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Cranial Nerve X - Vagus Nerve
Cranial Nerve X – Vagus Nerve
Figure 28. Cranial Nerve X - Vagus Nerve
The vagus nerve is the longest cranial nerve. It is like the facial and the glossopharyngeal in that it carries fibers of all types. Most of these fibers are either visceral motor or visceral sensory fibers and most are not found in the head and neck. Nevertheless, anyone who has taken any course in office emergencies is familiar with the term vasovagal syncope. So, despite the nerve having few branches in the area where dental personal work it does play a role in dental care.
As done previously with mixed nerves we will start as the cranial exit which in the case of the vagus nerve is the jugular foramen. Through the neck it is found in the carotid sheath with the common carotid artery and the internal jugular vein. Before it enters the sheath it sometimes gives off a branch that splits off an area where IX, X, and XI all join together. It then passes to the area around ear. We have already seen components of the trigeminal, facial and glossopharyngeal nerve in the vicinity of the ear so you can appreciate that the external ear is an area where several nerves are involved with general sensation. The vagus provides fibers to the external acoustic meatus but all of the nerves involved overlap to some extent.
Another early branch is the pharyngeal branch which carries fibers from taste buds in the pharynx. It is also of importance in the head and neck as it gives off branches that innervate all of the palatal muscles except the tensor veli palatini which, as mentioned earlier, is connected to the mandibular nerve. It also contributes to the pharyngeal plexus which supplies motor innervation to all the pharyngeal muscles except the stylopharyngeus and parts of the inferior pharyngeal constrictor which is supplied by laryngeal branches of the vagus. The stylopharyngeus muscle is supplied by the glossopharyngeal nerve.
The next branches are a couple of nerves that serve the larynx. The first is the superior laryngeal which gives rise to the external laryngeal, and the internal laryngeal nerves and the later branch is the recurrent laryngeal. The external laryngeal is a motor nerve to the cricothyroid muscle which allows one to vary the pitch of one’s voice. The internal laryngeal is a sensory nerve carrying fibers that originate in the tissues within the superior portion larynx and the laryngeal surface of the epiglottis. This will also carry taste sensation from the epiglottis. The recurrent laryngeal is motor to all the other small muscles in the larynx and sensory to the membranes found below the vocal folds. These nerves provide virtually all of the motor and sensory innervation to the larynx.
The vagus gives off no more branches in the neck and enters the thorax where it supplies pre-synaptic parasympathetic fibers to the viscera, most importantly the heart. It is these fibers that are implicated in vasovagal syncope as they slow down the heart rate. If these fibers provide vastly more stimulation than countered by the sympathetic fibers the heart rate slows down greatly and blood pressure falls precipitously. Not only are there autonomic motor fibers to the thorax but the vagus also has sensory fibers from all the thoracic organs.
After giving off branches to ganglia in the thorax the vagi (this is the plural of vagus to remind one that there are both a left and right vagus) follow the aorta into the abdominal cavity where it connects with the nerves that control the motility of the digestive system. It receives sensory information from the digestive organs except from the distal portion of the transverse colon to the rectum.