are released at a pre-synaptic neuron over a longer period of time)

42
Q

What occurs in skeletal muscle fibre when multiple action potential occur close together?

A

Frequency summation

43
Q

What occurs in skeletal muscle fibre when stimulation frequency is so high that individual contractions fuse?

A

Tetanus

44
Q

What is tetanus?

A

stimulation frequency so high that individual contractions fuse

45
Q

What is the load (tension)-velocity relationship?

A

• Velocity of shortening decreases as applied load increases

– Can lift a chip quicker than a sack of potatoes

46
Q

Why does the load(tension)-velocity relationship occur?

A

• Consider maximal load for a resting muscle
length (isometric condition)
• All available cross-bridges are engaged with resisting applied load, none available to shorten muscle
• At slightly smaller load but same muscle length, fewer cross-bridges needed to oppose load, so available to shorten muscle

47
Q

What is the velocity of shortening in skeletal muscle limited by?

A

by time it takes for ATP-consuming cross-bridge cycle to occur

Semester 2 Year 1 (82 decks)

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