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***Function - exercise, bursts of activity
 
***Function - exercise, bursts of activity
 
*Muscle activation is initiated by a nervous impulse crossing the Neuromuscular Junction
 
*Muscle activation is initiated by a nervous impulse crossing the Neuromuscular Junction
**The neurotransmitter, Acetylcholine (Ach), binds receptors in the muscle fiber to open Na+ channels
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**The neurotransmitter, '''Acetylcholine (Ach)''', binds receptors in the muscle fiber to open Na+ channels
 
**This causes a wave of depolarization along the sarcoplasmic membrane, further opening voltage-gated Na+ channels, which propagates the signal along the sarcolemma
 
**This causes a wave of depolarization along the sarcoplasmic membrane, further opening voltage-gated Na+ channels, which propagates the signal along the sarcolemma
 
**Depolarization of the sarcoplasmic reticulum causes Calcium to be released, which activates muscle contraction
 
**Depolarization of the sarcoplasmic reticulum causes Calcium to be released, which activates muscle contraction
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***Myosin heads bind Actin subunits, forming cross-bridges, hydrolyzing ATP and providing energy for contraction
 
***Myosin heads bind Actin subunits, forming cross-bridges, hydrolyzing ATP and providing energy for contraction
 
***Myosin heads undergo power stroke, displacing Actin and releasing ADP and Pi
 
***Myosin heads undergo power stroke, displacing Actin and releasing ADP and Pi
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*Control of skeletal muscle activity is regulated by the '''thin filament'''
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**In the off state, tropomyosin coiled-coils block the helical grooves of the actin filament
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**Tropomyosin movement relieves the steric block, allowing access to myosin binding sites
 
***Calcium regulates muscle contraction by binding troponin-C, which is attached to the thin filament
 
***Calcium regulates muscle contraction by binding troponin-C, which is attached to the thin filament
 
***This causes inhibition of the steric block keeping Actin and Myosin from interacting
 
***This causes inhibition of the steric block keeping Actin and Myosin from interacting
**Increased Calcium causes a negative feedback inhibition of Ca release, and it is pumped back into the sarcoplasmic reticulum by the Ca/ATPase pump
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*Increased Calcium causes a negative feedback inhibition of Ca release, and it is pumped back into the sarcoplasmic reticulum by the Ca/ATPase pump
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==Smooth Muscle==
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*Smooth muscle lacks troponin
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*Control of smooth muscle contraction involves '''thin and thick filament regulation'''
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* Thin filament regulation is carried out by the actin binding protein, '''Caldesmon''', which binds the thin filament and blocks myosin access
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**'''Calmodulin''' (activated by binding Ca) binds caldesmon and causes its dissociation from actin, enabling contraction
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*Thick filament regulation occurs by the '''phosphorylation''' of the '''regulatory light chain (RLC)'''
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**Calmodulin-Ca binds and activates myosin light chain kinase (MLCK)
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**MLCK then phosphorylates RLC, turning myosin on
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**Myosin light chain phosphatase (MLCP) switches myosin off
    
==Tendon==
 
==Tendon==
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