The
vagus, or tenth
cranial nerve, is a primary component of the
autonomic nervous system, which operates the internal organs. It transmits
parasympathetic signals to and from the heart, lungs, and digestive tract. The vagal system is claimed to be inhibitory of primal instincts by being part of the
parasympathetic nervous system, in opposition to the
sympathetic-adrenal system, involved in mobilization behaviors. It focuses on the structure and function of the two efferent branches of the vagus cranial nerve, which originate from the
medulla. Each branch is claimed to be associated with a different adaptive behavioral strategy; the ventral branches more restful in nature and the dorsal ones more active in nature. According to the theory, three organizational principles can be distinguished: • Hierarchy: The
autonomic nervous system reacts in three reaction patterns, which are activated in a specific order. • Neuroception: In contrast to
perception, it is here a cognition without awareness, triggered by a stimulus such as danger. •
Co-regulation: The need to feel safe enough to allow oneself to be in relationships, which is difficult for traumatized people. Porges describes the three neural circuits as regulators for reactive behavior. His findings were taken into account by some theorists of
childhood trauma, with related techniques used by trauma therapists such as
Bessel van der Kolk, These functions are claimed to follow a
phylogenetic hierarchy, where the most primitive systems are activated only when the more evolved functions fail. Polyvagal theory champions the measurement of
vagal tone as a new index of stress vulnerability and reactivity, including in populations with
affective disorders.
Proposed dorsal vagal complex (DVC) The dorsal branch of the vagus nerve originates in the
dorsal motor nucleus and is postulated by polyvagal theory to be the phylogenetically older branch. Under certain conditions, these animals "
freeze" when threatened, conserving their metabolic resources. This draws on the simplifying claims of the
triune brain theory which are no longer considered accurate due to the many exceptions to this rule (see ). That is, RSA is proposed as a measurable, noninvasive way to see how the vagus modulates heart rate activity in response to stress. If true, this method could be useful to measure individual differences in stress reactivity. RSA is the widely used measure of the amplitude of heart rate rhythm associated with the rate of spontaneous breathing. On the other hand, heart rate decelerations, which are also mediated by the vagus, are a sign of
fetal distress. More specifically, prolonged withdrawal of vagal influence on the heart creates a physiological vulnerability to the influence of the dorsal vagal complex, which in turn produces
bradycardia (very low heart rate). However, the onset of this deceleration is commonly preceded by transitory
tachycardia, which is reflective of the immediate effects of ventral vagal complex withdrawal. == Therapeutic applications ==