I noticed that Buddha Weekly are doing a series of articles called The Scientific Buddhist and have done a write up on the vagus nerve and the parasympathetic nervous system. Worth a read, and maybe some of you will be interested in the series.
Essentially, the Vagus Nerve reverses the flow of information — rather than orders flowing from your brain to your body, the nerve is instead taking some very strong suggestions from the body back to the brain. And, nine times out of ten, the brain listens. By lowering your breathing rate, your Vagus Nerve notes that things must be calm — you have no reason to be breathing hard and fast, and must therefore be able to relax. As it travels around your body and receives ‘relaxed’ messages from those organs over which you do have conscious control while meditating (your lungs, principally, but also your heart to a certain extent), it will infer that you are in no immediate danger, and have no need, therefore, to be stressed. It will convey this message to the brain, which (nine times out of ten) will then ease control over the to parasympathetic nervous system, allowing you to relax, rest, and digest.