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.
  
  
  
  
   

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).
   

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.