Mechanism of Muscle Contraction:
Mechanism of muscle contraction is best explained by the ‘Sliding Filament Theory’. It states that contraction of a muscle fibre takes place by the sliding of the thin filaments over/in between the thick filaments. It was proposed by Jean Hanson and Hugh Huxley.
The process of muscle contraction can be studied under the following heads:
i) Excitation of Muscle
Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A neural signal reaching the neuromuscular junction releases a neurotransmitter (acetylcholine) which generates an ‘actin potential’ in the sarcolemma. When the actin potential spreads to the triad system through the T tubules, the cisternae of the sarcoplasmic reticulum release calcium ions into the sarcoplasm.
ii) Formation of Cross Bridges
Increase in the Ca^(2+) level leads to the binding of calcium ions to the subunit Tn-C of the troponin of the thin filaments. This make troponin and tropomyosin complex to move away from the active sites of actin molecules. Now, the active sites are exposed to the myosin heads. Utilizing the energy released from hydrolysis of ATP, the myosin head now binds to the exposed ‘active sites’ on the acting molecules to form a cross bridge and P_i is released.
iii) Power Stroke
The cross bridge pulls the attached actin filaments towards the centre of the ‘A’ band. The ‘Z’ lines attached to these actin filaments are also pulled inwards from both the sides, thereby causing shortening of the sarcomere, i.e., contraction. During the shortening of the muscle, the ‘I’ bands get reduced in size/length (Z membranes of the sarcomere are brought closer), whereas the ‘A’ bands retain their size/length. It is important to note that myofilaments actually do not shorten. As the thin filaments are pulled deep into the A bands making the H bands narrow, the muscle shows the effect-contraction.
Cross-bridge Cycle
When myosin heads hydrolyses ATP into ADP and P_i the conformation of the myosin is changed to an active state so that it can perform the ‘power stroke’. When myosin head binds to actin (formation of ‘cross bridge’), it releases P_i and undergoes another conformational change, pulling the thin filaments towards the centre of the ‘A’ band/sarcomere. Thus the ‘power stroke’ is completed and the myosin head releases the ADP. At the end of the power stroke, the myosin head binds to a new molecule of ATP, which displaces/releases it from actin. This entire process is called ‘cross bridge cycle’. The combined power of several cross bridge cycles causes the muscle to contract. These cycles continues as long as the muscle receives stimuli.
iv) Recovery Stroke
The myosin goes back to its relaxed state and releases ADP. A new ATP molecule binds to the head of myosin and the cross-bridge is broken. The new ATP is again hydrolyzed by the ATPase of the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding.
v) Relaxation of Muscle
When motor impulses stop the Ca^(2+) ions are pumped back into the sarcoplasmic cisternae. It results in the masking of the active sites of the actin filaments. The myosin heads fail to bind with the active sites of actin. These changes cause the return of ‘Z’ lines back to their original position, i.e., relaxation.
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