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I was trying to answer this question when I remembered that the somatic axon is myelinated, while both sympathetic and parasympathetic preganglionic axons are also myelinated. Are they only myelinated or are they both myelinated and unmyelinated? Thanks

Edit:

This is a question from an old lecture quiz (previous year) and unfortunately I do not have the answers for the questions. The wording of the question was

Spinal Nerves are

a.) myelinated

b.) unmyelinated

c.) answers a and b

d) none of the above"

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  • $\begingroup$ Welcome to Stack Exchange Biology! If you have any questions, please visit biology.stackexchange.com/help. Perhaps I am missing something, but can you clarify what you are seeing that makes you think some of these nerves are unmyelinated? $\endgroup$
    – L.B.
    Commented Feb 11, 2019 at 16:48
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    $\begingroup$ I was answering a quiz on a lecture slideshow which was from a previous year course, which unfortunately lacks answers. The question was, 'Spinal Nerves are a)myelinated b)unmyelinated c) a and b d) none of the above." $\endgroup$
    – Vacuum
    Commented Feb 12, 2019 at 2:01
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    $\begingroup$ I added those details in an edit to the question. Hopefully no one treats it as a homework question since that is not what it is intended as. $\endgroup$
    – L.B.
    Commented Feb 12, 2019 at 15:16

2 Answers 2

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Spinal nerves are mixed nerves containing afferent and efferent neurons of various types. Anatomically, they protrude from the spinal column bilaterally at each vertebral level. They contain both myelinated fibers (e.g., A fibers) and unmyelinated fibers (e.g., C fibers).

The answer is (c): both myelinated and unmyelinated.

Please note that spinal nerves are NOT located in the spinal cord, despite the fact that neurons in each spinal nerve will either originate or terminate there. A spinal nerve is a peripheral structure. It starts at the point where the dorsal and ventral root converge (See April's Clinical Anatomy, Ch. 1). If you have the opportunity to observe back surgery or dissect a human cadaver, you can see and touch a spinal nerve. They are cable like structures containing bundles of many many many axons, but no neuron in its entirety. A nerve is not a neuron. This is a common misconception.

Just to add, the details here may seem like trivia, but they are not. Similar questions (and questions that require you to understand the distinction between a nerve and a neuron) are often asked on US medical licensing exams. This is because they are relevant to understanding and interpreting neurological symptoms and physical exam findings.

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Yes, there is grey and white matter in the spinal chord and the parasympathetic nervous system, so they are both myelinated and unmyelinated:

enter image description here

Here is an image of parasympathetic nerves: enter image description here

Primary afferent fibers of the dorsal roots are either myelinated or unmyelinated. In the rat, cat, and monkey, thin unmyelinated and thick myelinated fibers are located in the lateral and medial parts of the dorsal roots, respectively, in the dorsal root entry zone (Light and Perl, 1979a).

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  • $\begingroup$ Nothing is false in this answer, but it doesn't answer the question. Spinal nerves are not in the spinal cord, they are the nerves leaving the spinal cord. The presence of gray or white matter in the spinal cord is not relevant to the question. $\endgroup$
    – De Novo
    Commented Feb 14, 2019 at 4:50
  • $\begingroup$ thanks. He said: are they both myelinated and unmyelinated? I see what you mean. I updated the information. $\endgroup$
    – bandybabboon
    Commented Feb 14, 2019 at 4:52
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    $\begingroup$ I think you have a misunderstanding about what a nerve is. The boundaries of a nerve (e.g., a spinal nerve), a gross structure, are not the same as the boundaries of the neurons, a microscopic structure. A spinal nerve, at any level you measure it, will have fibers of different conduction velocity. Some of these (e.g. A fibers) are fast and heavily myelinated. Some of them (C fibers) are slow and unmyelinated. The gray matter in the spinal cord doesn't have anything to do with the question, which is about a spinal nerve, a PNS structure, not the spinal cord (a CNS structure). $\endgroup$
    – De Novo
    Commented Feb 14, 2019 at 5:00
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    $\begingroup$ Gray matter is not present in any nerve. Gray matter is a central structure that has nothing to do with the question. Nerves are not cells, they are bundles of fibers (a part of many many cells, but no neuron in its entirety). You can see an individual nerve, and distinguish it from surrounding structures (vessels, muscle, ganglia). You cannot see a neuron, or, as some may call it a "nerve cell". This is not over complicating it, but it is something that many students and neuroscientists who focus on cell biology miss. $\endgroup$
    – De Novo
    Commented Feb 14, 2019 at 5:13
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    $\begingroup$ It may be helpful to read the wikipedia entry for nerve $\endgroup$
    – De Novo
    Commented Feb 14, 2019 at 5:21

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