In the last decade, a heated discussion about the distinction between sympathetic and parasympathetic nervous systems errupted. These are commonly known as the “fight or flight” versus the “rest and digest” systems with roles like increasing or decreasing heart rate and pupil size (background on the divisions of the nervous system below). For about a century, the division of the two systems in the spinal cord and brainstem have been clearly laid out; most of the spinal cord has sympathetic neurons except for a small segment toward the butt. And then in 2016, a controversial paper came out claiming that all of the spinal cord just contains sympathetic neurons [1]. This group compared mRNA expression (structurally similar to DNA but is more of a precursor to proteins) and in various cell types. Prior to this, the location of sympathetic and parasympathetic cells were essentially based on classification of mostly anatomical data from the 1900’s by John Newport Langley (1852-1925; there were also functional data, but they’re not as clear-cut).
Anyway, the 2016 paper led to letters being published basically saying that it was nonsense [2][3] followed by a review paper in 2017 from the authors of the original paper describing how decades of research has hinted at this correction [4]. But this disagreement has fueled additional reviews [5] and commentaries[6]. You can get a sense from the progression of titles:
- Original paper [1]: The sacral autonomic outflow is sympathetic
- Review by authors of 2016 experiment [3]: The “Sacral Parasympathetic”: Ontogeny and Anatomy of a Myth
- A review by a different group [4]: Renaming all spinal autonomic outflows as sympathetic is a mistake
- A commentary by a different group saying that neither of these labels apply to the debated region [5]: The Sacral Autonomic Outflow Is Spinal, but Not “Sympathetic”
- Rebuttal review by an eLetter author [6]: The sacral autonomic outflow is parasympathetic: Langley got it right.
These texts have short angry sentences like, “We disagree.” [3] Others write, “In this commentary, arguments against this erroneous proposal are provided.” [5]… Anyway, it’s been 2 years since the last article came out and I’m excited to see what will come of this – will textbooks adapt?
[1] PMID: 27856909 (2016)
eLetters tagged on the original article [2] in Science under eLetter tab:
[2] Jänig W, Keast J, McLachlan E, Neuhuber W, Southard-Smith M (2016) E-Letter. In: Science
[3] Horn JP, de Groat WC (2017) E-Letter: functional criteria define divisions of the autonomic motor system. In: Science
[3] PMID: 29103139 (2017)
[4] PMID: 29453697 (2017)
[5] PMID: 28342217 (2017)
[6] PMID: 28566236 (2018)
Background on general nervous system structure: The nervous system is divided into the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerve fibers outside of these two structures). The peripheral nervous system can also be divided into somatic and autonomic nervous systems. The somatic system deals with movements, both voluntary and reflexes that involve skeletal muscles, while the autonomic system deals more with visceral reactions that are largely unconscious. The autonomic system can further be divided into enteric (related to the gut), sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) systems. There are a lot of divisions, but these classifications have been established for over a century.)
originally posted on Instagram August 4, 2020
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