Posted by DK on October 12, 2009 at 11:12:13:
In Reply to: Re: cranial nerves-Anyone please feel free to chime in posted by Mitch Sternlieb on October 11, 2009 at 21:00:54:
Ok, I think that there was a miscommunication on where I was speaking in generalities about the evolution of the parasympathetic/sympathetic nervous systems and where I was speaking about your specific question. My last paragraph was a general one. In it, I was simply trying to state WHY there aren't sympathetic fibers in the cranial nerves. I'm not saying there isn't sympathetic innervation of the head and neck, I'm simply saying it doesn't come from the cranial nerves. Instead, it originates from the sympathetic trunk.
Here's my main point: the cranial nerves do not contain sympathetic neuronal fibers, but they do contain parasympathetic neuronal fibers. This is because cranial nerves evolved as the body did, just like the parasympathetic nervous system. They do not contain sympathetic neuronal fibers, because it evolved later, and was pasted onto the system after the cranial nerves evolved.
: Hi DK,
: Thank you for your response. I appreciated the detail and the later structure-dictates function approach. I did find some clarity from the embryo reading; however, I continue to struggle with the overall organization of the nervous system, as this is my first venture into its complexities. I seem to recall that there are elements of sympathetic innervation in the head and neck, despite the limited course (T1-L3) of the sympathetic trunk. The explanation you gave that implied a need for short post-ganglionic nuerons in a region such as the head where the CNS-to-PNS distances are small as justification for having only parasympathetic innervation in the head and neck does not entirely click with me because I do not understand why a short pre-ganglionic neuron (hence, long post-ganglionic nueron) (sympathetic) would be any less favorable. I assume that the point that you are making here IS valid, I just don't understand why.
: Thanks,
: Mitch
: : Short answer: read embryo.
: : Not-as-short answer: SVE fibers are innervating muscles derived from pharyngeal arches. They will be found only in the trigeminal, facial, glossopharyngeal, vagus, and spinal accessory nerves. Since these are voluntary muscles, I have a hard time calling them parasympathetic or sympathetic.
: : GVE fibers are found in the oculomotor, facial, glossopharyngeal, and vagus nerves. We already know that the vagus does parasympathetic innervation of, well, a lot of the viscera. The facial and glossopharyngeal carry parasympathetic innervation from the superior and inferior salivatory nuclei, respectively. The oculomotor nerve does parasympathetic innervation of the eye. So, GVE fibers are only parasympathetic innervation.
: : Here's another way of thinking about it. The parasympathetic nervous system evolved first. That's why it is embedded in the other structures of the body (i.e. short post-ganglionic neurons). Therefore, anything in the cranial nerves that is part of the autonomic nervous system should be parasympathetic. The sympathetic nervous system evolved second and is just kind of pasted on to the body. Remember, sympathetic neurons only originate from T1-L3. To borrow from Dr. Evey, that's something you can hang your hat on.
: : : Hi,
: : : As I study the tutorial, I had a question regarding SVE and GVE nerves. I am having difficulty classifying SVE or GVE fibers as parasympathetic or sympathetic. I feel that I can find clarity if I can understand the answers to the following two questions:
: : : 1. Must all SVE or GVE functional components be composed of parasympathetic or sympathetic fibers as opposed to somatic fibers?
: : : 2. Can we assume, if the answer to question 1 is yes, that SVE or GVE fibers that stimulate digestion and slow non-digestion related functions (such as cardiovascular related functions) are all parasympathetic?
: : : Thanks,
: : : Mitch