What Is Stress?

This is part 2 of a series of blog posts discussing the ins and outs of stress: what it is, when it’s helpful and when is it harmful, how to optimize your stress response, and some strategies for living with a caveman nervous system in a modern, stressed-out world.

Stress is Ubiquitous

Caveman and sabertoothWhen the body is exposed to real or perceived harm or threat, the result is a cluster of physiological changes that are collectively referred to as the stress response, or just stress. All stressors, whether physical or psychological, whether real or imagined, produce a similar core pattern of physiological changes that prepare the body to either fight to the death, or run for it’s life (fight or flight). Remember from part 1, that our stress response has evolved very little since the days of our Paleolithic ancestors. What’s important to wrap our minds around, is that it does not matter what the particular stressor is. You can be fighting with your spouse, watching a scary movie, or being chased by a bear through the woods, it doesn’t matter…at least not to your nervous system. Your body only has one stress response with which it responds to all stressors. Thus, any perceived or real stress will always trigger the same nervous system response, commonly known as “fight or flight.”

Nervous System 101

“Stress kills.” We’ve all heard that saying. But do you know how and why stress kills? Did you also know that without stress, we would all be dead? In order to understand how stress can be so damaging yet essential for survival, it helps to have a basic understanding of the nervous system, for this is where the stress response originates.

Brain and nervous system

Our nervous system is made up of a central nervous system (brain and spinal cord) and the peripheral nervous system (everything outside of/peripheral to the brain and spinal cord). You can see on the awesome flow chart I made below (pats technologically-challenged self on back) that the peripheral nervous system is subdivided into the somatic and the autonomic nervous systems. Stated simply, the somatic nervous system is what’s active when we are voluntarily controlling our skeletal muscles (tell yourself to flex your arm, and your bicep contracts via the action of the somatic nervous system). In contrast, the autonomic nervous system (ANS) is always turned ‘on’ because it’s the branch of the nervous system responsible for controlling involuntary processes that are always humming along in the background…such as heart rate, body temperature, breathing, digestion, hormone release, and other processes required to maintain life. So the big takeaway point here is that the ANS is involuntary…you don’t have to remind your heart to beat, or your lungs to breathe…they just do it on their own, involuntarily.

Basic hierarchy of the nervous system

Notice that the autonomic nervous system (ANS) is further subdivided into two branches: parasympathetic (rest and digest) and sympathetic (fight or flight). Nerve fibers (originating in the central nervous system) diverge from the spinal cord to form the two branches of the ANS that innervate all of our internal organs and glands. Under the control of the hypothalamus, the two branches of the ANS work in opposition to each other, diligently maintaining allostasis (homeostasis) within our bodies, without us ever having to think about it. That’s pretty amazing when you think (or don’t think) about it!

Sympathetic vs Parasympathetic

Human nervous systemVia a complex network of neurons, hormones and chemicals, our central nervous system (CNS) is constantly monitoring our internal and external environments, and gathering information from our senses. The brain integrates our current situation with memories from previous experiences, and ultimately decides what it considers to be a threat in the external environment. Accordingly, the brain (more specifically, the hypothalamus) sends commands in the form of nerve impulses down the spinal cord, out the peripheral nerves, and to the various muscles, organs and glands of the autonomic nervous system (ANS).

The branch of the ANS that is active depends on whether or not our hypothalamus perceives any danger. In theory, there are really only two states that we can be in at any given moment: we are either stressed out (sympathetic nervous system is active) or we are not stressed out (parasympathetic nervous system is active). Of course things are actually a bit more complicated than that…but you really only need to understand the basics in order to know enough to effectively re-train your stress response, which is the ultimate goal here (I’m foreshadowing to a future blog post). :) In summary:

Neuron ConceptTwo branches of the involuntary Autonomic Nervous System (ANS):

1) Sympathetic Nervous System (SNS) = Stressed = Fight or Flight = Prepare for Exercise

2) Parasympathetic Nervous System (PNS) = Not stressed = Rest & Digest = Conserve and replenish energy = Growth & Reproduction

Fight or Flight – The Sympathetic Nervous System

Fight or Flight
During stress (be it physical or emotional, real or imagined), the sympathetic nervous system becomes activated, and the parasympathetic division powers down. High sympathetic activity prepares our bodies to ‘fight or flight’, and favors conditions that can support vigorous physical activity and the rapid production of energy required to support intense exercise, such as:


