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Mechanism of Muscle Contraction

Introduction:

  • Explained by H.E. Huxley and A.F. Huxley who proposed “the sliding filament theory”.
  • Sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin filaments (actin) over the thick filaments (myosin).
  • The sliding filament theory of muscle contraction can be divided into four stages-
    1) ATP hydrolysis
    2) Cross bridge formation
    3) Sliding of the filament
    4) Breaking of the cross-bridge

Explanation

1. ATP hydrolysis

  • By a signal sent by the central nervous system (CNS) via a motor neuron, muscle contraction is initiated. At the level of the neuromuscular junction or motor-end plate- (The junction between a motor neuron and the sarcolemma of the muscle fibre)
  • A neural signal reaching this junction causes the release of a neurotransmitter (Acetylcholine) from the synaptic knob, which generates an action potential in the sarcolemma.
  • Action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
  • An increase in Ca++ level leads to removal of the masking of active sites for myosin by binding of calcium with a subunit of troponin on actin filaments.

2. Cross bridge formation:-

  • The globular head is an active ATPase enzyme. It also provides the binding site for ATP and active sites for actin.
  • The myosin head now binds to the exposed active sites on actin to form a cross-bridge by the energy production from ATP hydrolysis.
    ATP → ADP + Pi + Energy

3. Sliding of filament:-

  • Formation of cross-bridge results in pulling of the attached actin filaments towards the centre of ‘A’ band.
  • The filaments move toward the M line and force is produced called the power stroke, as it is the step at which the sarcomere shortens and the muscle contracts.
  • The ‘Z’ line attached to these actins is also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
  • The ‘I’ bands get reduced, whereas the ‘A’ bands retain the length during the mechanism of contraction.


    sliding-of-filament:

     

4. Breaking of cross-bridge:-

  • In this stage, the myosin releases the ADP and Pi.ck.
  • A new ATP binds and the cross-bridge is broken.
  • Myosin head is back in a relaxed state.
  • The whole cycle is repeated itself for further muscle contraction.
  • This process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae.
  • Once the neural signalling stops, the calcium ions are pumped back in SR, resulting in the masking of actin filaments.
  • Masking of the "myosin-binding sites" on actin filaments causes the return of ‘Z’ lines back to their original position(relaxation).
     


cross-bridge

 

 

 

Frequently Asked Questions: FAQs

 

 

 

Q1 Who proposed the sliding filament theory?
Ans:

H.E. Huxley and A.F. Huxley proposed “the sliding filament theory”.

Q2 Mention the different stages of the sliding filament theory of muscle contraction?
Ans:

The sliding filament theory of muscle contraction can be divided into four stages-
1) ATP hydrolysis
2) Cross bridge formation
3) Sliding of the filament
4) Breaking of the cross-bridge

Q3 Both I and A bands reduce in length. True/ False?
Ans:
False
The ‘I’ bands get reduced, whereas the ‘A’ bands retain the length during the mechanism of contraction.

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