Posted by DK on October 15, 2009 at 14:36:47:
In Reply to: Re: Spinal Accessory Components posted by Julia on October 14, 2009 at 22:45:50:
Evey said earlier to a couple of us (and he said that he will probably make the comment to the whole class) that we shouldn't worry about CN XI. Something along the lines of it being pretty messed up and confusing (as has been determined in this thread).
: Our cranial nerve handout has CNXI as SVE, so I think it might be something we should worry about, and not just a confusion of terms with other nerves, although I'm not sure and don't have a real answer anyway. Did you see the link to Cranial Nerves, Gartner Chp15? It addresses the SVE confusion concerning the Spinal Accesory Nerve (cranial nerve XI), although it equivocates on the real answer (so don't get your hopes up). I copied the section below. The 2nd paragraph is particularly relevant.
: SPINAL ACCESSORY NERVE (CN XI)
: The spinal accessory nerve supplies motor innervation to the sternocleidomastoid, trapezius, and many of the intrinsic laryngeal muscles. In the early literature this nerve was described as consisting of two distinct parts: a cranial (bulbar) and a spinal root. It is now understood that the “cranial root” of the accessory nerve is composed of aberrant vagal fibers arising from the nucleus ambiguus in the medulla. These vagal fibers collectively form a distinct root as they emerge from the brainstem. On the other hand, the spinal accessory nerve derives its fibers from the spinal accessory nucleus residing in the posterolateral aspect of the ventral horns of cervical spinal cord levels C2–C5 (or C6). This nucleus is continuous superiorly with the nucleus ambiguus of the medulla. Delicate rootlets emerging from the surface of the lateral funiculus of the spinal cord (interposed between the dorsal and ventral spinal roots) converge and assemble to form the spinal accessory nerve. This nerve trunk ascends, enters the cranial vault through the foramen magnum, and proceeds on the lateral aspect of the medulla to join the aberrant vagal fibers as they emerge from the medulla. The two groups of fibers accompany one another for a short distance but then diverge to go their separate ways. The aberrant vagal fibers join the main trunk of the vagus nerve and follow those fibers of the vagus that are destined to supply most of the intrinsic laryngeal muscles. The spinal accessory nerve exits the cranial vault via the jugular foramen. It courses inferiorly to the deep surface of the sternocleidomastoid muscle providing it with motor innervation. It continues its inferior course to the posterior triangle of the neck and then proceeds to the deep aspect of the upper part of the trapezius muscle to supply it with motor innervation. In view of its origin, many neuroanatomists no longer consider the accessory nerve to be a true cranial nerve, but instead a unique type of spinal nerve.
: Additionally, there are differences of opinion relating to the classification of the functional components of the spinal accessory nerve. Some authors consider that this nerve carries branchiomotor SVE fibers since neurons of the spinal accessory nucleus develop in a manner characteristic of SVE, not GSE, neurons; whereas others believe that they are somatomotor, that is GSE.
: Recent literature supports that GSA proprioceptive fibers are carried by the spinal accessory nerve from the upper cervical spinal cord levels to the structures it innervates, but questions the branchial arch origins of the trapezius and sternocleidomastoid muscles.