The Parasympathetic Nervous System
In the previous post I explained how the sympathetic nervous system is used to increase energy levels and excitability to deal with environmental dangers or challenges, as well as the long-term negative health consequences of chronic stress and activation of the sympathetic nervous system (SNS). I will now be discussing the counterpoint to the SNS, which is the parasympathetic nervous system (PNS).
These two systems operate antagonistically. They both innervate the heart, but the SNS raises the heart rate, while the parasympathetic nervous system (PNS) lowers the heart rate. They both innervate the intestines, but the SNS reduces movement of the gut while the PNS increases it. The PNS causes the tear ducts by the eyes to release tears while the SNS inhibits that.
When someone begins to relax and unwind after experiencing an acute stress response, what is actually happening is the PNS is increasing its activity. This activation removes the allostatic load the SNS placed on the internal organs by shifting the homeostatic set points back to a range that is more conducive to rest, growth, digestion, calm, and even more sociable behavior. This is the state that our bodies prefer to be in the most. When the body is free from the need to use high amounts of force and energy to navigate the external environment to survive, the body’s attention is returned to looking after its internal state. If the SNS triggers “fight-or-flight,” the PNS is responsible for “rest-and-digest.”
The Structure of the Parasympathetic Nervous System
The word sympathetic comes from Greek, and means “with feeling.” The phrase was intended to describe bodily systems responding together in concert “sympathetically.” Para means alongside, and the parasympathetic system was so named because this collection of nerves was discovered in the early 1920’s and often innervated the same systems as the sympathetic nervous system, even though it took a different route through the body to do so.
The SNS consists chiefly of efferents (nerves sending signals from the brain to somewhere) and ganglia (nerve bundles) coming out of various vertebrae in the spinal cord and innervating the skeletal muscles and internal organs.
Contrasting the SNS, instead of coming out of vertebrae in the spinal cord the PNS consists mostly of various cranial nerves originating directly from the brain and brainstem.
These nerves send visceral efferents to all the same internal organs as the SNS, with the exception of the adrenal gland. The PNS also contains efferents to the eye, tear ducts, nose, mouth, heart, facial muscles, and other areas.
The Vagus Nerve
The largest of these cranial nerves is the tenth cranial nerve, also called the vagus nerve. Vagus is a Latin term meaning “wandering,” and it is called the wandering nerve because it the longest of the cranial nerves, innervating the heart, lungs, stomach, abdomen, liver, bile duct, pancreas and intestines. It is the highway on which the vast majority of parasympathetic signals are sent from the brain to the body.
When we perceive we are in a safe and non-threatening environment, and therefore a place where we can open up to and interact with others in a supportive way, the parasympathetic nervous system activates.
The vagus nerve lowers the heart rate, promotes the secretion of digestive juices, and moves food through the digestive system, increasing the assimilation of nutrients for the grow and repair that occurs when the body is resting. In doing so it helps the digestive system maintain homeostasis.
The vagus nerve also acts as the body’s social engagement system. The motor nerves of the vagus which connect to the pharynx, larynx, mouth, nasal region, and other parts of the head including the muscles of the middle ear, all promote social interaction. When we feel safe and relaxed we make more eye contact with people, turn our heads to them, make more facial expressions, and even our voice has more intonation and song-like quality (prosody). Having lots of time with friends and family is one of the best predictors of longevity, because these interactions increase our sense of safety and promote greater relaxation, digestion, and overall healing.
Artistic depiction of the vagus nerve.
Oxytocin
Of the hormones in the body, oxytocin is the most involved with the body’s parasympathetic activity. It also has a number of different functions related to reproduction, social bonding, increased listening to social cues, and aggression in defending loved ones.
It is released in the body in response to eating and even positive social interactions between people or animals. It is released most strongly in situations where care is being provided, for example during positive physical touch like that in cuddling, love-making, labour, and breastfeeding. Oxytocin is released in the gut through activity of the vagus nerve, and also from a brain region called the paraventricular nucleus. It then floods a variety of other brain regions which manage the sympathetic and parasympathetic activity of the body. Some of its activities include
stimulating the growth of new neurons in the brain, promoting increases in learning, memory, and creativity
reducing the perception of fear by quieting down the amygdala
increasing sense of safety and trust, and spontaneous social engagement
reducing the circulation of the stress hormones adrenaline and cortisol by inhibiting the activity of the hypothalamic-pituitary-adrenal axis
cooling down inflammation, which is a tissue- and cell-level stress response, restoring cellular homeostasis in many systems.
reducing sensitivity to physical pain and increasing the rate of healing of damage in the body, including that in the brain and heart.
promoting digestion and tissue growth and repair
increasing the release of more oxytocin through the vagus nerve (!)
With a resume such as the above, it really is no wonder why parasympathetic activity, mediated by and large by the vagus nerve and oxytocin, is associated with health and wellbeing.
Finishing Remarks
The parasympathetic activity of the vagus nerve and oxytocin both help to promote homeostasis in the body by reducing the activity of the sympathetic nervous system, promoting activities that help the body assimilate nutrients, grow and repair, and even support positive social interactions with others.
This raises the question, if our bodies have unconscious processes to foster relaxation, just as we do to foster activity and stress responses, why do we so often get stuck in a stress response instead of coming down?
The first reason is that the environment in which we as humans live is drastically different from the environment in which our nervous systems evolved. When the systems of the SNS and PNS evolved, animals did not at that time have the capacity for abstract reasoning, planning about the future, or rumination over the past. Without this capacity, their attention is on the present, which is the only time animals are able to act. Because of this, activation of the SNS occurs when there is an immediate requirement for action, and this helps the life form fulfill these aims.
Humans, with our enlarged neocortices, are a different animal. With our imagination, ability to reason abstractly about events, and knowledge about the past and future we can incidentally trigger SNS activity by imagining or recalling a stressful event even if there is no direct threat. We can remember a stressful event from years back and still feel angry about it, or we can hear about current events (e.g. destruction of farmlands and food processing plants), reason about how they may impact the future (food shortages and social unrest), and can experience our bodies reacting to that future as if it were occurring in the present. That is the power of imagination, and sometimes this can work against our own health and wellbeing if our homeostasis is constantly disrupted by reactions to events we are not presently facing.
The mind can also be used to relax instead of stress or rile up, but living things as a rule have a bias towards focusing on the negative in a situation instead of the positive, which again is due to each living thing’s preoccupation with survival. Ignoring something negative could be life-threatening, even if it often isn’t.
Knowing this tendency in ourselves to take on stress and to keep it around due to a lack of control of our minds and perception, no matter how useless it is to actual deal with the object of our activation, it makes sense that we learn and understand how to counterbalances this by learning how to decrease activation of the SNS and increase activation of the PNS in situations where greater homeostasis serves our long-term goals more than activation does.
