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).

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.