Notes: Sliding Filament Theory
[Muscle Contraction Physiology]
(1) Muscle Contraction
• Sliding Filaments = Muscle Contraction
• The Basic Steps:
1- Message sent
2- Neurotransmitter
3- Depolarization
4- Calcium + Troponin = Actin Exposed
5- Actin + Myosin (Contraction)
(2) Neuron  Muscle (Muscle Fiber)
• Cause:
– Conscious decision to move
– Homeostatic response
• Nervous System  Effector
– Neurotransmitter released (ACh, acetylcholine)
– ACh binds to sarcolemma of muscle fiber
– ACh initiates opening of sodium-potassium
channels
(3) Depolarization
• Cause:
– Binding of ACh to sodium-potassium channels
– Opening of channels + Movement of Na-K+ across
sarcolemma
• Movement of Charges:
– More Na+ moves in, Than K+ moves out
– Imbalance of charges  electrical current (action
potential)
(4) Sarcoplasmic Reticulum 
Calcium
• Cause: Depolarization
• Action Potential releases Calcium:
– Ca+ released from membrane of sarcoplasmic
reticulum to sarcomere
– Ca+ binds with troponin:
• Troponin-Tropomyosion conformation changes
• Troponin & Tropomyosin no longer cover Actin
• Actin is exposed
(5) Myosin  Actin
• Cause:
– Troponin & Tropomyosin unveils Actin
– Actin exposed
• Myosin releases Inorganic Phosphate + ADP
– Pi + ADP = ATP
– Myosin changes conformation
– Myosin binds to Actin
– Myosin and Actin slide towards each other
– WE HAVE A CONTRACTION! WOO WOO!
(6) Relaxation
• ATP binds back with myosin…
– Myosin detaches and moves away from Actin
– Troponin & Tropomyosin cover up Actin
– Ca+ moves back into Sarcoplasmic
Reticulum
– Repolarization  Sarcolemma stable again
(7) Why is it called the Sliding
Filament Theory?
• Movement of myofilaments causes
contraction
• Myosin grabs actin & SLIDES them
towards the center
(1-2) Depolarization of
Sarcolemma & Generation of
Action Potential
(3) Calcium Ions
released from
Sarcoplasmic Reticulum
(10) Everything moves back &
contraction done!
(4-6) Calcium binds to troponin,
tropomyosin moves, actin exposed
(9) Calcium removed by active transport
(7-8) Myosin attaches to actin, Myosin moves actin, ATP
released