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 solve the problem of arranging the steps involved in the mechanism of muscle contraction in proper sequential order, we will follow the logical sequence of events that occur during muscle contraction.

### Step-by-Step Solution:

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 action potential in the sarcolemma (Step VIII)**: The signal from the motor neuron leads to the release of the neurotransmitter acetylcholine at the neuromuscular junction, which generates an action potential in the sarcolemma (muscle cell membrane).

3. **Generation of the action potential in the sarcolemma (Step IV)**: The action potential spreads across the sarcolemma, leading to depolarization of the muscle fiber.

4. **Release of calcium ions in the sarcoplasm (Step I)**: The action potential travels down the T-tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.

5. **Binding of calcium with a subunit of troponin on actin filaments (Step III)**: The released calcium ions bind to troponin, causing a conformational change that exposes the binding sites on actin filaments.

6. **Binding of myosin head to the exposed active sites on actin to form a cross-bridge (Step V)**: With the binding sites on actin exposed, the myosin heads attach to these sites, forming cross-bridges.

7. **Pulling of the attached actin filaments towards the center of the 'A' band (Step VII)**: 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 move closer together, resulting in the shortening of the sarcomere and thus muscle contraction.

### Final Order of Steps:
The correct sequential order of the steps is:
II → VIII → IV → I → III → V → VII → VI