Contraction of muscle fibre takes place by the sliding of :-A Thin filaments over the actin filaments  B Thick filaments over the thin filaments  C Thin filaments over the thick filaments D Thick filaments over the myosin filaments

The correct answer is C. Thin filaments over the thick filaments.

Here's a step-by-step explanation:

1. Muscle contraction begins when the nervous system sends a signal to a muscle. This signal is transmitted through a type of nerve cell called a motor neuron.

2. The motor neuron releases a neurotransmitter called acetylcholine, which binds to receptors on the muscle cell and triggers an electrical impulse.

3. This electrical impulse travels through the muscle cell and causes the release of calcium ions.

4. The calcium ions bind to a protein called troponin, which is located on the thin filaments (made of a protein called actin) in the muscle cell.

5. When calcium binds to troponin, it causes a change in the shape of another protein called tropomyosin, which is also located on the thin filaments. This change in shape exposes binding sites for myosin, a protein that makes up the thick filaments.

6. The myosin heads bind to the exposed sites on the actin, forming a cross-bridge. This is powered by ATP, a molecule that provides energy for cellular processes.

7. Once the cross-bridge is formed, the myosin heads pull on the actin filaments, causing them to slide over the thick filaments. This is known as the power stroke.

8. The myosin heads then release the actin and return to their original position, ready to form another cross-bridge. This process is repeated many times during a muscle contraction.

9. When the nervous system signal stops, calcium is pumped back into the storage areas, and the muscle relaxes.