The Physiology of Fear


Introduction to Fear and Survival

In my “Caveman Brain” blog (4/18/2023), I discussed how the very survival of our most ancient ancestors, and for that matter most sentient forms of life on earth, depended on the organism’s ability to react to threats. I pointed to how this ancient system is still functioning in modern-day humans, and the problems this causes as we try to navigate our way through a very busy, tumultuous, and fast-changing environment.

 In my “Lions and Tigers” blog (5/17/2023), I went into more detail on how, at the sign of potential danger, two systems in our brain, the reptilian and mammalian, operate in unison, first to detect the presence of danger and then activate the body’s “fight or flight” systems, mobilizing our bodies to fend off existential threats.  In that blog, I emphasized the role of the limbic system, especially the amygdala, as the organ predominantly responsible for instigating a cascade of physiological events that culminate in the “fight or flight” response.

This blog will get down in “the weeds” and address what systems are activated when we go into the “fight or flight” response, and what happens when the opposing response, “rest and digest,” brings our physiology back to baseline. The understanding of how these systems work, and more importantly interact, has huge implications for managing the frequency, intensity, and duration of these genetically programmed responses.      

The Role of the Sympathetic and Parasympathetic Nervous Systems

The two main systems involved in activating and deactivating our response to stress are the Sympathetic and Parasympathetic Nervous Systems (SNS and PSNS respectfully).  Together, they make up the Autonomic Nervous System (ANS) which controls most of the body’s involuntary (without conscious effort) functions (e.g., heart rate, blood pressure, breathing, temperature, blood sugar, hunger, etc.). As mentioned in my “Lions and Tigers” blog, at the first sign of potential danger, the amygdala, in conjunction with the hypothalamus, sets off a sequence of physiological events that activates multiple organ systems. Interestingly, our body’s reaction to danger is so sensitive that these processes are often set in motion even before we are consciously aware of the source and nature of the threat. 

For example, have you ever nearly “jumped out of your skin” at the sound of a loud noise, even before knowing the source of that noise?  This is known as a “startle reflex,” is found across many of this planet’s species, and is one example of just how this SNS response, “fight or flight,” is “baked-in” to our DNA. 


OK, so let’s say you are out one evening taking a walk, you are relaxed, breathing-in the fresh evening air, watching the sunset on the horizon, and since you are feeling a bit hungry, thinking about where you might want to get a bite to eat after your walk.  Suddenly, out of the corner of your eye, you see something move and then dart out in front of you.  Without even thinking, you stop dead in your tracks, move back on your heels, your arms come up from your sides as if to protect yourself from an advancing threat.  After only a few seconds, you notice that it is just Muffles, your neighbor’s cat.  While cognitively you might experience almost immediate relief, if you were to do an immediate body scan you would notice that your pulse rate is up, your heart is pounding, your mouth is dry, and you feel breathless.  If you were to scan further, you might also notice that your hands are a bit sweaty, your legs feel a bit rubbery, and you are no longer hungry. What the heck just happened?

Well, in your peripheral vision, you caught a glimpse of something, an unknown something, and having perceived a potential danger, your amygdala signaled an alarm.  The amygdala then sent a distress signal to your hypothalamus that instantaneously signaled your adrenal glands to pump the neurotransmitter epinephrine (adrenaline) into your bloodstream. 

The surge of epinephrine caused your heart rate to increase and small airways in your lungs to open wide, providing more oxygen to your system, glucose (blood sugar) to be released into your bloodstream to supply your muscles with more energy, and all your senses to become heightened.  At this point, the hypothalamus secretes another hormone that then stimulates the pituitary gland, which in turn secretes another hormone that stimulates the adrenal glands to produce a hormone called cortisol.  I won’t burden you with the names and technical specifics of all these hormones, but it’s important to take note of the hormone cortisol.  While cortisol is critical to the healthy operation of our body, in excess, it can have negative long-term effects on our health.  (In a follow-up blog I will write more about cortisol, its function, its deleterious effects on our health, and how we can exert greater control over its secretion.)  

