# Neuromuskulär transmission och skelettmuskelkontraktion.pdf

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- Pages: 16
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## Page 1

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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## Page 5

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)
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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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## Page 13

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
1) Reduced force
2) Slower shortening velocity
3) Slower relaxation
Central and peripheral fatigue


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## Page 16

Muscle growth


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