Question

Which of the following occurs during muscle contraction?

Answer 1

The myofibril contains two major proteins, namely, Actin and Myosin. The light bands consist of actin, which is also known as I-band or Isotropic band. The dark band known as ‘A’ or Anisotropic band contains myosin. Both of these proteins are sequentially arranged in rod-like structures that are parallel to each other and also parallel to the longitudinal axis of the myofibrils. The actin filaments are thinner than the myosin filaments. Every ‘I’ band has an elastic fibre called ‘Z’ line in the centre that bisects it. The thin filaments are inflexibly connected to the ‘Z’ line.

Complete answer: The process of muscle contraction initiates when a signal is sent by CNS using a motor neuron. This neural signal reaches the neuromuscular junction which releases acetylcholine (a neurotransmitter). Acetylcholine creates an action potential into the sarcolemma and it spreads through the entire muscle fibre which causes the release of calcium ions into the sarcoplasm. This increase in Ca++ level results in the binding of calcium to a subunit of troponin present on actin filaments. This removes the covering of active sites for myosin. The myosin head then binds to the bare active sites on actin filament and forms a cross bridge. The attached actin filaments are pulled towards the centre of the ‘A band’. The ‘Z line’ which is attached to the actin filaments is also pulled inwards and as a result, it moves close altogether. This causes the shortening of the sarcomere and it results in a contraction.
So the correct answer is option B.
Additional Information:
The process of relaxation of muscle takes place when the stimulation of the nerve ends. Then the calcium ions are pumped back inside the sarcoplasmic reticulum. This breaks the link between the actin and myosin fibres. The actin and myosin both return to their original position which causes the muscle to relax.

Note: The muscle has a red-colored pigment called myoglobin which stores oxygen. The amount of myoglobin is more in some of the muscles which appear reddish. These muscles are known as Red fibres. They also have a large number of mitochondria which helps them utilize large amounts of oxygen for ATP production.