The oligodendrocytes makes the myelin sheath in CNS and schwann cells make it in PNS. What decides where the oligodendrocyte or Schwann cell will attach and start forming myelin sheath? Is it genetically determined? Is it random? Is there any disease associated with improper positioning of node of Ranvier?
A combination of differentiation site, chemical guidance during migration, and signaling cues form a variety of sources.
The final step in the generation of an oligodendrocyte is the development of a mature myelinating phenotype, and this is largely regulated by axonal signals. It seems likely that both soluble and cell mediated signals from adjacent axons are integrated into the developmental profile of oligodendrocyte precursors resulting in cell differentiation, up-regulation of myelin gene expression and formation of the myelin organelle. Candidates for axonally derived soluble factors include FGFs (Bansal et al., 1996 and Qian et al., 1997) and thyroid hormone (Barres et al., 1994a and Barres et al., 1994b), while axonal cell surface molecules such as L1, MAG, NCAM and N-cadherin may regulate formation of the myelin sheath (Payne and Lemmon, 1993 and Trapp, 1990).
I don't know of any diseases that just mess with positioning but there are diseases that kill oligodendrocytes (like multiple sclerosis).