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Physiology - Sliding Filament Theory


Components of Actin and Myosin:

1. bind to other myosin molecules to thick filament
2. has 2 binding sites
    a. bind to actin filaments
    b. bind to ATP
3. composed of 2 twisted protein with a head and a tail
4. gobular head acts as an ATPase, i.e. it hydrolyzes ATP to give ADP and a phosphate (Pi). (also puts myosin x-bridge in a "cocked" position)

1. 2 actin chains twisted together composes the thin Filament
2. interacts with regulatory proteins, troponin and tropomyosin
3. has a binding site for myosin

Cross bridge
 connection formed when mobile myosin heads bind to actin molecules in muscle

Muscle Fiber Contraction
1. Distal end of motor neuron releases acetylcholine (ACh)
2. ACh diffuses across the gap at the neuromuscular junction (synaptic cleft)
3. Muscle fiber membrane stimulated. Muscle impulse travels through transverse tubules. Reaches sarcoplasmic reticulum (SR)
4. Ca++ diffuse from SR to bind on troponin
5. Troponin and tropomyosin exposes binding site on actin
6. Actin + myosin filaments form links
7. Myosin cross-bridge pull actin filament inwards
8. Muscle fiber shortens
(Greater detail of steps 7-8 above)

1. Myosin head at 45 degrees relative to actin. Mysosin tightly bound to an actin (in rigor state)
2. ATP binds to binding site on myosin. Myosin detaches actin.
3. Myosin ATPase actively hydrolyzes ATP which remains bound to myosin
4. Energy is used to swing myosin head over and bind weakly to new actin molecule. Cross bridge is now 90 degrees relative to filament (relaxed state)
5. Release of Pi initiates power stroke. Myosin pushes actin filaments past it returning the myosin head to the 45 degree position
6. myosin head releases ADP and resume the rigor state

Muscle Fiber Relaxation
1. Acetylcholinesterase decomposes ACh. Muscle fiber not stimulated.
2. Ca++ transported back to SR
3. ATP causes links between actin and myosin to break (ATP doesn't break down)
4. Troponin and tropmyosin interact and block the binding site on actin
5. Muscle fiber relaxes
6. ATP breakdown "cocks" myosin cross bridge.


1. In your own words, explain the major events that occur during muscle fiber contraction

2. The union between a nerve fiber and a muscle fiber is
a. Motor neuron
b. Motor end plate
c. Neuromuscular junction
d. Neurotransmitter

3. When cross-bridge form between filaments of actin and myosin, the result is
a. Shortening of muscle fiber
b. Membrane polarization
c. Release of Ach

4. ______ stops the stimulation of a muscle fiber
a. ACh
b. Acetylcholinesterase
c. Tropomyosin
d. Ca++

5. The SR releases:
a. Sodium ions
b. Calcium ions
c. Chlorine ions

6. What does calcium bind to expose the binding site?
a. SR
b. Troponin
c. Tropomyosin
d. Myosin

1. See above
2. c
3. a
4. b
5. b
6. b


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