Axons and dendrites
An axon is not a bare fibre. In vertebrates it is wrapped — and the gaps in the wrapping matter as much as the wrapping does.
The myelin sheath
In vertebrates, a myelin sheath is situated around the axons. It is not continuous: it comes in segments laid one after another along the axon, with short bare stretches between them. The presence of the myelin sheath increases the speed of impulse conduction.
Nodes of Ranvier
The places where the myelin sheath is absent are called the nodes of Ranvier.
The gaps are the reason it is fast
An impulse cannot cross the insulated stretches, so instead of creeping the whole length of the axon it jumps from one node to the next. Fewer places to stop means a faster journey. A myelinated axon is quicker than a bare one precisely because most of it is skipped.
How long a neuron can be
The axon of a motor neuron running to a muscle in the foot starts in the spinal cord and ends at the muscle. It is a single cell, sometimes more than a metre long. Nothing else in the body approaches that — the cells of every other tissue in this chapter are measured in fractions of a millimetre.
Speed is the whole design
Compare the three ways a body sends a message. Blood carries hormones everywhere but slowly. Nerves carry impulses to one place, fast. Everything about a neuron — its length, the myelin, the gaps in the myelin, the one-way layout — exists to make that second option as quick as it can be.