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Exercise Physiology > Week 4 - Muscle Structure and Function > Flashcards

Week 4 - Muscle Structure and Function Flashcards

1
Q

What % of total body mass is made up of skeletal muscle?

A

40-50%

over 600 voluntary skeletal muscles

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2
Q

5 functions of skeletal muscle

A

1) Force production for locomotion and breathing
2) Force production for postural support
3) Heat production during cold stress to maintain internal body temperature
4) Acts as an endocrine organ
5) Support and stabilization of joints

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3
Q

Explain the 2 muscle actions.

A
  • Flexors (decrease angle joint)
  • Extensors (increase angle joint)

They attach to bones by tendons

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4
Q

Define the key words that make up the structure of skeletal muscle.
a) Epimysium
b) Perimysium
c) Endomysium
d) Basement membrane
e) Sarcolemma

A

a) Surrounds entire muscle
b) Surrounds fascicles (bunch of muscle fibres)
c) Surrounds individual muscle fibers
d) Below endomysium
e) muscle cell membrane

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5
Q

What do myofibrils contain?

A

contain contractile proteins; actin (thin filament) and myosin (thick filament) in the sarcomere

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6
Q

Sarcomere

A

The main functional/ contractile unit of a muscle fiber.
Includes Z line, M line, H zone, A band and I band.

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7
Q

a) Z line
b) M line
c) A band
d) I band

A

a) define the boundaries of a sarcomere
b) middle of sarcomere
c) contains actin and myosin
d) contains actin

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8
Q

Sarcoplasmic reticulum

A

storage sites for calcium and the terminal cisternae lies between each sarcoplasmic reticulum

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9
Q

Transverse tubules

A

extend from sarcolemma to sarcoplasmic reticulum

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10
Q

Role of satellite cells

A

Undifferentiated cells reside above sarcolemma - they have a key role in muscle growth and repair. During muscle growth, they increase the number of nuclei in mature muscles fibers (muscle hypertrophy).

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11
Q

Motor unit

A

motor neuron and all the muscle fibers it innervates

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12
Q

Neuromuscular junction

A

chemical synapse between motor neuron and muscle fiber - made up of axon terminal, synapse and motor end plate of muscle fiber.

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13
Q

Acetylcholine

A

neurotransmitter released from the motor neuron into the synapse where it binds to nicotinic receptors on the motor end plate of the sarcolemma that causes an end-plate potential (EPP) (influx of sodium ions into muscle fiber) thus depolarization of the muscle fiber and signaling for muscle contraction.

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14
Q

Myonuclear domain - What is it and why is it important?

A

the volume of sarcoplasm surrounding an individual nucleus and its important as a single nucleus is responsible for the gene expression for its surrounding sarcoplasm.

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15
Q

What does more myonuclei allow for?

A

greater protein synthesis resulting in muscle hypertrophy

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16
Q

Training adaptations of the neuromuscular junction

A
  • Increased size of NMJ
  • Increased number of synaptic vesicles releasing ACh
  • Increase number of ACh receptors on post-synaptic membrane
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17
Q

Sliding filament model of muscular contraction

A

refers to the formation of cross bridges which allows actin filaments to slide over myosin and the muscle shortens (power stroke). There is a reduction in the distance between the Z lines of the sarcomere.

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18
Q

ATP role in muscle contraction

A

ATP hydrolyzed via enzyme myosin ATPase (ATP –> ADP + P) provides energy required for cross-bridge formation and a power stroke.

Sources of ATP: PCr, glycolysis, and oxidative phosphorylation.

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19
Q

True or False - A band changes size during contraction

A

False - A band stays the same size

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20
Q

What % of resting length is a muscle shortened during a single contraction cycle? During a repeated contraction cycle, what % of resting length can muscle shorten?

A

1% + up to 60%

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21
Q

Brief outline of the 5 steps of crossbridge cycling.

A

1) ATP binds to myosin head and breaks link between actin and myosin.
2) ATP hydrolysis (via myosin ATPase) with ADP and Pi remaining attached to myosin head.
3) Pi dissociated from myosin which allows myosin to bind with actin binding sites and form a cross-bridge.
4) Power stroke occurs with the muscle shortening to generate force.
5) ADP dissociates from myosin and myosin awaits another ATP molecule.

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22
Q

What is excitation-contraction coupling?

A

The sequence of events where nerve impulses reaches the muscle membrane and leads to muscle shortening by cross-bridge activity.

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23
Q

What is released to provide the energy needed for the myosin head to pull on the actin filament and initiate a powerstroke?

