Fight-or-Flight as a Stress Response

In the last post I spoke about the concepts of homeostasis, allostasis, and allostatic load as they relate to stress. Now we will be applying these concepts to the human body, how it responds to stress, and what the long-term effects of chronic stress are on the body.

To talk about how the stress response manifests in the body I will first talk about the two major organ systems which coordinate those responses.

The Nervous System

The nervous system, consisting of the brain, spinal cord, and nerve fibres spread throughout and interlinking the many organ systems of the body, is what coordinates bodily actions in response to sensory stimuli. Nerve cells can span the entire length of the body and allow a cell at one end of the body to send a direct signal to a cell at the other end.

The nervous system allows very fast and precise responses to sensory inputs, which is why it is used to coordinate muscle movements. The simplest example of this is called a reflex arc, which is a chain of neurons connecting information from a sensory neuron (which carries information from a cell sensing the environment) to a motor neuron (which carries a signal for action to a muscle fibre) without going through the brain first.

A reflex test with a reflex hammer by a doctor is a common trope of routine medical checkups. When the force picked up by the sensory cell is of a sufficient intensity, it breaches a threshold set by nerve cells in the spinal column which immediately send a motor response to the leg to react. Another example of a reflex arc is withdrawal of the hand from a hot stove upon contact— the initiation of the motor response comes from the spinal cord directly instead of the brain.

The Endocrine System

In addition to the nervous system our body also has a slower form of body-wide communication which produces effects that are much more long-lasting than the single firing of a nerve fibre to stimulate a muscle contraction. This system is called the endocrine system, which consists of many glands throughout the body that produce and secrete hormones, or chemical messengers, which travel through the blood, lymph, and other fluids. When a hormone binds to a receptor for that hormone on a cell it alters the gene expression of the cell, changing its function and how it affects the environment around it. The endocrine system is slower than the nervous system but causes more long-term and systemic changes in how the body is responding to the environment than nerve cells do. Think for example of testosterone or estrogen, which are messengers which orchestrate the development of features unique to each sex which begin to show up during puberty.

Two Branches of the Nervous System

The endocrine and nervous systems exist because multicellular organisms need to coordinate responses in many organ systems from very few sensory inputs from other tissues and organs. Only your eye can perceive the external shapes in the environment via light, and the nervous and endocrine systems are what allow information inputs from the eye to influence other organs which cannot see the external world, yet need to be able to react to the eye’s visual stimuli to allow the organism to survive.

As mentioned in the previous post, all life tries to maintain homeostasis. The part of the nervous system that maintains this for the internal organs, glands, and some muscle groups is called the autonomic nervous system. It is called autonomic because it operates largely without conscious input.
Your nervous system is constantly fine-tuning many homeostatic set points in the body, including pH, electrolytes, blood sugar levels, and so on through the autonomic nervous system.
The autonomic nervous system has four divisions, two of which operate in a largely antagonistic manner to modify the functioning of tissues and organs in the body, similar to how a gas pedal and brake modify the speed of a vehicle. The name of these two branches are thesympathetic nervous system(the gas) and theparasympathetic nervous system(the brake). The term gas and brake is somewhat misleading though, because both can have activating or inhibiting influences on the tissues and organs they innervate. For example the sympathetic nervous system is responsible for increasing the heart rate, while the parasympathetic system lowers the heart rate; however, when it comes to the digestive system the sympathetic nervous system plays a mostly inhibitory role— reducing the activity of the stomach and upper intestines and reducing blood blow in those organs.

The Sympathetic Nervous System

The activation of the sympathetic nervous system, in the abstract, is about mobilizing the structures and energies of the body for high levels of action. Returning to the concept of homeostasis and allostasis, the sympathetic nervous system adjusts the set points of many physiological markers, such as heart rate, breathing rate, fat and glucose levels in the blood, and so on so that an individual is better able to act and move with a lot of force.

© Bill Watterson.

This is a handy system to have when an organism perceives the safety of the environment to be compromised, and requires a lot of action to maintain itself. Say for example if you were being chased by a tiger, a nearby avalanche was occurring, or if food was running away and you were hungry, a whole slew of coordinated responses kick in to allow you to survive to another day. Your heart rate and breathing rate increase, energy stores in the body become freed up, blood vessels constrict or dilate to direct blood away from the digestive organs and toward the heart, lungs, and skeletal muscles. You can experience tremors or shaking in the body, tunnel vision or limited loss of hearing in extreme cases. Even your blood coagulates faster in such a state in anticipation of injuries and blood loss. This is called the fight-or-flight response, hyperarousal, or the acute stress response.

Your mind also changes its function during a fight-or-flight response. In dangerous situations you can experience high levels of fear, anger, or other negative emotions. Thoughts race at a hundred miles and hour, no longer dispassionate but instead focusing heavily on what triggered the response in you, or alternatively focusing on avenues of escape. This is called a state of hyper-vigilance, or extreme attention to the details of the external environment.

These changes in the mind and body spur activity that, in the right situation, will literally save our lives. Our bodies possess an incredible ability to adapt to environments which demand a lot of us, especially those threatening to us, by engaging in allostasis and shifting the set points of the autonomic nervous system towards levels beneficial for high amounts of action.

This acute stress response is an example of an allostatic load, or a departure from homeostasis in some parameters, to maintain the health of an organism over the long-term. Usually the sympathetic and parasympathetic systems both fire regularly and their signals counterbalance one another. In cases of acute stress the sympathetic nervous system increases its activity and the parasympathetic nervous system decreases its activity, biasing the set points for all influenced cells toward sympathetic activation.

