“The Polyvagal Theory” by Stephen W. Porges

Subtitle: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-Regulation

Recommended to me by: Joshua Sylvae

This book is a chronological collection of Stephen Porges’ scientific research papers about vagal nerves and their functions, written in technical, medical language. Later papers summarize earlier research and even define some terms, so the book gets easier to read as it goes along.

The vagal nerve, also known as the tenth cranial nerve, originates in the brainstem and branches to the lungs, heart, digestive system, and face, independent of the spinal cord. It makes up most of the parasympathetic nervous system. It has both efferent (motor, from the brain to the periphery) neurons and afferent (sensory, from the periphery to the brain) neurons, creating a system that tends to stay in a given operating range (homeostasis) via negative feedback.

It is bilateral, one on each side of the body, and the two sides have slightly different functions, since we are not internally symmetrical, with the heart tilted to the left and the stomach on the left, etc.

As well as being bilateral, there are also two separate systems, thus “poly vagal theory”, many vagal nerves: an ancient system that all vertebrates have, and an additional newer system that mammals have. When the newer system is active, it suppresses the older system.

The ancient system is dorsal (originating toward the back of the brainstem) and unmyelinated (not sheathed).

Reptiles have this ancient vagal system, and a sympathetic system. They have a low resting metabolic rate. Under stress, their sympathetic system speeds up heart rate and breathing. If that doesn’t fix the problem, the dorsal vagal system puts them into freeze, dropping heart rate (bradycardia) and breathing rate (apnea). This works well to convince predators they are dead, or extend the time they can stay underwater.

The newer vagal system is ventral (originating toward the front of the brainstem) and myelinated (sheathed). It controls facial expressions, vocalizations (speech, singing, and other sounds), and coordinates breathing with vocalizing and swallowing. It tightens the muscles of the middle ear to filter out low frequency sounds that might drown out speech frequencies.

Mammals have a high resting metabolic rate, and a high requirement for a consistent oxygen supply. The newer vagal system is a “brake” on the sympathetic nervous system, gently reducing heart rate and breathing rate and allowing a focus on social signals. Under stress, the brake is removed, giving control to the sympathetic nervous system and instantaneously raising heart and breathing rate. If that does not take care of the problem, control goes to the ancient vagal system, sharply dropping heart rate (bradycardia) and breathing rate (apnea), which can be fatal for mammals.

The vagal brake can be engaged and disengaged at the speed of thought, unlike the sympathetic nervous system which works via adrenal hormones and other circulating chemicals that take a while to clear out of the body.

When the vagal system is busy telling the diaphragm to breathe in, the heart gets less of a “brake” signal and speeds up slightly. The brake is restored on the out-breath, slowing the heart slightly. This is RSA – Respiratory Sinus Arrhythmia. It can be used as a non-invasive indicator of vagal tone. The greater the difference in heart rate while breathing in versus breathing out, the more vagal tone there is.

The ancient vagal system has been partially recruited for pro-social immobility – accepting an embrace, for example.

The ancient vagal system also explains the immobility many people experience during rape. Understanding the neurological basis helps to reduce shame about not fighting back.

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