Which is true of muscle contraction?

Options

A. (a) Sarcolemma becomes permeable to `Ca^(2+)` ion
B. (b) Sarcolemma becomes permeable to `Na^(+)` ions
C. (c) Sarcolemma becomes nonpermeable to `Na^(+)` ions
D. (d) Concentration of `Ca^(2+)` ions is reduced in myoplasm

Answer

To determine which statement is true regarding muscle contraction, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Muscle Contraction**: Muscle contraction begins when a nerve impulse reaches the muscle fiber, leading to the release of a neurotransmitter called acetylcholine from the synaptic vesicles of the neuron.

2. **Analyzing Option A**:
- **Statement**: Sarcolemma becomes permeable to calcium ions.
- **Evaluation**: This statement is incorrect. The sarcolemma does not primarily become permeable to calcium ions during the initial phase of muscle contraction.

3. **Analyzing Option B**:
- **Statement**: Sarcolemma becomes permeable to sodium ions.
- **Evaluation**: This statement is correct. When acetylcholine binds to receptors on the sarcolemma, it increases the permeability of the sarcolemma to sodium ions, allowing them to flow into the muscle cell, which leads to depolarization.

4. **Analyzing Option C**:
- **Statement**: Sarcolemma becomes non-permeable to sodium ions.
- **Evaluation**: This statement is incorrect. The sarcolemma becomes more permeable to sodium ions during muscle contraction, not non-permeable.

5. **Analyzing Option D**:
- **Statement**: Concentration of calcium ions is reduced in myoplasm.
- **Evaluation**: This statement is also incorrect. During muscle contraction, calcium ions are released from the sarcoplasmic reticulum into the myoplasm, increasing their concentration.

6. **Conclusion**: Based on the evaluations, the correct statement regarding muscle contraction is **Option B: Sarcolemma becomes permeable to sodium ions**.

### Final Answer:
**The true statement about muscle contraction is that the sarcolemma becomes permeable to sodium ions (Option B).**

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