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The cervical and lumbar enlargements exist on the spinal cord as a result of the increased nerve input/output required for the arms and legs respectively.

However, I don't understand how the enlargements could exist by themselves without the rest of the spinal cord getting thicker.

The following is an exaggerated view of what I think a spinal cord should look like. The cervical region needs to be thicker as it has to accommodate the original nerves from the legs (blue) AND the new nerves from the arm (red). There are now ~2x as many nerves needed to go to the brain - thus the spinal cord above the cervical area should be twice as big. It is not - it just enlarges around the cervical area and returns to normal.

Why does the spinal cord not get thicker the further up we go? Doesn't it need to accommodate more nerve fibers and thus be thicker?

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  • $\begingroup$ Try this on a piece of paper: sketch out an outline of a human body, with head, arms, and legs. Start drawing fibers that travel from the brain to each of those areas. What does your "spine" that you draw end up looking like? $\endgroup$
    – Bryan Krause ♦
    Commented Apr 10, 2020 at 0:05
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    $\begingroup$ @BryanKrause I understand that there will be branching out from the central cord - is that what you're getting at? I'm asking why won't the central cord itself get thicker $\endgroup$
    – John Hon
    Commented Apr 10, 2020 at 1:42
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    $\begingroup$ Do you see how your drawn spinal cord (not the outline, the bundle of "wires") got thicker where your red lines come in? That's the cervical enlargement. $\endgroup$
    – Bryan Krause ♦
    Commented Apr 10, 2020 at 2:39
  • $\begingroup$ Also yes some nerves do go to the spine then turn around and go right back, look up something called a "reflex arc". $\endgroup$
    – John
    Commented Apr 10, 2020 at 3:08
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    $\begingroup$ @BryanKrause Yes I do see how it got thicker, but my question is that in diagrams it looks like a bump not a sustained increased in thickness. Given that there are now more nerves that must run to the brain after the cervical area, I don't see how the thickness of the spinal cord does not increase all the way to the brain (not just locally in the cervical area) $\endgroup$
    – John Hon
    Commented Apr 11, 2020 at 3:26

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