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Excitation Contraction Coupling and Muscle Contraction in Skeletal Muscles



Excitation Contraction Coupling

  • It is a series of events that links stimulation of cell membrane to initiation of muscle contraction.

↓ Generation of action potential at postsynaptic membrane

↓

Action potential spreads along the membrane

↓

Along T tubules, the action potential penetrates deeper into the muscle cell

↓

Opening of L-type of Ca channel on membrane of T tubules

↓

These L-type Ca channels on T tubules are in physical contact with Ca release channels on sarcoplasmic reticulum → opening of L-type of Ca channel on T tubules also opens Ca release channels on sarcoplasmic reticulum

↓

Entry of Ca into the sarcoplasm from two sources:

1. Entry of extracellular Ca via L-type of Ca channel on T tubules &

2. Entry of sarcoplasmic Ca via Ca release channels on sarcoplasmic reticulum

↓

Increased cytoplasmic Ca concentration

↓

Ca spreads into the sarcomere

↓

Binds with troponin

↓

Displaces tropomyosin into the grove in between two strands of actin molecules

↓

Uncovers myosin binding site on the actin molecule

↓

Myosin binds with actin and starts myosin cross-bridge cycle

↓

Muscle contraction


Mechanism of Muscle Contraction

Before Stimulation of the Muscle Fiber

  • The head of the myosin II is in cocked position i.e. its ready to bind actin molecule. In this state it is...

    • bound to ADP and phosphate.

    • aligned in line with the arm.

  • Tropomyosin molecule covers the binding site on actin → prevents binding of myosin to actin.

Stimulation of the Muscle Fiber →

uncovers myosin binding site on the actin as explained above

↓

Initiation of myosin cross-bridge cycle

↓

Muscle contraction

Myosin Cross Bridge Cycle

  • Its a cycle of movements repeated by myosin head that pulls thin filament over thick filament.

  • The events during a cycle are as follow:

Myosin head binds with the actin

↓

Triggers powerstroke i.e. the head bends and pulls thin filament over thick filament towards the center of the sarcomere

↓ Phosphate is released during Powerstroke and ADP is released right after it

↓

New ATP molecule binds with the head

↓

This binding reduces affinity of myosin for actin

↓

Myosin detaches from actin

↓

ATP is split into ADP and phosphate

↓

Released energy is used to bend the myosin head back into the cocked position

↓

The head binds with another actin molecule

↓

Cycle is repeated till myosin binding site on actin molecules is uncovered.


Muscle Contraction

With such repeated movements by all myosin heads in the entire sarcomere

↓

thin filaments are pulled over thick filament towards the center of the sarcomere.

↓

This results in shortening of sarcomere.

↓

Such shortening of all sarcomeres combined results in the shortening of muscle fiber

↓ i.e. muscle contraction

  • During contraction I band shortens and A band maintains the same length.

Termination of Action

Removal of Ca from cytoplasm by following mechanisms:

1. Na-Ca exchanger and Plasma Membrane Ca ATPase (PMCA) on plasma membrane pump Ca out of myofibril into the extracellular fluid.

2. Sarcoplasmic and Endoplasmic Reticulum Ca ATPase (SERCA) on sarcoplasmic reticulum membrane pumps Ca into the sarcoplasmic reticulum.

↓

Reduced cytoplasmic Ca concentration

↓

Ca dissociates from troponin

↓

Moves tropomyosin back to the resting position

↓

Covers the myosin binding site on the actin molecules

↓

Myosin head cannot bind with actin anymore

↓

Myosin cross-bridge cycle stops.

 

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