Chapter 4. peripheral factors in neuromuscular fatigue
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Transcript of Chapter 4. peripheral factors in neuromuscular fatigue
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Chapter 4. peripheral factors in neuromuscular fatigue
PF. Gardiner, Advanced neuromuscular exercise physiology
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Neuromuscular fatigue
• ↓ maximal force response in spite of continued supramaximal stimulus
• ↑effort necessary to maintain a submaximal contractile force
• During sustained submaximal muscle contraction– ↑ excitation of the motor pool – simultaneous ↓ in the maximal capacity of the
contractile system
• Neuromuscular fatigue: a condition that develops gradually as exercise continues
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Factors Affecting Performance
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Sites of Fatigue
• Central fatigue• Peripheral fatigue– Neural factors– Mechanical factors– Energetics of
contraction
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Changes in contractile properties during fatigue
Jones DA et al, JP 2006
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Changes in contractile properties during fatigue
Jones DA et al, JP 2006
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Changes in contractile properties during fatigue
Jones DA et al, JP 2006
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↑ neural signals when maintaining a submaximal contractile force
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Intramuscular factors: Interstitial K+
• MAJOR factor• ↑interstitial 細胞間 potassium (K+),
↓membrane excitability– critical interstitial potassium concentration at which
muscle tetanic force is affected similar to that in exercising human muscles (resting: ~4 mM; exercising: 10-13 mM)
• sodium-potassium ATPase (Na+/K+ ATPase)– Training ↑ Na+/K+ ATPase, ↓accumulation of
interstitiaI K, longer time to fatigue
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Na, K in nerve impulse
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Interstitial K+ and muscle force
Interstitial K+ in exercising human muscles: 10-13 mM
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Power output and interstitial K+ in human muscles
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Intramuscular factors
• ATP concentration only minor role– ATP usually NOT depleted during exercise– However, potential localized ATP depletion, especially in triad
region– Na+/K+ ATPase, use ATP generated by glycolysis– ↓ rate of ATP use by ↓crossbridge cycling, ↓ SR Ca2+ uptake
• ↑ calcium trapped in the cytoplasmic compartment– ↑ magnesium (Mg2+) ↓ calcium channel opening– inorganic phosphate enter sarcoplasmic reticulum and
precipitate with Ca– Minor factor : estimated <10% of maximum force
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Exer Biochem c6-high intensity ex 15
X: force, ����: PCr, :ATP after 10s and 20 s. open: type I, close type II human muscles
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Fatigue mechanism – increased H+
• Human muscle pH dropped from 7.05 to ~6.5 after exhaustive exercise– However, exhaustion in pH 6.8-6.9 in some situations– Force usually recover faster than pH– Ca2+ release from SR NOT inhibited even at pH 6.2– H+ has much less inhibitory effect in activation of the
contractile apparatus and Ca2+ release than previously assumed
• Low pH could inhibit glycolytic enzyme activities• However, alkalinizers DO increase performance in HIE
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Lactate metabolism
• Whenever glycolysis produce pyruvate, lactate also produced– Pyruvate synthesis rate >> pyruvate dehydrogenase activity– Lactate dehydrogenase activity high in skeletal muscle
• Fate of lactate– Leave muscle fiber via monocarboxylate transporter– Enter adjacent fiber with lower intracellular [lac]– Enter cells, used by heart, nonworking muscle (as fuel) or
liver and kidney (as sources for gluconeogenesis), intercellular lactate shuttle
• monocarboxylate transporter act as a symport, transfer lactate down gradient, accompanied by a H+
• Lactate is NOT responsible for muscle acidity, fatigue, or soreness
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Lactate synthesis REMOVE H+
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Major source of H+ during exercise
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Major mechanisms in muscle fatigue
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Fatigue mechanisms
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Structures other than muscleNeuromuscular transmission failure
• failure of a nervous impulse to be translated into sarcolemma
• Neurotransmitter depletion: acetylcholine– ↓ in max force: stimulated by its motor nerve >
stimulated directly to muscle– 3,4-diaminopyridine ↓ force difference between
indirect and direct stimulation– 3,4-diaminopyridine↑acetylcholine release
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Neuromuscular transmission failure
• Postsynaptic membrane failure– Prolonged exposure to ACh desensitize ACh receptor
• Failure of axon branches to pass on action potentials– Action potential generated in axon is NOT propagated
into all of the branches extending to muscle fibers
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Difference between direct and indirect stimulation
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Inhibition of motoneurons
• Inhibition of motoneurons: ↓motoneuronal excitability, ↓ firing rates during fatigue at maximal and submaximal force
• Afferent nerve (sensory nerve) signals– Demonstrated under ischemia conditions– Receptors for metabolic by-product concentrations?
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Inhibition of motoneurons
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Skinned muscle
• skinning the muscle fibers allows us to set the intracellular concentrations of molecules– no longer a semipermeable barrier or transporter
system that can become limiting