Okay, let's tackle this question. The question is about where the maximum number of sodium channels per square micrometer is found. The options aren't listed, but since it's from March 2009, maybe I can recall the standard answer for this.

Sodium channels are crucial for action potentials. Different tissues have different densities. For example, in neurons, the nodes of Ranvier have a high concentration because they're where the action potential is regenerated. But what about other tissues? Cardiac muscle cells also have a lot of sodium channels for their action potentials, but maybe less than the nodes of Ranvier.

Wait, I remember that the axon initial segment and nodes of Ranvier have the highest density of voltage-gated sodium channels. These are parts of neurons where rapid depolarization is essential. So if the options include nodes of Ranvier, that's likely the correct answer. If the options are between nodes of Ranvier and cardiac muscle, then nodes would have more per unit area.

Now, the distractors might include other areas like the cell body of a neuron, which has fewer sodium channels compared to the nodes. Or maybe other structures like the internodes of myelinated axons, which have very low density because the myelin sheath insulates them, and the action potential jumps between nodes.

So, the correct answer is probably the nodes of Ranvier. Let me confirm: the nodes have the highest density to allow for saltatory conduction, which speeds up signal transmission. Each node is a gap in the myelin where sodium channels cluster densely. Therefore, the maximum number per square micrometer would be there.

The other options might be things like the cell body (soma), dendrites, or even cardiac muscle. Cardiac cells do have sodium channels but not as concentrated as nodes of Ranvier. The cell body has some, but not as many. Dendrites typically have even fewer. So the answer is nodes of Ranvier.

**Core Concept**
Voltage-gated sodium channels are essential for rapid depolarization during action potentials. Their density varies across tissues, with the highest concentration in specialized structures requiring rapid signal propagation, such as the nodes of Ranvier in myelinated axons.

**Why the Correct Answer is Right**
The **nodes of Ranvier** in myelinated axons contain the highest density of voltage-gated sodium channels (approximately 10,000–20,000 channels/µm²). This clustering enables saltatory conduction, where action potentials "jump" between nodes, significantly increasing conduction velocity. The high density ensures rapid depolarization at each node, despite the myelin sheath insulating the axon and preventing continuous conduction.

**Why Each Wrong Option is Incorrect**
**Option A:** *Cardiac muscle cells* have fewer sodium channels compared to nodes of Ranvier, as their action potentials rely on slower Ca²⁺ influx for prolonged depolarization.
**Option B:** *Neuronal soma* has sparse sodium channels, as action potentials are initiated at the axon hillock/initial segment, not the cell body.
**Option C:** *Unmyelinated axons* lack nodes of Ranvier and distribute sodium channels uniformly, resulting in lower density per unit area.

**Clinical Pearl / High-Yield Fact**
Remember **"Saltatory conduction = sodium channel

✓ Correct Answer: D. Nodes of Ranvier