The power of the Vagus Nerve (deep dive)
Introduction to the vagus nerve
The vagus nerve is the tenth cranial nerve and is the longest and most complex of the cranial nerves. The vagus nerve runs from the brain through the face and thorax to the abdomen. Vagus is latin for wandering or straying which is an appropriate term for this nerve as it wanders right through the entire body and connects our brain and all of our primary organs. The old anatomical drawing below illustrates this really well.
The vagus nerve is a key component of our autonomic nervous system (ANS). The ANS forms our fast stress response and it is involved in regulating (without the need for conscious control) most of the autonomic processes, such as heart rate, blood pressure, digestion, pupil dilation and so on.
The ANS is divided into two branches, the sympathetic (SNS) – quick response mobilising system, also known as the fight & flight response – and the parasympathetic (PNS) – more slowly activated dampening system, also known as the rest and digest and calm & connect – branch.
Both branches are always active and mostly have an antagonist relationship with a constant “fight” for control and one being more dominant than the other in different contexts, having a different overall effect on us.
Chronic stress can significantly influence and upset this natural balance and lead to the SNS becoming chronically dominant with the PNS (our brake) being pushed further and further into the background.
The vagus nerve is the primary nerve through which the PNS exerts its control. When the vagus nerve is functioning well and is nice and active we have what we call good vagal tone. Good vagal tone results in good dampening, calming and slowing down of our physiology which is vital to our ability to recover following stress, restore and heal our body, keep inflammation in check, recharge our "battery", re-energise and also for our brain to operate optimally.
Importantly, it is not enough to just have a vagus nerve, make time to relax and sleep 8 hours - for actual restoration and recharging you need this nerve to function really well; to have good vagal tone. It's a bit like a car with a poor brake - you can pump the break as much as you like but until the brake works properly you are not going to slow down.
So the vagus nerve doesn’t simply just produce good vagal tone. It needs good looking after, including a sufficient supply of its neurotransmitters (messengers) from our diet. As alluded to above, chronic stress can gradually reduce vagal tone and reduce the resilience and functioning of the ANS.
Measuring vagal tone
The PNS, through the vagus nerve, is also responsible for slowing down our heart rate. Interestingly, if left to its own devices the sinus node (the heart's natural pacemaker) fires at around 100 beats per minute. It is the vagus nerve connected to the sinus node that slows down the heart to a much slower rate of around 70 or less, when we are at rest.
Contrary to popular belief, a normal healthy heart doesn’t beat evenly like a metronome, but instead, when looking at the milliseconds between heartbeats, there is constant variation. This is called heart rate variability (HRV).
Various processes are involved in creating this interbeat variability, including the autonomic nervous system, blood pressure regulation and breathing and it is also influenced by exercise, hormonal reactions, metabolic processes, cognitive processes, stress and our mental state.
HRV can be non-invasively measured and these measurements produce a variety of HRV metrics that represent the activity of different processes. One of these metrics, called the Root Mean Square of the Successive Differences (RMSSD), measures the successive differences between neighboring heartbeats. These short term differences are mostly the result of the PNS and as such this RMSSD HRV metric accurately reflects vagal tone.
HRV is generally considered an indicator of a healthy heart and the more vagal tone the higher the HRV thus high HRV is very good for us and an objective indicator of our overall health & well-being, our capacity for performance, our resilience and ability to recover well as well as our capacity for adapting to and coping with stress.
In fact, there is a vast body of research that has provided overwhelming evidence of the link between low HRV and poor functioning, morbidity, poor future health outcomes and mortality as well as poor mental health, for example depression and anxiety.
Lifestyle and vagal tone
With normal and healthy functioning, HRV should increase during relaxing activities, for example meditation or sleep, when the parasympathetic nervous system starts to dominate. On the other hand, HRV naturally decreases during exercise and stress, when elevated sympathetic activity helps the body respond to the challenge and keep up with the higher demands. Therefore, HRV is typically higher when the heart is beating slowly, and HRV is lower when the heart starts to beat faster.
HRV levels changes naturally from day to day, based on the level of activity and amount of, for example, work-related stress, but if a person is chronically stressed or overloaded – physically or mentally – the natural interplay between the two systems can be disrupted, and the body can get stuck in a sympathetically dominant fight & flight state, with low HRV and high stress hormone levels, even when the person is resting. This places a chronically high demand on our bodies and consumes body resources at a pace higher than our capacity to restore them. This, over time, can lead to various mental and physical health problems.
Genetics determine only about 20 to 30% of someone’s HRV level. The good news is that the remaining 70 to 80% is under our direct control through our lifestyle and through specific practices we can adopt.
By taking active steps to improve our lifestyle we can improve our HRV. This includes the various domains covered in a Firstbeat Lifestyle Assessment, including improving our fitness, enhancing recovery from good sleep and relaxation, eating well and working towards a better balance between stress and recovery.
You can find your HRV (RMSSD) value in your Firstbeat Lifestyle Assessment for each of the measurement days in the "Restorative effect of sleep section" under the "Quality of recovery (Heart rate variability)" heading. This night-time HRV is a very reliable indicator of your capacity for vagal tone. Keeping your HRV in the good range (if it already is) or adopting lifestyle changes and specific practices to get it into the good range (if it currently is in the low or moderate range) is a great investment in a "long life well lived".
