Autonomic nervous system (deep dive)
The autonomic nervous system is part of the peripheral nervous system and is involved in regulating a huge part our physiology without the need for conscious awareness or control.
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 they exert strong influences on our entire system, including heart rate, respiration, hand temperature and sweat gland activity. For example, the PNS slows down our heart rate whereas the SNS speeds it up, causing continuous variation in inter-beat intervals (as in, the heart doesn’t beat as a metronome). Refer to the What is heart rate variability post.
The SNS does not have its own “brake” and therefore only has an accelerating effect on our physiology (including heart rate) and indirectly our psychology. We rely therefore on the braking mechanism of the PNS to downregulate us (and slow down our heart rate).
When we are presented with a stressor, challenge or threat our autonomic nervous system responds first and fast. The SNS kicks into gear and messages the adrenal gland to release adrenaline and noradrenaline. Adrenaline decreases fast (less than 10 minutes) so if we need to sustain an increased level of activation when the stressor, challenge or threat remains, the HPA-axis takes over and triggers the adrenal gland to release cortisol (see the HPA-axis article for more detail).
When the SNS is dominant we move into what we call a catabolic state - the breakdown of molecules into smaller components which releases energy but also breaks down our cells. PNS dominance shifts us into an, opposite, anabolic state - the rebuilding of cells, recovery of energy and healing of our body. A good analogy, which is reflected in the body resources chart in your Firstbeat Lifestyle Assessment, is that of a battery. The SNS uses up and depletes battery power, whereas the PNS recharges the battery. Easy to see then that good balance in the nervous system is vital.
The nervous system is sophisticated in its operation but doesn’t distinguish that well between a physical stressor (e.g. exercise) and other sources of stress (e.g. a stressful meeting at work). These other stressors can trigger the SNS in the same way that a workout can. Aside from using up energy and motivation, the body also mobilises the fight & flight resources at a time when they aren’t necessary (no physical exertion). This can lead to body fat storage, changes in insulin sensitivity and other harmful side effects.
The sources of non-physical stressors are widespread and include social, relational, environment, emotional, mental, nutritional, social media / information, financial, work / study and performance stresses. As there is no shortage of stress in our modern live there is a constant demand on our SNS making it easy for the SNS to dominate and for the important PNS brake to withdraw. In sharp contrast, the SNS was only ever designed as an acute / emergency response with fast but temporary activation.
A chronic state of stress and SNS dominance therefore has a very harmful downstream impact on our health, well-being and performance. It becomes a downward spiral as a dysregulated nervous system also makes it less effective in responding to stress and reduces its ability to put the brake on.
The PNS operates primarily through the vagus nerve and the more active the PNS (higher vagal tone) the more variation is produced in our heart rate as the PNS continuously puts the brakes on the SNS. Low variation indicates a more dominant SNS and / or chronic stress, systemic overload, insufficient capacity for recovery or inability to relax, suppressed immune system, chronic inflammation, imbalances / dysregulation in the ANS and other (typically not yet noticeable or detected) underlying health issues.
Importantly, the vagus nerve connects key parts of our brain directly with our entire physiology and the communication within the vagus nerve is bi-directional. Messages run down from the brain to “direct” physiological change (e.g. responding to a stressor) but most of the communication fibres actually go the other way and as such our physiology is continuously sending sensory information up to the brain, either messages of health, calm and harmony or message of unwellness, stress and alarm.
Indeed, higher heart rate / higher vagal tone influences our psychology in four important domains; psychological well-being, cognitive performance, emotional self-regulation and our capacity for social engagement. For example, imbalances in our ANS are often a significant contributor to depression and anxiety. In a nutshell, good ANS functioning is key to optimal functioning, health and well-being and in fact a vital foundation to attain and sustain it.
There are some schools of thought that include a third branch, the enteric or intrinsic nervous system (ENT) that resides in our digestive system. Its is also called the second brain as its make up is very similar to our brain. The enteric nervous system is embedded in the lining of the gastrointestinal system, beginning in the esophagus and extending down to the anus. The enteric nervous system is capable of operating independently of the brain and spinal cord but is influenced by the sympathetic and parasympathetic branches, in particular via the vagus nerve.
Source: The enteric nervous system: “A little brain in the gut” Annahazi A (2020)
The neurons in the enteric nervous system control the motor functions of the system, in addition to the secretion of gastrointestinal enzymes.These neurons communicate through many neurotransmitters similar to the brain, including acetylcholine, dopamine, and serotonin. Interestingly enough the neurotransmitters serotonin and dopamine which we rely on so much for good mood have a high concentration in the ENT. In fact more than 90% of the body's serotonin and approximately 50% of the body's dopamine are found in the ENT, hence the strong connection between gut health / nutrition and our psychological well-being.
