Which is true for muscle contraction :-

Options

A. Sarcolemma becomes permeable to `Ca^(++)` ions
B. Sarcolemma becomes permeable to `Na^(+)` ions
C. Sarcolemma becomes non-permeable to `Na^(+)` ions
D. Concentration of `Ca^(++)` ions is reduced in sarcoplasm.

Answer

To determine which statement is true regarding muscle contraction, we can analyze each option provided:

1. **Understanding Sarcolemma**: The sarcolemma is the cell membrane of a muscle cell. It plays a crucial role in muscle contraction by facilitating the movement of ions across its membrane.

2. **Role of Acetylcholine**: When a nerve impulse reaches the muscle, acetylcholine (ACh) is released from the synaptic vesicles into the synaptic cleft. This neurotransmitter binds to receptors on the sarcolemma.

3. **Permeability Changes**:
- When acetylcholine binds to its receptors on the sarcolemma, it causes the sarcolemma to become more permeable to sodium ions (Na+).
- This increased permeability allows sodium ions to flow into the muscle cell, leading to depolarization of the sarcolemma.

4. **Depolarization and Action Potential**: The influx of sodium ions makes the inside of the sarcolemma more positive compared to the outside, creating an action potential. This is crucial for muscle contraction.

5. **Evaluating the Options**:
- **Option 1**: "The sarcolemma becomes permeable to the calcium ion" - This is not true; the initial change is related to sodium ions.
- **Option 2**: "The sarcolemma becomes permeable to the sodium ion" - This is true; the binding of acetylcholine increases permeability to sodium.
- **Option 3**: "The sarcolemma becomes non-permeable to the sodium ion" - This is false; it becomes more permeable.
- **Option 4**: "The concentration of the calcium ion too is reduced in the sarcoplasm" - This is misleading; calcium ions play a role later in the contraction process, but the initial change involves sodium ions.

6. **Conclusion**: Based on the analysis, the correct answer is **Option 2**: "The sarcolemma becomes permeable to the sodium ion."