The myelin sheath does not cover
the entire axon; it leaves small sections uncovered. These
small exposed sections are called nodes of Ranvier. They
are spaced from 0.2 to 2 millimetres apart.
The reason that the myelin sheath speeds up neural conduction
is that the action potentials literally jump from one node
of Ranvier to the next. In fact, these nodes are the only
places where the ion exchanges that generate the action
potential can take place.
This process is called saltatory conduction (from the Latin
saltare, meaning “to jump”), as opposed to the
much slower, continuous propagation that occurs in non-myelinated