  • Peripheral blood vessels constrict, which raises our heart rate and blood pressure to increase the delivery of blood and oxygen to the brain, heart, and skeletal muscles.
  • The muscles involved in exercise (i.e. skeletal muscles, heart, liver, adipose tissue) all dilate, to allow a greater influx of blood and oxygen.
  • Respiration rate increases due to the airways dilating, which allows for faster movement of air into and out of the lungs.
  • Blood glucose rises as the liver starts breaking down its glycogen reserves to liberate glucose into the bloodstream, which is used for energy.
  • The pupils of the eyes dilate, which narrows our focus.
  • Processes that are not essential for ‘fight or flight’ are shut down. For example, blood is shunted away from the gastrointestinal tract and  kidneys, and towards the skeletal muscle in preparation for ‘fight or flight’. This effectively inhibits digestion and urine production, as these activities are not essential during an emergency.
  • Long-term, energy-intensive, building processes (such as growth and repair of tissues, and reproduction) are halted during sympathetic nervous system activation.

Stressed outAs you can see, the function of the SNS is quite literally to get us ready to either fight to the death, or run for our lives. It does this by quickly preparing our bodies for intense exercise, and by shutting down any processes that are not required for immediate survival. This series of events is called the acute stress response, and its effects are systemic, of short duration, and gradually subside when the stressor is no longer present. Remember from part 1, that the human nervous system and its stress response have evolved very little, if at all, since the dawn of humanity. Caveman stress was short-lived, and consequently the SNS is really only designed to deal with acute stressors. If the stress keeps on going…becoming chronic…this is when we start running into health problems associated with “stress.”


Rest and Digest – The Parasympathetic Nervous System

During the quiet intervals between periods of exercise, when the brain perceives no threats to be present, the SNS can take a break, and the parasympathetic branch of the autonomic nervous system becomes dominant. As you can see from the list below, the parasympathetic response is essentially the opposite of the sympathetic response.

The parasympathetic nervous system (PNS):Rest and digest (2)

  • Is nicknamed “rest and digest” because it’s active when we are at rest, relaxing, and digesting our food
  • Supports body processes that conserve and replenish energy reserves during times of rest and recovery
  • Supports the growth and repair of tissues, and also supports a reproductive state
  • Shunts blood away from skeletal muscles and towards the digestive organs, providing the energy needed to properly digest and absorb the nutrients from food
  • Decreases heart rate
  • Constricts the airways
  • Constricts the pupils
  • Increases salivation, digestion, defecation and urination

Yin yang of homeostasis and allostasisAre your eyeballs rolling back into your head yet? Take a deep breath (to activate your PNS), and meditate upon the yin-yang-like nature of the autonomic nervous system. The SNS and PNS work in tandem to create either stress or relaxation, respectively. When the SNS is active, we are meant to exercise. When the PNS is active, we should be at rest. Now can you see why it’s a really bad idea to eat when we are stressed out? Our bodies are not designed to digest or absorb nutrients from food when the SNS is active, which is why activating our PNS by developing an attitude of gratitude before every meal is one of the BEST things we can do to improve our digestion, and truly our overall health.

So What is Stress?

I’ve sort of shot myself in the foot by titling this blog post “What is Stress”, because while it sounds like a simple question, the answer is inevitably much more complex than one might initially think. Stated simply, “stress” is the body’s response to the brain perceiving a threat. The body’s stress response is largely mediated by the nervous system, as we’ve just discussed. But the story doesn’t end there, and in the next article of this series, we’ll dive into the notorious “stress hormones” and discuss how ongoing, chronic stress can lead to chronic inflammation and chronic disease.

Red flip flops on the beachBefore signing off, I want to reiterate that the inevitable stressors of life (AKA things that stress you out) are NOT the primary issue when it comes to stress-related diseases. Rather, our interpretation of, and reaction to these various stressors is  ultimately what’s responsible for triggering our stress response.  Mark Sisson made me laugh when he said: “Stressing over stress is what makes stress so stressful.” It definitely pays to have the skills to manage our stress, and we’ll talk about that in a future blog post too. But for now, let’s give it a rest (and digest)! :)

In good health,

Kinsey Jackson, MS, CNS®




  1. Probably the most harm comes from chronic stress. Those issues that stress people out day after day that don’t get addressed.

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