Myelinated and Non-Myelinated Nerve Fibres

Neurons can also be classified by whether their axon carries a myelin sheath or not:

Myelinated (medullated) nerve fibres carry an insulating fatty myelin sheath around the axon, secreted by the Schwann cells, which gives the fibre a whitish appearance. This sheath is not continuous — it is interrupted at intervals by the Nodes of Ranvier — and the whole axon is additionally covered by the neurilemma. Because the nerve impulse effectively 'jumps' from one Node of Ranvier to the next rather than travelling continuously along the membrane, conduction is much faster in myelinated fibres; this jumping mode of conduction is called saltatory conduction. The myelin sheath also helps prevent the impulse from weakening (loss of signal) as it travels. The cranial nerves of vertebrates are myelinated.

Non-myelinated (non-medullated) nerve fibres lack this fatty sheath — here, the Schwann cell wrapping the axon does not secrete a myelin layer — so these fibres appear grey rather than white. Conduction of the impulse is correspondingly slower in a non-myelinated fibre than in a myelinated one. The nerves of the autonomic nervous system are non-myelinated. …