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I'm reading Periphery by Moses V. Chao, and the author explains that the peripheral nervous system consists of the portion of the nervous system outside the brain and spinal cord. But, if I understand correctly, the cell bodies of motor neurons are in the spinal cord (or even further upstream?), whereas the nerve fibers themselves are just bundles of really long myelinated axons going all the way from the spinal cord to the nerve endings. So these seem to be cells whose cell bodies are part of the central nervous system, but whose axons are part of the peripheral nervous system. Is that how neurologists think about it? I would have thought things would be subdivided in a way that each cell is in one system or another.

I see this related question, but I don't think its answer is a clear answer for this question. That answer does seem to suggest that the cell bodies of the motor neurons are part of the PNS, not the CNS, even though they're physically in the spinal cord, but it's not really explicit about that point.

Alternatively, it's perfectly reasonable to have overlapping systems. I'd just like to know the way the lines are drawn.

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  • $\begingroup$ The answer in your link does answer your question: "The distinction [central vs. peripheral] is slightly arbitrary... the most important part is that together they are parts of the nervous system, rather than being distinct systems..." IOW, yes, the motor neurons are just long mylinated anons. $\endgroup$ Commented Feb 8 at 5:38
  • $\begingroup$ Does this answer your question? Why are spinal nerves considered a part of PNS while the spinal cord is a part of CNS? $\endgroup$ Commented Feb 8 at 5:39
  • $\begingroup$ The answer in the link says: " 'CNS consists of brain and spinal cord, and PNS consists of everything else.' That's really it. The distinction is slightly arbitrary, I'd think of them as the central (nervous system)..." I could read that answer two ways. One is that the cell bodies of the motor neurons aren't really considered "part of" the spinal cord, since those cells are part of the PNS. The other is that the bodies are part of the spinal cord, and it's only the axons that are part of the PNS. $\endgroup$ Commented Feb 8 at 5:51
  • $\begingroup$ Have you considered the ganglia? This might be what's tripping us both up re. our communication. $\endgroup$ Commented Feb 8 at 6:00
  • $\begingroup$ Yes, but I don't think all peripheral nerves go to them. Do they? Also, it occurred to me that, if the nerve itself is only the bundle of axons and not the cell bodies, then that other answer did answer my question, but then my question becomes, "Is a nerve just the axons, or is the collection of cell bodies part of the nerve too?" $\endgroup$ Commented Feb 8 at 6:04

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Answering my own question:

Yes, cells can straddle anatomical systems in the body.

In particular, the spinal cord is part of the central nervous system while spinal nerves are part of the peripheral nervous system, as described in this question. And nerves are bundles of axons, supported and held together by additional cells. The cell body to which the axon belongs is not part of the nerve. Instead, for spinal nerves, it's a part of the spinal cord and hence in the CNS.

It was surprising to me that the boundaries of anatomical structure don't always correspond to the boundaries between cells, but that makes sense when I consider that the field of anatomy began as a macroscopic endeavor, a scale on which it makes sense to treat the boundary between nerve and spinal cord as the point where they meet.

Thanks to anongoodnurse who basically led me to this answer in the comments.

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