It is important to note that HRV naturally declines as we age and the low, moderate or high range is therefore age-adjusted. Given both the genetic and age influence on HRV, means that comparing your HRV with others is not that useful.
What is far more useful is tracking your HRV and working towards an upward trend over time. It is very common for the people I work with to achieve a 20% or higher increase in HRV in a few months, “simply” by using the data insights from their Firstbeat Lifestyle Assessment to identify and make a small number of strategic lifestyle changes and adopting some new practices (refer below).
In addition to lifestyle factors, HRV is strongly influenced by certain breathing patterns, mindset and our mental state.
We are not acutely aware of this but our heart speeds up during an inhale and slows down during an exhale (producing HRV), a phenomenon called respiratory sinus arrhythmia (RSA). This synchronous fluctuation happens because the SNS is activated with each inhalation and the PNS is activated with each exhalation.
When RSA is optimal, HRV is at its highest and this high HRV is good for us as explained earlier. We can maximise RSA and therefore HRV by breathing at a particular frequency, called resonance frequency (RF). This HRV Breathing practice provides acute benefits (for example helps you downregulate after a stressful activity) and by adopting a daily practice you exercise and strengthen your autonomic nervous system reflex and increase HRV chronically. For a more detailed explanation of RSA and a guide for HRV Breathing download this resource.
Our mindset and mental state also have a powerful influence on HRV. Negative emotions or constant worry “block” the vagus nerve, reduce HRV and create irregular heart rate waveforms. Positive emotions and a more mindful, in the moment, mindset “allow” the vagus nerve to fire on all cylinders, raise HRV and create a smooth heart rate waveform.
Mindfulness meditation is a practice that nurtures the development of more positive emotions and a more “present to the now mindset” and as such has shown great promise in raising HRV (with a regular and sustained practice). A wide range of mindfulness practices are available and this Introduction to Mindfulness provides some more info to get you started.
The vagus nerve’s far reaching power
The vagus nerve and the interesting correlation between HRV and health, resilience and performance has kept researches very busy, albeit this research does not appear to have reached the public domain all that well.
I like look at the vagus nerve as having not just one but three powerful functions, beyond just its basic function (which is of course not that basis at all) of regulating our autonomic processes as described earlier.
These two other functions relate to two fascinating findings from research. The vagus nerve’s deeper function which involves its regulating influence on our immune system, specifically including the inflammatory response. And the vagus nerve’s higher function which involves its “upward” influence on our brain.
The vagus nerve’s deeper function
The vagus nerve is connected to the immune system and for example innervates the spleen and as such various research studies have explored the involvement of the vagus nerve in regulating our immune system and in particular its important function in keeping our inflammatory response under control.
In a variety of autoimmune diseases, tumour necrosis factor alpha (TNF-alpha) has been reported to be over-produced. TNF-alpha is an inflammatory cytokine produced by macrophages/monocytes during acute inflammation and is responsible for a diverse range of signalling events within cells.
Research by the Feinstein Institute for Medical Research demonstrates that inflammation can be controlled through the vagus nerve, the body’s main “highway” of neural information that prompts and curbs inflammation. Specifically, bioelectrical vagus nerve stimulation in their research has been shown to turn of TNF production in the spleen. Similarly, in my private practice, clients with autoimmune / inflammatory illnesses have greatly benefited from HRV Biofeedback training protocols in combination with targeted lifestyle changes.
The below TEDMED talk by Dr Kevin Tracey provide a summary of the two TNF-alpha studies.
The vagus nerve’s higher function
Contrary to popular belief (that our brain dominates control), more than 70% of the fibres in the vagus nerve lead upwards and signal important information back to the brain. When there is good vagal tone and a smooth heart rate waveform, the messages that go up to the brain signal a sense of safety, calm and balance telling the brain it can turn-off its fight & flight centre and open up its higher functions that power our performance and ability to engage with others.
Various research indicates that greater vagal tone positively impacts cognitive performance, emotional self-regulation, psychological well-being and capacity for social engagement (Shaffer F et al. 2014 and Geisler FC et al. 2013 and McCraty R et al. 2014 and Williams et al. 2015 and Thayer JF et al. 2009).
This further validates the findings from other research that has repeatedly established a correlation between HRV / vagal tone and conditions such as depression and anxiety. It is therefore estimated that for many people suffering from depression or anxiety the root cause lies in an imbalance in the autonomic nervous system (as a result of for example chronic stress) as opposed to a chemical imbalance in the brain. This might explain also why only about 1/3 of people with depression experience benefit from anti-depressants!
It also explains why so many elite armed forces units and elite athletes measure and train to influence HRV. They know from experience that in life-threatening tactical and high performance contexts, high HRV, good vagal tone and the ability to use the vagus nerve to control their physiological and mental state can make the difference between dying and living and between losing and winning.
“Fifty percent of psychology,
sits below the neck”
-Dr Martin Seligman
The below video by Dacher Keltner provides a good overview of these less well-known functions of this fascinating nerve.
In conclusion
As Dacher Keltner says so well: "The vagus nerve is one of the greatest human nexus". It is fundamental to our entire functioning, can be easily and non-invasively measured and a wide range of options are available to us to restore or sustain its optimal functioning.
By enabling the three functions of the vagus nerve to function optimally, we can directly influence our physical and mental health, performance and engagement with others.