Wk7: Neuromuscular responses to acute exercise

Question 1

What is the definition of muscular fatigue?

Answer 1

the decline in muscle tension or force capacity with repeated stimulation or during a given time period.

Question 2

What are the 4 main components voluntary muscle actions exhibit?

Answer 2

1. CNS
2. PNS
3. Neuromuscular junction
4. Muscle Fiber

Question 3

Is muscle weakness reversible?

Answer 3

No

Question 4

Is muscle fatigue temporary?

Answer 4

Yes

Question 5

What is the relationship between fatigue and motor units?

Answer 5

• Fatigue is highly dependent on the motor unit
• Fast isoforms are type 2a and type 2s and type 2b
• Initially recruit slow twitch muscle fibres and then progress to fast.
• Slow to fast the structure and metabolic properties change
• Slow – higher capillarisation, mitochondrial densitive, oxidative capacities, fatigue resistant, can’t generate high muscle contractions, much slower shortening velocity.
• Fast – increased fibril cross sections (motor neurone neuromuscular junction has an increased density), causes large amounts of fatigue, generate high muscle contractions, fast shortening velocity.
• Nutrition and hydration status as a large influence on fatigue

Question 6

Outline the neuromuscular transmission pathway?

Answer 6

• Motor cortex originates impulse and travels down spinal cord
• Motor neurone sends an action potential to the neuromuscular junction
• Muscle action potential initiated
• Releasing calcium into the cytoplasm
• neuromuscular transmission pathway: centrally or peripherally located

Question 7

What is loci of failure?

Answer 7

• central is originated from the motivational drive, principally motor cortex down to the motor neuron. Travels down the peripheral nerve and downstream
• you can electively stimulate the muscle to see the sights that cause muscle fatigue.

Question 8

Define central fatigue

Answer 8

when the force goes back to what it was

Question 9

Define peripheral fatigue

Answer 9

when the force doesn’t go back to where it was

Question 10

What is the role of central fatigue?

Answer 10

to prevent peripheral muscle injury

Question 11

What is neuromuscular junction failure?

Answer 11

Failure of nerve impulse to elicit normal muscle action potential, which may occur at pre- or post-synaptic terminal

Question 12

What causes neuromuscular junction failure?

Answer 12

neuro fatigue

Question 13

what is implied sarcolemma/t-tubule function?

Answer 13

A reduced sarcolemmal action potential can block t-tubule transmission that could inhibit Ca2+ release to impair force output.

reduced sarcolemma AP may be due to changes in Na+/K+, increased K+ or impaired Na+/K+ pump during repeated contracts, which may lead to cell depolarisation.

Question 14

What is impaired Ca2+ handling?

Answer 14

• impairments to Ca2+

handling:
1) Rrelease of calcium from the sarcoplasm into the cytosol
2) Binding (to troponin c) of calcium
3) Reuptake - slowing the pump of calcium into the sarcoplasmic reticulum can impair muscle fatigue.

Question 15

How does central fatigue occur?

Answer 15

if the failure to contract muscles occurs in the CNS or the motor neurone

Question 16

How does peripheral fatigue occur?

Answer 16

if the failure to contract muscles occurs in the PNS or in the Ca2+/cross bridge cycle

Question 17

what would happen if the failure to contract a muscle occurred in the nervous system?

Answer 17

no AP’s would form

Question 18

What leads to a decreased force of contraction?

Answer 18

• electrical properties of the muscle fibre
• excitation-contraction coupling failure
• impaired cross-bridge function changes in the metabolite levels

Question 19

what happens if the concentration of potassium becomes too high?

Answer 19

what happens if the concentration of potassium becomes too high?
it leaks into the T tubules which affects the DHPR
this affects Ca2+ into the cell

Question 20

what happens if Ca2+ release from the SR fails?

Answer 20

Ca2+ can’t enter the cell which affects the cross bridge cycle

Question 21

what can cause a decrease in Ca2+ release?

Answer 21

RYR1 failure

Question 22

what does a decrease in Ca2+ responsiveness cause?

Answer 22

increase in H+ which decreases Ca2+ troponin binding

Question 23

what does a decrease in Ca2+ sensitivity cause?

Answer 23

for any given concentration of Ca2+ there is less force of contraction compared to normal

Question 24

what is a slowed reaction caused by?

Answer 24

a decrease in SR Ca2+ uptake and ATPase activity

Question 25

what does impaired cross bridge cycling cause?

Answer 25

decrease in velocity of shortening and slowed relaxation

Question 26

what can NMR be used for?

Answer 26

to assess the levels of metabolites present during contraction

Question 27

what changes can be seen in metabolite levels during contraction?

Answer 27

PC levels fall as it binds to ADP to form ATP

Pi levels increases as ATP is broken down to produce energy

ATP levels stay constant

Question 28

what does the release of Pi cause?

Answer 28

increases the concentration which inhibits further Pi release as there is no gradient for it to travel down
the cross bridge cycle can’t occur so there is a decrease in force

Question 29

when an action potential occurs how does the concentration of potassium in the t tubule change?

Answer 29

it goes from 4mM to 9mM

Question 30

what effect does a high potassium concentration have on action potentials?

Answer 30

it changes the shape of the action potential, there isn’t as large an amplitude, the duration of the action potential is longer

Question 31

What are reactive oxygen species?

Answer 31

are highly reactive oxygen-derived molecules (e.g., H2O2) that are elevated during fatiguing contractions

Question 32

What is the effect of acute increases in reactive oxygen species?

Answer 32

damage myofibrillar proteins and reduce Ca2+ sensitivity