To slightly supplement alex.tan's wicked-cool neuroanatomy node, the reason that this nerve is so important that it is mostly in control of all of the fun fun things that you do with your mouth, like speaking, swallowing, vomiting, coughing and salivating.

In addition, this nerve provides critical visceral feedback information to the brain. For example, without the vagus nerve, your body wouldn't be able to measure its own blood pressure (there are pressure-sensitive neural mechanisms in the heart) and wouldn't be able to tell how full or empty the various parts of the GI tract are.

One astounding new treatment for clinical depression involves the implantation of a specialized cardiac pacemaker designed to intermittently electrically stimulate these sensory fibers of the vagus nerve. The theory is that depression may have a lot to do with how the actions of stress hormones in the body (e.g. on the viscera) feedback to the brain; the visceral organs are all targets for the major stress hormones, and it is thought that a lack of feedback signal from these organs via the vagus nerve might be responsible for some of the physiological and hormonal changes typical to depression. The theory behind the pacemaker treatment, (currently in clinical trials reserved for only those that don't respond to anti-depressants,) is that by stimulating the sensory portion of the vagal nerve, you create artificial feedback which helps adjust the patients hormonal response to more normal, pre-depression functioning. The treatment is surprisingly successful, considering that it is essentially a very clever, very specific, very subtle form of electro-convulsive therapy.

So, next time you lean back in your chair, feeling the happiness that is a sated, happy stomach full of yummy food - or feel your guts twist and churn as you watch your latest WU plunge into negative rep -- thank your hard working vagus nerve.