The nodes of Ranvier, located between
the myelinated sections of the axon, are areas of low electrical
resistance where almost all of the axon’s sodium channels
are concentrated. These nodes are where action potentials
can regenerate after the ion currents associated with them
have propagated passively down the insulating myelin sheath
between one node and the next.
This saltatory propagation lets the neuron preserve its
energy, because these narrow nodes are the only places where
active excitation is needed to propagate the impulse.
This method of propagation also saves a great deal of space.
The speed of conduction of a nerve fibre is proportional
to its diameter if that fibre is myelinated, but proportional
only to the square root of its diameter if it is not. This
means that to conduct a nerve impulse at the same speed as
a myelinated fibre with a diameter of 20 micrometres (i.e.,
100 m/s), a non-myelinated fibre would need to have a diameter
of several centimetres.