Stephen Porges' book 'Polyvagal Theory' describes the vagus nerve's role in regulating bodily state and calming down.

“I think he did an incredible job of talking about the dorsal motor nucleus of the vagus, which I'll talk about in a few moments, as a pathway for regulation of bodily state, for calming down, about the role of parent-child relationships in infancy, and how the vagus nerve pathways are present and can be activated early in life, without any learning or plasticity, and how that's so critically important to the bond that's formed between caretaker and infant. And there's just a beautiful set of studies and a beautiful set of clinical data that he describes in that book, 'Polyvagal Theory', as it relates to things like PTSD, et cetera.”