This is a relatively recent theory. This video gives a very brief intro to the polyvagal theory. This theory is the brain child of Stephen Porges, PhD. What Dr. Stephen Porges proposes in his polyvagal theory is that the vagus nerve has more function than we previously thought and that the sympathetic/parasympathetic nervous systems are only part of the equation in how people react to the environment and trauma.
The polyvagal theory is opening up our understanding of how people affect each other and what we can do to better treatment for people suffering from PTSD. To learn more about the polyvagal theory and Stephen Porges, visit http://www.nicabm.com/treating-trauma