Why are sodium channels packed into the nodes of Ranvier? · Grey Matter

In a myelinated axon the spike is regenerated only at the nodes, short gaps about a micrometre long, so each node must produce enough inward current to charge the next internode to threshold.

Why are sodium channels packed into the nodes of Ranvier?

In a myelinated axon the spike is regenerated only at the nodes, short gaps about a micrometre long, so each node must produce enough inward current to charge the next internode to threshold. Clustering sodium channels at high density there, held in place by scaffold proteins and flanked by junctions that seal the sheath to the axon, provides that current from a very small area of membrane. It also saves energy, since sodium enters only at the nodes and the pump has less to restore. Varying node length and channel number is one of the ways axons tune their conduction speed.