Mechanisms of Acute Stress Response

As mentioned earlier, the nervous system and the endocrine system are the means by which perceptions of the environment can change behaviour. In the example of a tiger attack in the acute stress response, your eyes or ears may pick up on the presence of a predator nearby focusing on you. A section of the brain called the amygdala (that means almond because of its shape) is responsible for emotionally charging perceptions of the environment with a negative or “unsafe” quality.
If this hits a certain threshold, two systems are activated in the brain: the sympathoadrenal system, and the hypothalamic-pituitary-adrenal (HPA) axis.

Three mammals with high sympathetic activation. © Getty Images

The sympathoadrenal system sends a signal via the sympathetic nervous system to a part of the adrenal gland called the adrenal medulla, which releases epinephrine (adrenaline) and norepinephrine. Within a very short time these hormones wash through the body via the blood and activate a wide variety of changes, including increase in heart rate, lung ventilation, blood flow to muscles, release of sugars from the liver, and other physiological changes. These effects can last up to an hour after the initial release. This is why even after danger passes you may still feel your heart racing, for example after experiencing a near miss in a potential car collision.

The hypothalimic-pituitary-adrenal axis is a more complex case. The amygdala communicates to a part of the brain called the hypothalamus, which releases a hormone called corticotropin releasing factor (CFR). When CFR reaches a part of the brain called the anterior (“in front”) pituitary gland, this gland releases adrenocorticotropic hormone (ACTH). This hormone, upon reaching the kidneys, causes the adrenal cortex to release steroid hormones, particularly cortisol.

Cortisol is a hormone which is released in the body every morning in tune with the circadian rhythm. It is usually highest when we wake up. Saliva tests of cortisol levels are often used as an indicator of activity of the HPA-axis, and by extension stress levels more generally. It modifies the metabolism of the body by helping produce and release glucose from the liver, fats from adipose (fat) tissue, and also raises amino acid levels. It has an inhibitory effect on the immune system and on anabolic (or tissue-growing) activities, including those of muscle or bone. This is done because when one is in a stressful situation the body needs to spend energy on staying active and aware of the environment, instead of on growth and repair. The body is even prepared to break down muscle or other tissues, converting them to forms that can be used for energy, in order to maintain a certain level of energy in the blood.

In the right situations, this can save our lives. What can be problematic is when an organism has elevated levels of adrenaline and cortisol for prolonged periods when the objective situation does not call for it. For example, people who have stressful work hours or are experiencing financial insecurity can have these elevated cortisol levels. Obviously attempting to keep energy levels in the body raised above a given homeostatic set point is not needed in the developed world where there usually is little requirement for immediate action and food is relatively abundant. But the perception of stress by the body can create a state of almost semi-permanent perceived crisis, where the body accumulates this allostatic load of stress hormones and adaptations which do not objectively serve the person, and which eventually wear down the body.

Effects of Chronic Stress

The consequences of chronic stress and elevated stress hormones and sympathetic nerve signalling is a difficult list to write exhaustively.

Mobilizing high levels of energy to the muscles which is never discharged can lead to tenseness in the muscles and habituate them to tightness. Conversely high stress can also lead to atrophy of the muscles as well, so there can be an increase in tenseness with a corresponding loss of strength or endurance. Osteoporosis can result as a consequence of chronically drawing down on the minerals, proteins, and other resources that make up bones. This raises the risk of accidental injury.

The respiratory system can become geared towards hyperventilation, where breathing is predominantly in the chest and through the mouth instead of nose. Sensory nerves in the chest themselves feed information back to the brain, and full chest inhalations stimulate the sympathetic nervous system further. Chronic chest breathing can lead to a positive feedback loop, where chest breathing stimulates release of stress hormones, which then increases the amount of chest breathing. The excessive ventilation lowers CO2 levels such that the pH of the blood shifts to be more alkaline, which requires the body to modify kidney function and mobilize calcium stores from the bones and other tissues to help balance the blood pH. This can have long-term negative effects on bone density.

Chronic stress disrupts the regular metabolic processes, which can lead to obesity, diabetes, chronic fatigue, and other metabolic syndromes. The digestive process and appetite itself can be suppressed by chronic stress, thereby making assimilation of nutrients from food itself much slower or less frequent.

The immune system can become suppressed or dysregulated and make an individual more vulnerable to infections, autoimmune diseases, and cancer.

The increased heart rate, blood pressure, and blood thickness (from increased rates of coagulation) can raise the risk of heart attacks and strokes.

Chronic stress has a host of negative effects on the reproductive systems, including libido, menstruation, and pregnancy. It has a negative influence on the developing fetus, and can lead to lower birth weight or even embryo loss or miscarriage.

Concluding Thoughts

Bringing this back to homeostasis. The temporary shift of set points in the body toward those favouring higher energy for action can have long-term negative consequences on the body and wellbeing of the individual if this allostatic load is maintained for longer than is required to deal appropriately with the cause of the stress. All of the negative side effects of stress are a direct consequence of the nervous system applying a high allostatic load to the body for too long.

Often times the stress response, while appropriate in the wild, is not as appropriate for the situations or conditions which trigger a stress response in us anyhow. This makes developing awareness around sources of stress in our lives, how they affects us, and how to reduce the allostatic load on our systems critical for maintaining our long-term health and quality of life.

In the next entry I will be going into detail about the parasympathetic nervous system: the system responsible for down-regulating the stress response and cooling the fires it stokes in our bodies. Hope to see you there.

Thanks for reading Living Breathwork! Subscribe for free to receive new posts and support my work.