Long-term Effects of Chronic SNS Activation

 

In a nutshell, the SNS shunts resources away from long term projects – like building a strong immune system, or digesting food, or making babies – in favor of short term crises, like getting away from an attacking lion a million years ago. Crises that were usually resolved quickly. One way or another. 
 
But long after the lion has pounced on someone else and left you alone, you’re still shaking like a leaf! That’s because the effects of the SNS diminish gradually, while the effects of the PNS diminish abruptly. For example, in a frightening situation (= SNS arousal), it takes your heartbeat awhile to go back to normal even after the danger is over. But when you wake up – and are no longer so regulated by the PNS – your heartbeat increases briskly.  
 
One reason for this is that, unlike many other hormones, the dominant SNS hormones – epinephrine (adrenaline), norepinephrine, and catecholamines (which include dopamine) – do not exert any negative feedback to reduce their own synthesis.  
 
Bottom-line, lighting up your SNS is not just a fleeting experience, but something that has a real stickiness to it, a lasting impact.  
 
For example, chronic activation of the SNS burdens five major systems of your body: gastrointestinal, immune, cardiovascular, endocrine, and nervous. Let’s look at the lingering effects of that wear and tear for each system, with an emphasis on the nervous system, since that’s where it feels like we live as conscious beings.  
 
Gastrointestinal

Chronic stress and other sources of SNS activation increase your risk for to ulcers, colitis, irritable bowel syndrome, diarrhea, and constipation. 
 
Immune

Routine SNS arousal weakens your body’s defenses in numerous ways. This finding is well-documented in numerous studies, and we’ve all had the personal experience of catching a cold when we’re run-down. 
 
Cardiovascular  

Hardening of the arteries and heart attacks are all more likely if you experience chronic stress – especially when combined with a steady dose of hostility. 
 
Endocrine

A steady diet of SNS increases risks for Type II diabetes, especially when combined with lots of sugary and refined carbohydrate foods. In the erotic department, it leads to impotence in men and low desire for both sexes. 
 
And it’s probably not very good for your longevity, either. The body makes cortisol (one of the stress hormones) and DHEA from the same raw materials. DHEA is sometimes called the “anti-aging hormone” due to its beneficial effects. But under stress, production is shifted toward cortisol, so there’s less DHEA.  
 
Nervous  

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The amygdala.

Repeated experiences of fear (and perhaps other negative emotions such as disgust or anger) increase what’s called the Long-Term Potentiation (LTP) of neurons in the amygdala; in other words, the synaptic connections there are strengthened. Further, repeated stressful experiences lead amygdala neurons to grow more connections with each other. As a result, in a vicious cycle, repeated experiences of fear and stress make the amygdala more sensitive to and more reactive to fear- and stress-related information.  
 
Now, the amygdala plays a central role in the formation of implicit memories: the registration of lived experience (especially the emotional and sensate parts) beneath conscious awareness. When the amygdala has become sensitized and energized in a fearful and negative direction, then it shifts implicit memory that way. Over time, this “dark shading” can lead you to feel a free-floating anxiety, depressed mood, and irritability.  
 
Your implicit memories and negative emotions also create the conceptual lenses through which you see the world. These perspectives seem self-evident – of course most relationships are disappointing, of course you’ll get in hot water if you say what you really feel, etc. – and are thus typically unquestioned, which is what makes them most problematic; like the proverbial fish, we swim in the waters of our belief systems without realizing we’re soaked in assumptions.  

 

When you peer at the world through subtly shadowed glasses, it looks more daunting and less friendly, which naturally makes you too cautious or too aggressive . . . sometimes with serious consequences. And in a feedback loop, any “tinted” information coming through your lenses increases the negative sensitization of the amygdala, which in turn darkens your worldview, leading to incoming information that’s even more shaded.   
 
The hippocampus.

Compared to the amygdala, stress hormones – notably cortisol – have an opposite effect on the hippocampus, a part of the brain that is vital for forming explicit memories of events: a clear record of what actually happened. In other words, stress hormones reduce long-term potentiation (LTP) in the hippocampus. In the extreme, intense and longstanding stress or trauma can literally shrink the hippocampus.  
 
