Johan Dahlin
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Neuromuskulär transmission och skelettmuskelkontraktion.pdf
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Skeletal muscle physiology Block 2 Nervcellsfysiologi Eric Hanse
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Motor unit The -motorneuron and the muscle fibrers innervated by the -motorneuron
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The skeletal muscle consists of sarcomeres in series and in parallel
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The neuromuscular junction A very large synapse consisting of several hundreds of release sites High ”safety” – produces a suprathreshold postsynaptic potential AchE hydrolyses most of the Ach within 1 ms
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Pharmacological aspects • Postsynaptic AchR antagonists Curare (Eg. Tubocurarine, pancuronium, vecuronium, atracurium) Myasthenia Gravis (antibodies against nAChR, α1- subunit) AchR agonists Desensitisation of AChR (Eg. Suxamethonium) AchE-inhibitors Reversible (eg. neostigmine och physostigmine)
- Treatment of av mysthenia gravis Irreversible –”nerve gases” (eg. sarin, Dyflos, ecothiopate, parathion) • Presynaptic Inhibition of choline transport (Ex. hemicholinium) Lambert-Eatons syndrome (antibodies against presynaptic voltage-gated calcium channels) Botulinus toxin – proteolysis of SNARE proteins
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Developmental aspects Innervation promotes clustering of AChR and switch of subunits -reversible upon denervation Polyneuronal innervation initially
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Calcium, from the sarcoplasmatic reticulum, is necessary for myosin binding to actin
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ATP transfer energy to myosin, and decreases the affinity between myosin and actin The contraction force is proportionell to the number of parallel actin-myosin bindings at any given moment
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The muscle is strongest at an optimal length
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The higher contraction velocity, the lower force Koncentrisk Excentrisk
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M Linari et al. Nature 528:276 10 DEC 2015 Higher tension recruits more myosin filaments
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Calcium makes the titin stiffer Herzog 2014 J Appl Physiol 116:1407
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The nervous system regulates contraction force by summation and recruitment Summation -summates contractions mainly because of higher calcium concentrations in response to higher frequencies of action potentials Recruitment of motor units according to a pre- determined order, type I (S) first
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Different motor units
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Fatigue Peripheral fatigue
- Reduced force
- Slower shortening velocity
- Slower relaxation Central and peripheral fatigue
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Muscle growth