Figure 1. Simplified model of ion channel organization of the mammalian myelinated fibre. Sodium channels (g_Na) are aggregated in high density in the axon membrane at the node of Ranvier, but are present in much lower densities in the paranodal and internodal axon membrane, under the myelin. Fast potassium channels (g_K) have a complementary distribution and are present in the axon membrane under the myelin sheath. This complex pattern of ion channel organization has important implications for impulse conduction, and for the abnormalities of conduction that occur in axons following demyelination.

Figure 2. Relationship between conduction velocity and fibre diameter for myelinated and nonmyelinated fibres at 37°C. Above a critical diameter of approximately 0.2 m, myelinated fibres conduct impulses more rapidly than nonmyelinated fibres of the same size; interestingly, myelinated fibres as small as 0.2 m are observed in the CNS, but myelinated fibres smaller than this are not present.