Muscle IV Flashcards
What is muscle fatigue
Descrease in muscle tension as a result of previous contractile activity that is reversible with rest
What are 2 categories of muscle fatigue
Central and peripheral
What is central fatigue?
Feeling of tiredness and a desire to cease activity
- precedes physiological fatigue (CNS, primarily brain)
- muscles could continue
What influences the sensation of fatigue perceived by brain
Low pH from acid production during ATP hydrolysis
- H+ removed to ECF causes decline in pH
What is peripheral fatigue
Fatigue outside CNS
What is the theory of peripheral fatigue at the neuromuscular junction?
- proposed ACh synthesis can’t keep up with neuron firing rate, decreased neurotransmitter release> decrease AChR activation on muscle> muscle fails to reach threshold for firing AP
UNLIKELY
Alpha motor neurons can keep up
What are the 3 theories about peripheral fatigue with excitation-contraction coupling?
Not depletion of ATP or glycogen stores
- issues with AP propagation (at t-tubule or sarcolemma)
- issues with calcium storage/uptake/release
- issues with cross bridge cycling
How are there issues with AP propagation
- with repeated AP firing, K+ builds up in t-tubules (Extracellular space) changing threshold for APs in muscle fibre
- generated on sarcolemma but failed down t-tubules
What is the peripheral fatigue theory within the muscle fibre?
Build up of inorganic phosphate, ADP, H+, lactate
- substances can act directly or indirectly to cause fatigue
What causes peripheral fatigue
- Failed excitation-contraction coupling at the t-tubule (increase in Extracellular K+)
- Accumulation of phosphate, acid, ADP, lactate
What does accumulation of phosphate, acid, ADP, lactate cause?
- Decreased in rate of Ca release, reuptake, and storage by SR
- Decreased activation of thin filament proteins by Ca2+
- Direct inhibition of binding and power stroke motion of myosin cross-bridges
What are the differences in skeletal muscle fibres?
- Size
- Colour
- Vascularization
- # mitochondria
What are 2 classifications of skeletal muscle
1, maximal velocity of shortening (fast or slow)
2. Primary pathway they use to form ATP (oxidative or glycolytic)
What is type I muscle fibre
Slow fibres containing myosin with slower ATPase activity (hydrolyze ATP slower, cross bridge slower)
What is type II muscle fibre
Fast fibres containing myosin with more rapid ATPase activity (hydrolyze ATP faster)
What are aerobic (oxidative) fibres?
Fibres containing large amount of mitochondria have high capacity for aerobic metabolism
What are aerobic (oxidative) fibres?
Fibres containing large amount of mitochondria have high capacity for aerobic metabolism
- surrounded by blood vessels and contain large amount of myoglobin to aid in oxygen delivery
What are glycolytic fibres?
Fibres containing few mitochondria but an abundance of glycolytic enzymes and large store of glycogen
- look white
- only anaerobic metabolism
What are the 3 types of skeletal muscle fibres?
- Slow oxidative (type I)
- aerobic, small, red - Fast oxidative-glycolytic fiber (Type IIa)
- anaerobic + aerobic, medium, pink, normal functioning - Fast glycolytic fiber (Type IIx)
- fast, anaerobic, large, white
What type of fibre creates most tension per one twitch?
Type IIx has large increase in tension, high fatiguability
What fiber type is most fatiguable to least?
Type IIx, Type IIa, Type I
What are the determinant of muscle force/tension development in a muscle cell?
- Fibre diameter (larger- more actin + myosin, more cross bridges)
- Fatiguability (amt of time fibre can develop max tension)
- Initial resting length
- Frequency of activation
What is the amount of tension developed directly proportional to?
Number of cross bridges formed
What does too much or too little overlap of thick and thin filaments in resting muscle result in?
Decreased tension
What happens if sarcomeres are compressed before
Less force, distance to shorten is small
What is optimal resting length?
Individual sarcomere has optimal overlap
- needed for adequate cycling and shortening
What is a muscle twitch
Single action potential in a single muscle fibre results in an individual muscle twitch
- muscle relaxes completely between
Is a single twitch the maximal force that a muscle fibre can develop
No summation of twitches can cause maximal
How long does a single AP last for?
1-3ms but a muscle twitch may last up to 100ms
What is summation
Subsequent AP occurs before the muscle fibre is allowed to relax a more forceful contraction occurs
What happens during 1 AP that makes summation possible?
- A single AP does not cause release of entire Ca store from SR
- Ca from 1 AP may not activate all troponin complexes for sufficient time, some regions may be covered my tropomyosin before cycling begins
How is summation possible?
- 2nd AP causes second wave that may keep additional troponin complexes activated (more cross bridges)
- Ca remains elevated for longer period allowing increased cross bridge cycling and further shortening of sarcomeres
What is tetanus
Maintained contractile response to repeated stimuli
What are the two forms of tetanus
Unfused tetanus and fused tetanus
What is unfused tetanus
Reaches steady state of contraction but stimuli are far enough apart that the muscle fibre slightly relaxes between stimuli
- slight relax between twitches
- not maximal force
- approx. sustained level of tension
What is fused tension
The stimulation rate is fast enough that the fibre does not relax, instead it reaches maximum tension and remains there
- looks like continuous contraction
How much more tension is fused tetanus compared to twitch
2-4x more
What is one way to increase tension developed my a single muscle fibre
Increase rate at which APs occur in fibre
What is a motor unit
Single motor neuron and all muscle fibres it innervates, one motor neuron innervates one fibre type
What is motor neuron pool
Group of all motor neurons innervating a single muscle
- in ventral horn of spinal cord
What is motor units are recruited first when weak stimulus
- weak stimulus from upper motor neuron to motor neuron pool recruits smallest motor neurons first
- as increases, additional larger motor neurons recruited