A

the release of Pi from the myosin head

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24
Q

Sequence of events leading to muscle contraction

A
  1. Signal from motor nerve reaches axon terminal
  2. Stimulates synaptic vesicles to release ACh across synaptic cleft and to the ACh receptors on the sarcolemma of muscle fibre.
  3. The release of ACh causes excitation of muscle fiber (sodium ions cause depolarization).
  4. This causes the propagation of AP which travels along the sarcolemma and down the T tubule which causes depolarization of muscle fibre.
  5. This results in opening of calcium ion channels from sarcoplasmic reticulum and terminal cisterna of SR. This increase intracellular Ca2+ concentration in the muscle fiber (calcium released into sarcoplasm).
  6. Calcium ions binds to troponin (causing conformational change) which causes tropomyosin to move from the actin-binding sites. (ATP used)
  7. Energized myosin heads bind to the actin binding site to form crossbridges and pulls on the actin molecule to produce a back-and-forth movement (power stroke). This causes muscular contraction.
  8. When ACh release/nerve impulse to muscle stops (muscle fiber is repolarized) and Ca2+ is removed by reuptake into the SR, crossbridge cannot form and the muscle relaxes.
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25
Q

Fatigue

A

a decline in muscle power output

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26
Q

Why does a decline in muscle power output occur?

A

Due to:
- a decrease in muscle force production at cross-bridge level
- a decrease in muscle shortening velocity

27
Q

Possible causes of fatigue during heavy, very heavy and severe exercise (1-10min)

A
  • Decreased Calcium ion release from SR
  • Accumulation of metabolites that inhibit myofilament sensitivity to calcium (Pi, H+, free radicals)
28
Q

Describe the key metabolites that contribute to fatigue at high/heavy intensities.

A

Pi and free radicals modify cross-bridge heads and reduce number of cross-bridges bound to actin.

H+ ions bind to Ca2+ binding sites on troponin, preventing Ca2+ binding and contraction.

29
Q

Describe possible causes of fatigue during moderate intensity exercise (>60min).

A

Increased radical production
- Radical accumulation modifies cross-bridge heads and reduces number of cross-bridges bound to actin.

Glycogen depletion
- Depletion of muscle glycogen reduces TCA cycle intermediates and decreases ATP production via oxidative phosphorylation.

30
Q

Does the accumulation of Pi and H+ contribute to fatigue during moderate intensity exercise?

A

NO

31
Q

Define exercise-associated muscle cramps (EAMS)

A

spasmodic, involuntary muscle contractions during exercise

32
Q

Are exercise-associated cramps caused by electrolyte or dehydration imbalance?

A

No -

  1. dehydration and electrolyte imbalance would likely impact all skeletal muscle, whereas muscle cramp is isolated to the peripheral muscle that are exercising.
  2. Muscle cramp also occurs with repeated electrical stimulation despite there being no change in blood concentration of electrolytes.
  3. Static stretching of the cramping muscle often relieves the cramp.

However, in some extreme conditions (prolonged exercise in a hot environment) electrolyte imbalance could cause EAMS.

33
Q

What is EAMS likely cause?

A

Hyperactive motor neurons in the spinal cord
- High-intensity exercise can increase excitatory activity of muscle spindles and reduce inhibitory effects of the Golgi tendon organ. This leads to increased activation of muscle and uncontrolled contraction (cramp).

34
Q

Strategies to alleviate EAMC

A
  • Passive stretching and massaging
  • Reduce exercise intensity
  • Activating ion channels in mouth/throat could send inhibitory signals to spinal cord thus inhibiting overactive motor neuron (pickle juice).
35
Q

Define the 3 types of muscle action/contraction

A

Concentric - muscle contracts with force greater than resistance and shortens

Eccentric - muscle contracts with force less than resistance and lengthens

Isometric - muscle contracts ( and develops tension) but does not change length

36
Q

Isotonic

A

Isotonic is where muscle tension remains the same but muscle length changes (eccentric/concentric)

Iso - same
Tonic - tension

37
Q

Isokinetic

A

Isokinetic is where muscle length decreases with constant velocity.

Iso - same
Kinetic - velocity

38
Q

What type of muscle action occurs during dynamic exercise and static exercise?

A

Dynamic exercise - concentric and eccentric
Static exercise - isometric

39
Q

3 types of muscle fibers

A

1) Type I: Slow-twitch, slow-oxidative fibers

2) Type IIa: Intermediate fibers, fast-oxidative glycolytic fibers

3) Type IIx: Fast-twitch, fast glycolytic fibers

40
Q

What are the 4 key biochemical characteristics important to the function of muscle fibers?

A

1) Oxidative capacity - quantity of mitochondria, capillaries, and myoglobin

2) Type of myosin isoform expressed - 3 types that differ in activity due to ATPase they express (e.g. rate of ATP breakdown)

3) Abundance of contractile protein within fiber - amount of actin and myosin

4) Type of motor neuron innervation

41
Q

In arm and leg muscles, what % of type I fibers are there?

A

45-55%

42
Q

What is the % of slow fibers and fast fibers for
a) Distance runners
b) Track sprinters

A

a) 70-80% slow, 20-30% fast
b) 25-30% slow, 70-75% fast

43
Q

Contractile properties of muscle fiber types

A
  • Maximal force production
  • Speed/velocity of contraction (regulated by myosin ATPase activity)
  • Maximal power output (force x shortening velocity)
  • Fatigue resistance
  • Muscle fiber efficiency (amount of ATP used to generate force)
44
Q

What does muscle contraction speed (shortening velocity) depend on?

