The Vagus Nerve
What are the Cranial Nerves?
In order to understand what a cranial nerve is, it is first important to understand what any nerve is. Nerves are bundles of axons that carry electrical signals to or from the brain. Axons can carry signals to the brain like axons carrying visual, sensory, and auditory information, or axons can carry information to the body, like those that initiate muscle movement, digestion, and some hormone release.
(from http://218.189.204.42/~bio/news/spinal%2520cord/nerve.jpg)
Nerves extend out of and enter into a number of different parts of the central nervous system (brain and spinal cord). Nerves are often named after the part of the central nervous system they connect with. For instance, spinal nerves are nerves going to or coming from the spinal cord. The cranial nerves get their name because they pass through the cranium (skull) to connect to the brain.
Where do the Cranial Nerves Connect?
The image below (from http://soma.npa.uiuc.edu/courses/bio303/Ch3.html) shows a ventral view of the brain with the connections of the 12 cranial nerves. The cranial nerves enter the brain in the brain stem. This is where many of the cell bodies of outgoing nerves and where synapses of incoming nerves are located for the cranial nerves. Cranial nerves 1 through 4 enter the midbrain, 5 through 8 enter through the pons, and 9 through 12 enter through the medulla – all different parts of the brain stem. Notice how cranial nerve 1 is the most rostral (forward) of the nerves and cranial nerve 12 is the most caudal (towards the tail).
(Click on image to enlarge)
(Image from http://www.rch.org.au/cep/treatments/index.cfm?doc_id=3245)
Unfortunately, the organization of the cranial nerves is not as organized as their naming system. However, they aren’t that disorganized either. The cranial nerves can be classified into three categories: sensory, motor, and mixed. Sensory cranial nerves convey only sensory information from the periphery (sight, sound, touch, heat, etc.) to the brain. Motor cranial nerves carry signals from the brain to move muscles (both the kind you control consciously and the kind you don’t; somatic and autonomic, respectively). Mixed cranial nerves, on the other hand, have both sensory information entering them and signals leaving them to control muscles.
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Other Subsections:
What are cranial nerves? (top of this page)
What is vagus nerve stimulation?
How does vagus nerve stimulation work?
