Understanding the autonomic nervous system

Introduction

The autonomic nervous system relates to the involuntary or unconscious side of the nervous system. It does not require activation or stimulation. It is automatic in the sense that it does not require effort in order to stimulate it.

Previously it was thought humans had no influence over the autonomic nervous system. We now know that it is possible to affect it through practices such as yoga, mindfulness and conscious breathing. Such practices result in the slowing of heart beat and expansion of smooth muscle tissue, producing profound physiological effects.

Brief overview of the autonomic nervous system

The autonomic nervous system is part of the nervous system that relates to the peripheral nervous system. This part of the nervous system governs processes such as homeostasis. It also regulates organs and metabolism. The central nervous system relates to activity in the brain and spinal cord.

The autonomic nervous system has three primary branches: the ‘sympathetic’, the ‘parasympathetic’ and the ‘enteric’ (or visceral) nervous system.

The sympathetic nervous system originates from around the first vertebrae of the middle (thoracic) spine and ends around the middle of the lower (lumbar) spine.  The parasympathetic nervous system is found at the polar ends of the spine – the base of the skull and the spine (sacrum). ‘Para’ means around, so the parasympathetic nervous system surrounds the sympathetic nervous system. The enteric nervous system is found in the gastroenterological tract and communicates with the brain via the vagus nerve (more about that below).

How the autonomic nervous system responds

The physiological responses of the sympathetic and parasympathetic nervous system are quite distinct. Generally, we can think of the sympathetic nervous system as ‘stimulating’ and engages the stress response. Actions such as rage, fear, excess worry, a feeling of being out of control, bright lights and loud noises, strenuous exercise, looming deadlines, phobias, stimulants such as coffee, refined sugars, alcohol, nicotine and narcotics, scary movies, video games are all known to stimulate the sympathetic nervous system. It is commonly known as the ‘flight or flight‘ response.

The parasympathetic nervous system is ‘down-regulating’ and helps to bring about the ‘rest and relaxation‘ response. Actions such as soothing touch, meditation and gentle yoga, conscious breathing, submersion in water, massage, hypnotherapy, acupuncture and acupressure, essential oils, lying down, restful sleep, certain foods such as celery, darkness, slow-tempo, classical music such as Bach or Mozart are known to activate the parasympathetic nervous system.

The effect of the activation of the sympathetic and parasympathetic nervous system upon certain areas of the body is set out in the table below.

Although activities which work to produce a parasympathetic nervous system response are preferable, in the case of burnout or adrenal exhaustion, there may be an excessive parasympathetic response. This may result in feelings of ennui, listlessness, loss of joy, lack of motivation, brain fog and lethargy.

Furthermore, in extreme cases, there be an immobilisation in the body. Therefore, in such cases, practices which work to stimulate the sympathetic nervous system may be necessary.

Stimulating the parasympathetic nervous system

Although the parasympathetic nervous system is largely involuntary, micro-movements of the upper neck, jaw, and tongue (via salivation) may work towards stimulating it. These include practices such as ‘sounding’ (including vocalisation, humming, chanting etc..). Spontaneous sounds emitted whilst moving, such as cries, moans and sighs help to discharge strain from the body, in particular the gut, nerves and connective tissues. This is particularly helpful in the case of difficult emotions which may be held tightly in the body, such as in the psoas muscle.

Yawning is also another sign of autonomic release.  This occurs where there is an excess of carbon dioxide in the blood. Yawning works to expel the carbon dioxide whilst at the same time oxygenating the blood. It regulates stress by modulating body temperature. It cools the air and regulates flow of blood between the lungs and the brain. Tearing of the eyes may accompany yawning and further contributes to the cooling of the brain.

Other common actions which work to support the autonomic release in the body include:

• trembling, tingling, quivering and shaking
• laughter
• ‘goose bumps’ on skin
• belching and flatuence
• perspiration
• Bodily temperature changes
• intestinal gurgles

The wandering (vagus) nerve

One of the most important nerves in the body is the ‘vagus nerve’. Given its name because of its tendency to ‘wander’ from the brain stem through the torso (ie like a ‘vagabond’), the vagus nerve connects the brain to the rest of the body. Specifically, it is the 10th cranial nerve and originates from the brain stem, exiting the skull just by the ear and passes along the neck before working its way down to the abdomen.

Therefore, practices that work to mobilise the neck, throat and tongue stimulate the vagus nerve.

Polyvagal theory

In 1994, Dr Stephen Porges presented a revolutionary paper, known as the ‘polyvagal theory’. Briefly, Porge’s theory describes that in humans, there are 3 basic subsystems that underpin the nervous system. The systems then dictate emotions and behaviours in a distinct hierachy.

‘Old vagus’

The first subsystem is the most primitive and derives from early fish species (500 million years ago) (the ‘old vagus’). The function of this system relates to immobilisation, metabolic conservation and shutdown. It specifically targets the internal organs.

From a physiological perspective, the old vagus is unmyelinated, which means it does not contain a neural sheath or covering. In this state, there is the tendency to withdraw or even curl up into a foetal ball in order to feel safe.

‘Flight or fight’

The next part in the evolutionary development is the sympathetic nervous system. It originates from the reptilian period around 300 million years ago. Unlike the old vagus, the goal of the ‘new vagus’ is mobilisation and arousal. It principally targets the limbs of the body.

In this more primitive bodily response, the periphery of the body becomes active. Adrenalin and other hormones circulate. This results in more impulsive and instinctive decisions.

‘New vagus’

Finally, the next part in the development of the nervous system is the ‘new vagus’, and is myelinated (ie it has a neural sheath). It is estimated to be 80 million years old and exists only in mammals. Its principle role is in social engagement and helps us to be more present.

The new vagus is seen as a branch of the parasympathetic nervous system, which is linked to facial and vocal expression. In particular, through the interaction with the facial nerves, the new vagus affects the muscles of the throat, face, middle ear. It also innervates the heart and lungs.

Through the new vagus, bonding, attachment and relationships arise as well as the cultivation of emotional intelligence.

Therefore, rather than thinking about the autonomic nervous system as principally either the ‘sympathetic’ or ‘parasympathetic’, the new way is to think of it is in a hierarchy of responses.  From a position of discernment and conscious decision making to more primitive, instinctive responses.

image: https://corkpsychotherapyandtraumacentre.ie/trauma/polyvagal-theory/

Understanding the autonomic nervous system

The autonomic nervous system is a core part of the nervous system. Through the two branches of the sympathetic and parasympathetic nervous system, life becomes a continual dance between ‘activation’ and ‘relaxation’. Yoga in general works to produce a parasympathetic response, although in more trauma-informed practices, practices that are stimulating are also helpful.

Polyvagal theory has given us a new way of thinking about the autonomic nervous system, with its most primitive form at the bottom of a proposed hierarchy, with the more mammalian and evolved form sitting at the top. It is here the more positive qualities of humanity emerge through social interaction, relationships and the cultivation of emotional intelligence.

If I can help you experience less stress in your personal or professional life, please feel free to get in touch or email me at scott@yogibanker.com

References:

Peter Levine, 2010, In an Unspoken Voice, 1st edition, North Atlantic Books, UK.

With gratitude to Tias Little and Charlotte Watts through their dedication to teaching the art and science of yoga.