The Physiology Of The Stress Response
Author: Emile
Stress: A man frustratedly looking at the receiver of a phone

In order to understand the stress response, it is vital that we have a working knowledge of the nervous system, and particularly of the peripheral nervous system. So I apologise if the first part of this sounds suspiciously like Biology 101!

Don’t panic! Just refer to the nervous system diagram if you are getting a bit lost. Ultimately we are interested chiefly in the balancing act that goes on between the sympathetic and parasympathetic nervous system when the stress response is induced.

The Nervous System

The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. This contains both the central nervous system and the peripheral nervous system.

The Central Nervous System (CNS)

The central nervous system is made up of the brain and spinal cord. Here goes the impossible, where I get to try to summarise the functions of the nervous system into one sentence: it samples the environment and body and makes useful adjustments within the body, utilizes controlled movements based on inborn and acquired (learned) information, integrates instinct and experience with the facts of the present situation, and generates an appropriate response.

The Peripheral Nervous System

The peripheral nervous system is the part of the nervous system outside of the brain and spinal cord. It serves as a relay of information to and from the CNS and the limbs, organs and other parts of the body. The peripheral nervous system is broken down into two parts, the somatic nervous system and the autonomic nervous system.

The Somatic Nervous System

The somatic nervous system is responsible for coordinating body movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.

The Autonomic Nervous System

The autonomic nervous system is that part of the peripheral nervous system that controls the visceral processes through largely unconscious (involuntary) mechanisms. Although largely outside of our direct control it nevertheless can work together with the somatic nervous system, which is that part of the nervous system that we have voluntary (conscious) control over. The autonomic nervous system controls functions such as pupil dilation, heart rate, respiration rate, digestion, sexual arousal, swallowing and urination.

The autonomic nervous system is split into the enteric, sympathetic, and parasympathetic nervous system. Let’s take a brief look at these three systems so that we are better able to understand the stress response.

The Enteric Nervous System

The enteric nervous system controls the function of the gastrointestinal system. It is actually embedded in the gastrointestinal system itself. Although it usually receives some input from the autonomic nervous system it can actually operate independently of the brain and spinal cord! In general, the sympathetic nervous system causes inhibition of gastrointestinal secretion and motor activity, and contraction of gastrointestinal sphincters and blood vessels. The parasympathetic nervous system stimulates these digestive activities.

The Parasympathetic Nervous System

The major function of the parasympathetic nervous system is to keep the body in its normal state; to return the body to equilibrium. So it is responsible for the unconscious ‘rest and digest’ functions of the body. The parasympathetic nervous system enables the body to perform simple but vital tasks such as breathing regularly, excreting hormones, eating and digesting food. Rather critically, it also keeps a person alive whilst sleeping!

The Sympathetic Nervous System

The sympathetic nervous system usually functions to produce localized adjustments (e.g. sweating to reduce an increasing body temperature) and reflex adjustments of the cardiovascular system in order to maintain homeostasis. However, it is also critical in regulating the neuronal and hormonal flight-or-fight stress response. In a sense, the parasympathetic and sympathetic nervous systems work in balance with each other and directly or indirectly affect almost every structure in the body.

The Stress Response

The stress response begins when we are faced with potential stressors. We determine whether a particular situation is ‘dangerous’ based on sensory input, memory and reasoning. Sometimes conscious reasoning is a core part of this process and sometimes it happens much more unconsciously. If the situation is perceived to be ‘dangerous’ (i.e. stressful) then the hypothalamus is activated.

Stress Hormones

The biological response to stress starts in the brain. Prolonged stress causes the brain to signal the adrenal glands (above the kidneys) to release the stress hormones epinephrine (adrenaline) and cortisol. These are the hormones that prepare the body for flight or fight! Sustained elevation of these stress hormones begins to negatively impact the body. Let’s go through this process a little more slowly. This stress response can be artificially broken down into two parts.

Release Of Epinephrine

Much of this initial release of epinephrine happens before we are even consciously aware of ‘danger’, with the amygdala and hypothalamus already in action before the brain’s visual centres have even finished processing the information! Obviously, this enables a much quicker response to any danger.

The amygdala initiates the process by sending a distress signal to the hypothalamus, which then activates the sympathetic nervous system by sending signals to the adrenal glands. These adrenal glands respond by pumping the hormone epinephrine into the bloodstream (think adrenalin rush!). The epinephrine in the bloodstream leads to exactly the type of sympathetic nervous system physiological reactions that one associates with adrenalin:

  • Increased heart rate,
  • More blood to muscles,
  • heart and other vital organs,
  • Increased blood pressure,
  • Increased pulse rate,
  • Opening up of small airways in the lungs so it can absorb additional oxygen, which then travels to the brain to increase alertness,
  • Sharper hearing, sight and other senses,
  • Release of glucose and fats from temporary storage sites into the bloodstream, supplying additional energy to the body.

