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The sensory and motor neurons comprising the spinal cord and brain stem have the interesting property that different structural components belonging to the same neuron can occupy both the PNS and CNS.

Here are several examples:

  1. Motor neurons' somas are in the CNS while the majority of their axonal projections are in the PNS

  2. Sympathetic preganglionic neurons' somas are in the CNS while the majority of their axonal projections are in the PNS.

  3. Pseuduniopolar sensory neurons' have part of their axons in the CNS, but their somas and other part of their axons are in the PNS.

The general dogma of myelinating cells states that Schwann cells myelinate the PNS axons while oligodendrocytes myelinate the CNS axons. However, I'd like to know which cells are responsible for myelinating the segments of axons that are just exiting or entering the spinal cord. Does there exist an abrupt demarcation point where the myelinating cells suddenly cease to operate / exist?

Referring to the provided picture below:Schwann Cell vs Oligodendrocyte Myelination

Does this picture reflect what is truly observed? Is there really a distinct "outer limit" of oligodendrocyte myelination?

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