A

the rate of cross-bridge cycling which depends on the myosin ATPase isoform

45
Q

How does contraction/shortening influence the I band and A band?

A

I band shortens in length whereas A band stays the same size (A bands of different sarcomeres come closer together as the I band decreases)

46
Q

How are muscle fibers typed?

A
  • Contractile properties
  • Muscle biopsy (small pieces of muscle removed <50mg)
  • Oxidative capacity (mitochondria, capillaries, myoglobin)
  • Staining for type of myosin ATPase
  • Immunohistochemical staining (selective antibody binds to unique myosin proteins and fiber types differentiated by colour difference)
  • Gel electrophoresis (identify myosin isoforms specific to different fiber types)
47
Q

Categorize the 3 fiber types (I, IIa, IIx) into low, moderate, high for…

  • Number of mitochondria
  • Resistance to fatigue
  • Predominant energy system
  • ATPase activity
  • Vmax (speed of shortening)
  • Efficiency
  • Tension
A

Type I - high, high, aerobic, low, low, high, moderate

Type IIa - high/moderate, high/moderate, combination, high, high, moderate, high

Type IIx - low, low, anaerobic, highest, highest, low, high

48
Q

Muscle twitch

A

contraction resulting from single stimulus
one complete cycle of contraction and relaxation

49
Q

Describe the events that occur during a muscle twitch, referencing the length of these periods (ms).

A

Stimulus causing twitch
- Latent period (5ms) which corresponds to the depolarization of muscle fiber.
- Contraction (40ms) when calcium is released from SR resulting in crossbridge binding.
- Relaxation (50ms) due to reuptake of calcium in SR and crossbridge detachment.

50
Q

What two factors explain why speed of shortening is greater in fast fibers?

A
  • SR release Ca2+ at a faster rate
  • Higher ATPase activity
51
Q

Describe the 4 factors that influence force regulation in the muscle

A

1) Number and types of motor units recruited: more motor units = greater force

2) Muscle length: ideal length for force generation due to increased cross-bridge formation

3) Firing rate of motor neurons: frequency of stimulation (simple twitch, summation, tetanus)

4) Contractile history of muscle: rested muscle vs muscle exposed to fatiguing exercise

52
Q

In a motor unit, a single impulse from a motor neuron stimulates…

A

all fibers simultaneously

53
Q

Within a muscle what is there a varied quantity of? What does this enable?

A
  • Motor units
  • Number of fibers within each motor unit

This enables a variety of fine motor control (small motor units - less fibers per motor unit) and large force production (large motor units - more fibers per motor unit).

Eye - Large amounts of motor units/ 10fibers per motor unit (small motor units)

Gastrocnemius - 600 motor units / 2000 fibers per motor unit (large motor units)

54
Q

True or false - All muscle fibers that belong to a single motor unit are of the same fiber type.

A

True

55
Q

What do motor neurons supplying larger faster motor units have?

A
  • Larger cell bodies
  • Larger diameter axons
  • Greater number of axonal branches
  • More complex and extensive motor end plate of NMJ
56
Q

Henneman Size Principle of motor units

A

Motor units recruited in consistent pattern according to their size.

  • Small slow units recruited first at lesser stimulus strengths - they produce low force. These motor units have easily excited motor neurons.
  • As stimulus strength increases, larger fast motor units are recruited to produce higher force. These motor units have higher threshold motor neurons - harder to excite.
57
Q

How does muscle length influence force regulation in muscles?

A

There is an ideal length for force generation where you have the optimal amount of overlap between the thin and thick filament. This results in increased cross-bridge formation and force production.

58
Q

Summation

A

sum of individual twitches (muscle stimulation) - no time for muscle fiber relaxation before next twitch (stimulus) so they add together.

59
Q

2 ways to increase twitch force

A

1) Recruit more motor units to fire simultaneously
2) Increasing frequency of APs in motor axons

60
Q

Tetanus

A

A state of sustained maximal muscle contraction.

  • at high stimulation frequencies, the muscle has no time to relax between stimuli which results in a smooth contraction that is much stronger than a single twitch.
61
Q

Incomplete tetanus

A

occurs when stimulation frequency is high but at a rate that still allows partial relaxation of muscle between twitches.

62
Q

In relation to muscle force-velocity relationship:

Describe the relationship.
When is maximum velocity of shortening greatest?
At any given force exerted by a muscle, the speed of the movement is greater in what type of muscle?

A

It describes the relationship between the speed of muscle shortening (velocity of movement) and muscle force production.

  • Maximum velocity of shortening is greatest at the lowest force.
  • At the lowest force for both slow and fast fibers.
  • Muscles with a higher % of fast-twitch fibers.
63
Q

Muscle force-power relationship:

Describe the relationship.
At a given velocity of movement, the peak power generated is greater in what type of muscle?

A

Power increases with velocity up to 200-300 degrees/second but decreases at higher velocities because force decreases with increasing movement speed.
Muscles with a higher % of fast-twitch fibers.

64
Q

What fiber type contains the most fibers?

A

Type IIx fibers

Exercise Physiology (35 decks)

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