Which of the following is a true statement about cardiac muscle cells?

Options

A. They lack an orderly arrangement of actin and myosin filaments
B. They have less extensive sarcoplasmic reticulum and thus contract more slowly than do smooth muscle
C. They are connected by intercalated discs, through which action potentials spread to all cells in the heart
D. They have a relatively low sodium permeability and resting potentials more positive than an action potential threshold

Explanation

To determine which statement about cardiac muscle cells is true, let's analyze each option step by step:

### Step-by-Step Solution:

1. **Evaluate the first statement**: "They lack an orderly arrangement of actin and myosin filaments."
- **Analysis**: Cardiac muscle cells do contain an orderly arrangement of actin and myosin filaments, which are essential for muscle contraction. Therefore, this statement is **false**.

2. **Evaluate the second statement**: "They have less extensive sarcoplasmic reticulum and thus contract more slowly than do smooth muscles."
- **Analysis**: While it is true that cardiac muscle cells have a less extensive sarcoplasmic reticulum compared to skeletal muscle, they actually contract more rapidly than smooth muscle. Thus, this statement is also **false**.

3. **Evaluate the third statement**: "They are intercalated discs through which action potential generate or spread to all cells in the heart."
- **Analysis**: This statement is true. Cardiac muscle cells are connected by intercalated discs, which contain gap junctions that allow for the rapid spread of action potentials across the heart, ensuring coordinated contractions. Therefore, this statement is **true**.

4. **Evaluate the fourth statement**: "They have a relatively low sodium permeability and resting potential more positive than the action potential threshold."
- **Analysis**: This statement is misleading. While cardiac muscle cells do have specialized pacemaker cells that control heart rate, the statement about sodium permeability and resting potential is not accurate in the context provided. Therefore, this statement is **false**.

### Conclusion:
The only true statement about cardiac muscle cells is that they have intercalated discs through which action potentials spread to all cells in the heart.

### Final Answer:
**They are intercalated discs through which action potential generate or spread to all cells in the heart.**

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