How Does Autonomic Nervous System Affect Health?

If you’ve been following my blog, you may already know that I’m very keen on learning about and improving my HRV through a continuous HRV monitoring. Why focus on HRV, you may ask.

It seems intuitive that autonomic nervous system (ANS), that regulates body’s vital functions like breathing, temperature and digestion, affects the rest of our body. But in which ways does it affect the body’s overall health? Does low HRV (dominant sympathetic nervous system) affect the immune system and how? Keep reading, if you, like me, have ever wondered these questions.

During a period of physiological or psychological stress, so-called stress hormones, like adrenaline and cortisol, rush into bloodstream, activating the sympathetic nervous system (SNS) and putting our body into “fight or flight” response. Under stress, the body prioritizes our immediate survival, pushing blood to muscles, heart, and other vital organs, opening wide small airways in the lungs to enable increased oxygen supply to the brain and releasing extra glucose to bloodstreams to supply energy to all parts of the body [1].

Sounds great – to a certain degree. While more blood and oxygen to our vital organs may be a good thing during an acute stress, long-tern chronic stress can lead to a serious health issue. To get the extra resources for “fight or flight” response, the body depletes resources for functions non-vital for immediate survival. Stress reduces sex hormones in the body such as estrogen and testosterone [2], potentially leading to a suboptimal reproductive function. Chronic stress can also inhibit normal differentiation of stem cells, impairing repair from injury [3]. Additionally, chronic stress suppresses or dysregulates immune responses, [4] altering the balance between different types of immune cells.

Among different types of white blood cells, neutrophil and lymphocyte are most common and together amount to approximately 90 % of the white cell population. A fascinating 2014 study showed that an elevated neutrophil-lymphocyte ratio (NLR) in mice correlated with their behavioral indicators of chronic stress [5]. Other studies demonstrated that a higher NLR was correlated with severity of depression in human subjects [6] and stress level in MS patients [7].

For us trying to optimize health through self-quantification, NLR can be a cheap and readily available means to quantify the balance between SNS and PSN. Assuming an absence of a disease that affects NLR, such as cardiovascular diseases, the ratio should be between 1-3. If your ratio is higher, it may be an indication of SNS dominance during 3-4 weeks prior to the test.

In addition to SNS and parasympathetic nervous system (PSNS), ANS has a third, perhaps less know, main division: enteric nervous system (ENS). ENS exists in the gut lining and controls gastrointestinal behaviours independently of central nervous system. A 2011 paper suggests that abnormalities of the ENS can play an important role in patients with chronic constipation [8].

Along increasing awareness of the importance of the gut health, we now know that chronic constipation can be both a cause and symptom of a serious illness, and that having a bowl movement less frequently than every day should not be dismissed as “just normal”. While trying to improve your gut health, your nervous system may not be one the most obvious places to look for improvement but given the intimate connection between the ANS and the gut, it’s worth paying an attention to the health of your ANS. For example. There are a few anecdotes that a stimulation of vagus nerve helps to normalize bowel movement [9].


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