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Abstract
The evolution of the autonomic nervous system provides an organizing principle to
interpret the adaptive significance of physiological responses in promoting social
behavior. According to the polyvagal theory, the well-documented phylogenetic shift
in neural regulation of the autonomic nervous system passes through three global stages,
each with an associated behavioral strategy. The first stage is characterized by a
primitive unmyelinated visceral vagus that fosters digestion and responds to threat
by depressing metabolic activity. Behaviorally, the first stage is associated with
immobilization behaviors. The second stage is characterized by the sympathetic nervous
system that is capable of increasing metabolic output and inhibiting the visceral
vagus to foster mobilization behaviors necessary for 'fight or flight'. The third
stage, unique to mammals, is characterized by a myelinated vagus that can rapidly
regulate cardiac output to foster engagement and disengagement with the environment.
The mammalian vagus is neuroanatomically linked to the cranial nerves that regulate
social engagement via facial expression and vocalization. As the autonomic nervous
system changed through the process of evolution, so did the interplay between the
autonomic nervous system and the other physiological systems that respond to stress,
including the cortex, the hypothalamic-pituitary-adrenal axis, the neuropeptides of
oxytocin and vasopressin, and the immune system. From this phylogenetic orientation,
the polyvagal theory proposes a biological basis for social behavior and an intervention
strategy to enhance positive social behavior.