So, you just stumbled across Muffles the man-eating kitten which put you on a runaway anxiety train fueled by powerful neurotransmitters and hormones.  Somehow, you need to apply the brakes, but how? 

Activating the Rest and Digest Response

Luckily, evolution has not only supplied us with an on switch for our “fight or flight” defensive response, it has also equipped us with an off switch.  This off mechanism, often referred to as the “rest and digest” response, is the second component of our Autonomic Nervous System (ANS), known as the Parasympathetic Nervous System (PSNS).  The PSNS works in opposition to the SNS, lowering blood pressure, slowing our heart rate and respiration, and effecting many other physiological processes associated with a state of decreased arousal.  And while the PSNS is the predominant system responsible for our body’s “normal” day-to-day functioning, as we have seen, it is quickly dominated by the faster SNS during conditions of both real and perceived threat.   The good news is twofold; 1) Our “fight or flight” response is limited in the time that it can remain activated, and with the perception that the “threat” has abated, our PSNS kicks-in to unwind the effects of the autonomic nervous system, and 2) we have the ability to elicit the activity of the PSNS, thereby moving ourselves in to a more relaxed physiological state.

Health Benefits of Activating the PSNS

Over the past several decades, there has been a great deal of research done on our ability to activate the PSNS mechanism and its health benefits. In fact, most disciplines of modern medicine, (e.g., cardiology, oncology, rheumatology, pain management, etc.), have embraced these practices to assist their patients in reducing the effects of disease, as well as promoting overall physical health.  Additionally, practices that aid in the activation of the PSNS have been proven to assist in the improvement of most all known psychological disorders, including but not limited to, anxiety, depression, personality and mood disorders, attention and sleep disorders, substance abuse as well as eating disorders.  Likewise, in the general population, practices that stimulate the PSNS are highly correlated with improvement of mental health and overall experiences of well-being.

Practices to Activate the PSNS

So, you may ask, what are these practices? What is “magic sauce” that improves both physical and mental health?  Below are activities most highly associated with activation of PSNS:


      • Yoga

      • Deep Breathing

      • Getting out in nature

      • Warm baths, showers, hot tubs, steam showers

      • Guided Imagery and Visualization

      • Progressive Muscle Relation

      • Exercise

      • Tai Chi

      • Listening and Playing Music4

      • Massage

      • Singing/Chanting

      • Reading

      • Gardening

      • Mindfully engaging in almost any activity

    For those who have not read my “Monkey Mind” blog (April 13, 2023), my two favorite definitions of mindfulness are:

    Bringing one’s complete attention to the present experience on a moment-to-moment basis.  

    -Marlatt & Kristeller

    Paying attention with openness, curiosity, and flexibility.

    -Russ Harris

    Woman hiking in mountains

    The Importance of Mindfulness

    The most important aspect of all the above-listed activities is, that they are engaged in mindfully.  That is, with the intention to be fully present, and aware of all thoughts, emotions, physical sensations, memories, urges, and anything else that “shows up,” while engaged in the activity. As an example, one can take a walk in the woods, mindfully, noticing sights and sounds, how you are breathing, what thoughts you are having, how your feet land on the ground and how they leave a trace behind, etc.  Or, you can walk in the woods while wrapped up in thought, ruminating about problems at home or work, rehashing an event that did not turn out as you hoped.  That is, not noticing or experiencing anything about the present moment.  

    In future blogs I will further explore how we can become active participants in our physical and mental health and overall well-being.  Keep in mind, that the skill we most want to develop is our ability to be aware of and open to all that is present, both internally (thoughts, emotions, memories, etc.) and externally (physical sensations, the nature of our surrounding environment, etc.), as we engage in our day-to-day activities, That is, we most want to practice being mindful. 

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