Further, recent evidence has shown that at least some portions of the brain actually do grow brand-new neurons (contrary to long-held belief), including the olfactory bulb (for smell) . . .  and the hippocampus. But glucocorticoids due to stress prevent the birth of new neurons in the hippocampus, impairing its ability to produce new memories.  
 
The effects of all this can be quite extreme. For example, in people who have a history of severe depression – which could be regarded as both a result and a cause of stress and painful feelings – the hippocampus can shrink by as much as 10 – 20%. This shrinkage could be one of the reasons for the poor memory associated with depression. Unfortunately, hippocampus atrophy persists after depression resolves; it appears to be a permanent consequence of intensely painful experiences.  
 
 The amygdala-hippocampus one-two punch. When the amygdala is over-sensitized and the hippocampus is compromised, it’s a horrible combination: painful experiences can get recorded in implicit memory – with all the distortions and turbo-charging of an amygdala on over-drive – without an accurate explicit memory of them! Then it may feel like: “Something happened, I’m not sure what, but I’m really upset.”  
 
This could be a reason why victims of trauma sometimes feel dissociated from the actual events surrounding their trauma, yet are very reactive to any trigger that reminds them unconsciously of what once happened. 
 

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Meditation: Exercise

Meditation activates the PNS for many reasons, including pulling attention away from stressful subjects and activities, sitting quietly, relaxing, and bringing awareness into the body.  
 
An interesting, possible additional reason has to do with a common method of meditation: paying attention to the sensations of the breath around the nostrils and upper lip. In your brain, the olfactory bulb – which receives sensory signals from the nostrils – sends neuronal projections directly to the amygdala, probably due to the evolutionary importance of detecting disgusting, frightening – and sometimes, pleasant – aromas. When you bring your attention to the breath around the nostrils, you activate the sensory networks in that area, including the olfactory system. 

 

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As a result, you are flooding the amygdala with information that has a neutral quality to it (that quality is called “feeling” – distinct from emotion – in Buddhism), or a positive quality if you meditate with incense. That would tend to crowd out unpleasant information within the amygdala. It could also sensitize the amygdala increasingly over time to neutral information, leading its processing to be increasingly dedicated to neutral information compared to negative information. 

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Positive Emotion: Exercise 

Positive feelings activate the PNS directly by lowering cardiovascular reactivity. They also do so indirectly by priming a person to experience life in more optimistic and pleasant ways, and the effects of that include reducing the sensitization of the amygdala to negative events. Anything that gives you a positive feeling – especially of a more relaxed sort, like contentment, gratitude, lovingkindness, or tranquility – will usually arouse your PNS.  
 
Yes, sometimes it is hard to have positive emotions. And that difficulty alone can cause some negative emotion! But just do what you can. There are two great wings to psychological growth and spiritual practice: being with and working your inner and outer worlds. While being with is primary, there is still a great role for working with, including the cultivation of positive feelings. (For more on this point, please see the Two Wings article in Bulletin #2, at http://www.wisebrain.org/bulletin.html.) 
 
If you like, experiment with cultivating positive emotion for a few moments, and whatever you experience is really fine. 
 
Perhaps focus on what you feel grateful for.  Or feelings of lovingkindness, perhaps for yourself or some people you are close to.  

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Concluding Perspectives 
Over the Long Haul many of us balance a driven and routinely stressful way of life with vacations or the occasional day off. This is a kind of “binge and purge” approach to stress management, but it is not at all effective. You cannot undo the accumulating effects of chronic stress with intermittent respites, even in Tahiti.   
 
There is no way around it: each of us needs to have minimal chronic stress combined with a steady state of relaxed, alert, contented coping that emphasizes PNS activation with just enough SNS arousal to get the job done – whatever it is. In a single sentence, that’s your best-odds prescription for a long, productive, and happy life.  

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(You can download to read the full length article by Dr Rick Hanson here.)