The steps involved in the mechanism of muscle contraction are given below.
Arrange the steps in proper sequential order and select the correct option.
I. Release of calcium ions in the sarcoplasm.
II. Signal sent by the CNS via a motor neuron.
III. Binding of calcium with a subunit of troponin on actin filaments.
IV. Generation of the action potential in the sarcolemma.
V. Binding of myosin head to the exposed active sites on actin to form a cross-bridge.
VI. Inward pulling of ‘Z’ line attached to actin causing shortening of the sarcomere.
VII. Pulling of the attached actin filaments towards the centre of 'A' band.
VIII. Release of neurotransmitter acetylcholine and generation of the action potential in the sarcolemma.

Options

A. III, II, V, I, IV, VI, VII, VIII
B. II, VIII, IV, I, III, V, VII, VI
C. I, VIII, VI, II, V, IV, III, VII
D. II, IV, V, I, III, VII, VIII, VI

Answer

To arrange the steps involved in the mechanism of muscle contraction in proper sequential order, we can follow these steps:

1. **Signal sent by the CNS via a motor neuron (Step II)**: The process begins when the central nervous system (CNS) sends a signal through a motor neuron to initiate muscle contraction.

2. **Release of neurotransmitter acetylcholine and generation of the action potential in the sarcolemma (Step VIII)**: The motor neuron releases the neurotransmitter acetylcholine, which binds to receptors on the muscle cell's membrane (sarcolemma) and generates an action potential.

3. **Generation of the action potential in the sarcolemma (Step IV)**: The action potential travels along the sarcolemma, leading to further events in the muscle contraction process.

4. **Release of calcium ions in the sarcoplasm (Step I)**: The action potential triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm (the cytoplasm of the muscle cell).

5. **Binding of calcium with a subunit of troponin on actin filaments (Step III)**: The released calcium ions bind to troponin, a regulatory protein associated with actin filaments, causing a conformational change.

6. **Binding of myosin head to the exposed active sites on actin to form a cross-bridge (Step V)**: This conformational change exposes the active sites on the actin filaments, allowing the myosin heads to bind to these sites, forming cross-bridges.

7. **Pulling of the attached actin filaments towards the centre of 'A' band (Step VII)**: Once the cross-bridge is formed, the myosin heads pull the actin filaments inward, towards the center of the sarcomere.

8. **Inward pulling of ‘Z’ line attached to actin causing shortening of the sarcomere (Step VI)**: As the actin filaments are pulled, the Z lines (boundaries of the sarcomere) are drawn closer together, resulting in the shortening of the sarcomere and thus muscle contraction.

Putting it all together, the correct sequential order of steps is:
**II → VIII → IV → I → III → V → VII → VI**