Also called the tenth cranial nerve.  It regulates the autonomic nervous system in three primary states.  1) Ventral Vagal Complex (VVC) which cues social engagement, 2) The Sympathetic Nervous System (SNS) which responds as fight or flight, and 3) Dorsal Vagal Complex (DVC) the body signals freeze, withdrawing in collapse usually when confronting extreme danger.  The vagus nerve has two two branches, the ventral and dorsal branch.  When we are able to feel safe and calm, the autonomic system operates through integration of the ventral branch of the vagus nerve.  We have relaxed muscles, we’re able to be social and engage others, blood flows to the skin, and we are capable of pleasurable emotions.  We have access to our prefrontal cortex when we feel safe and calm.  Especially in the event of trauma, the dorsal branch gets activated.  When an event either is or seems so threatening that we become overwhelmed, it can cause the body to freeze and go into collapse.  The body becomes flacid, heart rate slows, breathing is shallow, and we become disassociated with our executive function.  It is a state that can mimic death.  These two branches of the vagus nerve create what is known as the polyvagal theory developed by Steven Porges.  The ventral branch (at the front of the body) is activated when we are calm.  The dorsal branch (at the back of the body) is activated when we are so overwhelmed that the body freezes and collapses.  

Some of the most interesting work being done to help better understand how the vagus nerve interrelates with our emotional and psychological health is being done by Babette Rothschild. You can find more information here: