Editor's Note: A warm welcome to guest author Michael Shea, PhD, who collaborated with Dr. Alexander on this article.

The autonomic nervous system (ANS) is composed of two distinct divisions; the sympathetic and the parasympathetic nervous systems. The enteric nervous system is now referred to as separate from the ANS since it has its own independent reflex activity.¹

Restoring vagal tone and outflow is a vital ingredient to your clients making progress toward improving their quality of life. This is especially true for those clients with chronic dysfunctions and those who have recently faced environmental disasters.

Seventy-five percent of the parasympathetic nervous system is composed of the vagus nerve.² The vagus nerve is the tenth cranial nerve (CN X) and has two divisions. It arises from two nuclei in the brain stem. The first is the dorsal motor nucleus, and the second is the nucleus ambiguous.

The Evolutionary Vagus

Dr. Porges suggests that the dorsal motor nucleus portion of the vagus nerve is the newer evolutionary vagus nerve and is composed of myelinated fibers. He asserts that this newer version of the vagus nerve stays above the diaphragm between the face and the heart. It is associated with what he calls the "social nervous system."

He states that there are three autonomic subsystems that have evolved during the eons and are behaviorally linked to three general adaptive domains of behavior:

• Social communication
• Defensive strategies of fight or flight
• The defensive immobilization of withdrawal and dissociation

His view is that the face is a predominantly parasympathetic organ. This likely accounts for the state of relaxation many people experience when receiving facials at a spa. Porges believes the vagal innervation of the face allows human beings to discern their level of safety when in visual contact with others. He calls this neuroception. He also believes it adds to our human capacity of discerning empathy and love.³

Heart Rate Variability

This new vagus helps control heart rate by slowing the heart down after it speeds up from sympathetic driven stimulation. Conceive of a spectrum between feeling stimulated by anxiety versus that of excitement. The actual strength of this vagal response is called vagal tone. It is typically measured by what is called heart rate variability (HRV). The older evolutionary vagus makes its way down the esophagus as well as to the heart and then below the diaphragm. Porges states that the fibers of the older vagus are unmyelinated. The relationship of the so called old and new vagus when they meet at the heart is not defined in the literature.

The new vagus is believed to promote feelings of safety, positive feelings towards others such as connectedness and the potential to respect both the suffering and joy of others.^4-5 It is important to look at the subdiaphragmatic vagus with a new metaphor — conceive of it as a master gardner and lover.

Above the diaphragm the vagus is a social animal, while below the diaphragm the vagus is tending a garden and even lower in the pelvis becomes a lover as it relays sexual orgasm signals
to the brain. The subdiaphragmatic vagus  has numerous important metabolic (gardening) functions:

1. The Vagus modulates chronic inflammation, and helps regulate the immune system via microbiome-brain communication.^6-9 It resets the immune system and switches off production of proteins that fuel inflammation. Low vagal tone (difficulty in lowering heart rate) associated with heart problems makes this function less effective. The subdiaphragmatic vagus provides the communication network between the brain and gut to differentiate between harmful bacteria that play a role in inflammation. Discrete signals from the subdiaphragmatic vagus initiate an anti-inflammatory reflex.
2. Compelling evidence indicates that the gut microbiota can activate the subdiaphragmatic vagus and play a critical role in mediating effects on the brain and behavior including moods and emotions.¹⁰
3. The Vagus helps regulate glucose metabolism through its connections to the liver and portal vein.¹¹
4. The subdiaphragmatic vagus has a motor component associated with peristalsis in the small and large intestines. This relates to its connection to the enteric nervous system.

In order to evacuate the bowel regularly,  a person usually needs to feel safe and have a sense of privacy in order to relax the anal sphincter(s). In this way, the social nervous system and its acquisition of safety above the diaphragm is connected to the subdiaphragmatic vagal role in producing a bowel movement.

You are encouraged to research Stephen Porges' Polyvagal Theory in greater depth. You may find his ideas to fill in many gaps toward understanding how physiology and psychology combine in the dance of improving the quality of your clients lives. Most importantly, please add the notion of enhancing vagal outflow and tone to your treatment strategies.

References

1. Wikipedia. "Parasympathetic nervous system." Wikipedia.com, 2017.
2. Ken Hub. " Parasympathetic nervous
   system." KenHub.com, 2017.
3. Stellar JE, et al. "Affective and physiological responses to the suffering of others: Compassion and vagal activity." Journal of personality and social psychology, 2015; 108(4): 572-585.
4. Whipple B, et al. "Sexual response to self-stimulation in women with complete spinal cord injury." The Journal of Sex Research, 1996.
5. Whipple B, et al. "Brain (PET) responses to vaginal-cervical stimulation in women with complete spinal cord injury." Journal of Sex and Marital Therapy, 2002; 28:79-86.
6. Pavlov VA, et al. "The vagus nerve and the inflammatory reflex—linking immunity and metabolism." National Review of Endocrinology, 2012 December; 8(12): 743–754.
7. Forsythe P, et al. "Vagal pathways for microbiome-brain-gut axis communication." Advances in Experimental Medicine and Biology, 2014; 817:115-33.
8. Carabotti M, Maselli MA, et al. "The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems." Annals of Gastroenterology, 2015; 28, 203-209.
9. Gordan R, Gwathmey JK, et al. "Autonomic and endocrine control of cardiovascular function." World Journal of Cardiology, April 26 2015; 7(4): 204-214.
10. Collins SM, Kassam Z, et al. "The adoptive transfer of behavioral phenotype via the intestinal microbiota: experimental evidence and clinical implications."  Current Opinion in Microbiology, 2013; 16(3) 240–245.
11. Ribeiro IM, Ferreira-Neto HC, et al. "Subdiaphragmatic vagus nerve activity and hepatic venous glucose are differentially regulated by the central actions of insulin in Wistar and SHR." Physiological Reports, 2015; 3(5), e12381.