Hypothalamic-Pituitary-Adrenal Axis (HPA) And Release Of ACTH

As the initial surge of epinephrine subsides, the hypothalamus activates the second component of the stress response system — known as the HPA axis. This network consists of the hypothalamus, the pituitary gland, and the adrenal glands.

The hypothalamic-pituitary-adrenal axis (HPA axis), is a major part of the neuroendocrine system and contains a complex set of direct influences and feedback interactions among three endocrine glands: the hypothalamus, the pituitary gland and the adrenal glands. The HPA axis controls reactions to stress, regulates many body processes including digestion, the immune system, emotions, sexuality and energy storage and expenditure. The HPA axis a regular mechanism for interactions among glands, hormones and parts of the midbrain that mediate the general adaptation syndrome (GAS).

The HPA axis depends on a series of hormonal signals to keep the sympathetic nervous system at full force! If the brain continues to perceive something as dangerous, the hypothalamus releases corticotropin-releasing hormone (CRH), which travels to the pituitary gland, triggering the release of adrenocorticotropic hormone (ACTH). This is the hormone that travels to the adrenal glands, prompting them to release cortisol. In ‘normal’ stress responses the threat passes, cortisol levels recede and the parasympathetic nervous system kicks in and diminishes the stress response.


Cortisol levels naturally vary throughout the day. Higher levels typically occur in the mornings and decline during the night. As described, the brain monitors signals from the body and can turn up or down the production of cortisol when needed. Excess cortisol stimulation can become a problem under conditions of prolonged physical, physiological and/or emotional stress, essentially where there is a continuous or chronic stress response. In this situation, the excess cortisol that is secreted can interfere with glucose (required for fuel) from getting to important tissues in the body.

Excess cortisol over time can cause any of the following: depression, accelerate ageing, depressed immune system, chronic exhaustion or chronic fatigue syndrome, poor sleep habits and insomnia, obesity and weight-related problems, high blood pressure, diabetes – constant stress messages cause excess glucose in the bloodstream, which then increases insulin levels and can lead to insulin resistance, increased blood glucose and diabetes, Increased cardiovascular disease – the body attempts to store this excess glucose by forming triglycerides that then eventually lead to an increase in body fat, and these prolonged levels of triglycerides in the bloodstream can lead to other heart and blood vessel problems.

DHEA (Dehydroepiandrosterone)

The same repeated and prolonged stressful events that cause cortisol to increased will cause DHEA levels to decline. DHEA is the “antidote” to cortisol. When DHEA levels decrease this can cause several metabolic and behavioural problems. Cortisol and DHEA levels act in opposite directions, so high cortisol means low DHEA.

Prolonged low DHEA is associated with any of the following:

  • cancer
  • decreased immunity to illness,
  • poor coping ability,
  • memory loss,
  • fatigue
  • diabetes
  • obesity
  • asthma
  • chronic fatigue,
  • lupus and many other chronic illnesses.

We Live In Denial Of How Dangerous An Ongoing Stress Response Can Be For Our Health

People are always talking about how psychological stressors can lead to actual physiological damage and indeed chronic illness. I think often people hear this in a very selective way, a little like many out there take in information on how sugar is dangerous or how we should exercise more. With a pinch of salt! This is due to the highly stressful world that we live in and a level of denial about many of the risks that we face. To a degree, this is a good thing!

Can you imagine, for example, what your life would be like if you woke up every morning wondering if you were going to die in a car crash on the way to work? Or indeed when you are presenting to the board on a corporate merger and you start considering not only how anxious you are that you are about to get fired, but also that you have been working on this presentation for over a month, which means stress levels have been consistently massive and sleep at a bare minimum, which means that you might have cancer to contend with also if this keeps up, and you are less than sure if your disability insurance is as great as you had hoped, and then again would the wife really stay with you anyway if you got really sick, and of course you still have to survive the car drive home……. GASP!

Drumming Home The Mind-Body Link

Talk about a way to maintain a continuous stress response and bring on chronic illness! So we certainly should not spend our life invoking a panic attack. However, I am hoping that this brief introduction to the stress response is helpful in making the mind-body link just a little bit more real for you.

You will see in other articles that we will make use of psychological, physiological and biochemical treatment modalities as a way of relieving or reducing unhealthy stress, as well as in the treatment and management of stress-based illness.

Just remember, it is a lot easier to prevent chronic illness than it is to manage or cure it once it presents!

Author: Emile

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