**Core Concept**
The "all or none law" refers to the principle that a cardiac muscle cell (cardiomyocyte) can only contract with maximum force or not at all. This is due to the unique properties of the cardiac muscle cell's action potential and the underlying ion channels.

**Why the Correct Answer is Right**
The all or none law is a fundamental concept in cardiology. When a cardiac muscle cell is stimulated by an action potential, it can only generate a full contraction, regardless of the intensity of the stimulus. This is because the action potential triggers a rapid influx of calcium ions (Ca²⁺) through voltage-gated calcium channels, leading to a rapid increase in intracellular calcium concentration. This, in turn, triggers a full contraction of the muscle cell. The all or none law ensures that the heart contracts with a consistent force, regardless of the frequency or intensity of the stimuli.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the all or none law specifically applies to cardiac muscle cells, not smooth muscle cells or skeletal muscle cells.

**Option B:** This option is incorrect because the all or none law is a fundamental property of the cardiac muscle cell's action potential, not a result of the nervous system's control over the heart.

**Option C:** This option is incorrect because the all or none law is a principle that applies to individual cardiac muscle cells, not the entire heart.

**Clinical Pearl / High-Yield Fact**
The all or none law has important implications for the treatment of cardiac arrhythmias. For example, the use of anti-arrhythmic medications that alter the action potential can lead to a loss of the all or none law, resulting in irregular heart rhythms.

**Correct Answer: C**. Cardiac muscle cells.