@kremer1 @upstatephil The diagram is called a dermatome map. Here is a link to an explanation of dermatomes.
https://www.ncbi.nlm.nih.gov/books/NBK535401/figure/article-29335.image.f1/
This is useful, but it only describes the nerves that exit the spine at nerve roots between vertebrae in the space called the foramen. This predictable map applies if there is impingement of a nerve in the foramen from bone growth, extruded disc material and possibly narrowing if the disc height has been lost or vertebrae slipping in a way that narrows the space between them.

Consider the situation where there is compression of the spinal cord inside the central spinal canal. Do you know what nerve functions will be affected? It's anybody's guess. The spinal cord is also mapped in the body. All of the nerve cells are arranged in a specific path from the brain to the organ, and these join together in a huge bundle, and that bundle can float and move inside the spinal canal as you move. It has to be able to move for flexibility of the spine. If the spinal cord is impinged or "tethered" at a particular level inside the canal, it compresses a big bundle and you don't know specifically what will get squeezed more. This can also compromise the blood supply to the spinal cord itself. If this spinal cord compression happens in the neck, it can affect anything below that level, so anything in the body. Typically it can cause gait imbalances (walking with a limp), and bowel or bladder dysfunction. It gets more confusing when a patient has both nerve root compression AND spinal cord compression.

I have noticed that many surgeons tend to focus on the predictable and think in terms of the dermatome map, so they may tell a patient that their leg pain is NOT related to the cervical disc that has collapsed, and would therefore be a lumbar spine problem. This is the misinformation that was given to me by several surgeons. When I saw the 6th surgeon (because the first 5 would not help), I asked for MR imaging of my entire spine, and there was no compression of nerve roots that would affect my legs, but I did have cord compression at C5/C6. I found my answer in medical literature that described my condition as "funicular pain" or what they call tract pain that originates in the tracts of the spinal cord. Here is the link to that medical literature.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3111492/