Bam! Pow! You hear these noises as you walk home late at night. You have a sudden urge to run away as quickly as you can. Where did this impulse originate? What led you to run and not stay? Why did your heart rate heighten, and now you are breathing quickly? Our fight-or-flight response and the role of adrenaline is the reason for these reactions!
- We are going to explore the nervous system and the fight-or-flight response and the role of adrenaline.
- First, we will establish how the sympathetic nervous system is part of the fight-or-flight response.
- Then, we will discuss how the sympathetic and the parasympathetic nervous systems affect our fight-or-flight response.
- Following this, we will explore what it means when a fight or flight response is underactive.
- Throughout the explanation, we will discuss the various examples of the flight or flight response.
Fig. 1: The fight or flight response is a famous response in the world of psychology and biology.
Fight or Flight Response: Nervous System
When we detect a threatening stimulus such as a scream or a dog barking, the body sets off a complex chain reaction involving the endocrine and nervous systems.
The fight or flight response is activated by releasing hormones and activating the nervous system, preparing the body for action (typically through fight or flight, aka, running away).
What is the sequence of events and the stress hormones that cause fight-or-flight reactions?
- Senses receive input: you hear or see something alarming, and the information is sent to the brain via the sensory neurones. The hypothalamus (part of the brain in charge of the stress response) activates.
- The autonomic nervous system, which includes the sympathetic nervous system, is aroused.
- The sympathetic nervous system signals the adrenal medulla in the adrenal glands to produce adrenaline and noradrenaline.
- Adrenaline and noradrenaline activate the fight-or-flight response.
Another way of understanding the responses of fight or flight is to think of the three stages that are involved. The three stages of fight-or-flight are:
- The alarm stage: At this period, the central nervous system is alarmed and activated, preparing your body to fight or run away from potential danger.
- The resistance stage: Our body attempts to normalise and recover from the initial fight-or-flight response.
- The exhaustion stage: If the first two stages repeatedly occur over time (for those who suffer from chronic stress), this will cause the body to feel exhausted and break down.
The Endocrine System in the Fight or Flight Response
The stress hormones adrenaline, noradrenaline, and cortisol are the messengers that trigger the physiological fight-or-flight response in the various organs, thanks to our endocrine system.
Adrenaline is one of the catecholamines produced in the adrenal glands. Adrenaline helps relax the smooth muscles of the lungs to aid in breathing. It is responsible for the rising heart rate, the constriction of peripheral blood vessels, and the constriction of skin cells, causing hair to stand up (causing goosebumps).
Fig. 2: Adrenaline is released during the fight-or-flight response.
It also helps the liver break down glycogen to glucose and access sugar stores for immediate access to energy.
Cortisol is a stress hormone produced in reaction to chronic stress. Our bodies’ response to chronic stress is called the hypothalamic-pituitary-adrenal system (HPA axis). This system kicks in after the initial fight-or-flight response if stress is prolonged.
Fight or Flight Response: Sympathetic Nervous System
The sympathetic nervous system controls our flight or flight response. The sympathetic nervous system is activated like an alarm when there is perceived stress or danger. It’s a stress response that mobilises the body’s resources to deal with danger through our hormones and physical changes.
Fig. 3. The sympathetic and parasympathetic nervous systems and their functions.
Once activated, it takes 20 minutes to an hour for the body to return to its normal state.
The fight-or-flight response, communicated via the sympathetic nervous system, affects many body parts. The body then prepares for the eventuality of either running away from the danger (flight) or fighting the danger (fight).
Biological Reactions of the Sympathetic Nervous System Activation:
- Heart - There is an increase in heart rate. Why? An increased heart rate improves oxygen delivery to parts of the body.
- Lungs - Because there is an increase in oxygen, your airways will relax, allowing for easier delivery of all that oxygen.
- Eyes - Our pupils will increase in size so that more light is allowed in and vision is improved.
- Digestive tract - Our digestion will slow down so that all our energy can be focused on other areas of our body.
These effects help us in precarious situations where we must act and think quickly.
Your sympathetic nervous system activates our fight-or-flight reaction and affects our immune system. These effects are useful in repairing an injury quickly if we get hurt.
Fight or Flight Response: Sympathetic or Parasympathetic?
Many processes happen in both a sequence and in tandem when our fight-or-flight response is triggered.
For example, we can see opposite roles in our sympathetic and parasympathetic nervous systems. Well, they sound similar, so what makes them different?
Remember, our sympathetic nervous system is the bells and whistles that ring in alarm when there is a perceived danger. Our sympathetic nervous system is in control by activating our fight or flight response for as long as it is deemed necessary.
On the other hand, your parasympathetic nervous system sends signals when it’s okay to relax all those reactions and systems. When the parasympathetic nervous system is in control, our heart rate slows down along with our breathing rate. Other changes at this time are lower blood pressure and normal digestive rates.
To summarise, the sympathetic nervous system is involved in fight-or-flight, and the parasympathetic nervous system is involved in rest-and-digest.
Underactive Fight or Flight Response
Although it seems inconvenient to have a fight-or-flight reaction to public speaking or seeing a spider, a long time ago, when dangers were more apparent and life-threatening, a fight-or-flight response was (and still is) useful to the organism’s survival. Just imagine for a moment if someone were to have an underactive fight-or-flight response at the sight of a tiger or a fire. It would take them more time to perceive the danger, and their reaction would be slow and sluggish.
The seconds the body saves in being ready for action might mean the difference between life and death.
In this case, it was being able to dodge a tiger in time or run away from a fire.
Only our ancestors with these quick reactions survived long enough to pass on their genes.
Fight or Flight Response Examples
Our life is full of events or happenings that could be perceived as stressful or fear-invoking. There is a multitude of examples that perhaps we have all collectively experienced.
An example of the fight-or-flight response in action can be seen if you are walking along a street during a dark night. In this scenario, you notice a rather scary figure following close behind you.
Your heart rate begins to climb, your palms grow a little sweaty, and your breathing quickens. The urge to run climbs, and as the figure gains on you, they break out into a run.
You run, too, muscles ready to go and full of energy, your heart beating fast to provide oxygenated blood to your limbs. More subtle changes less apparent can be seen in your gut.
In this example, we can see how the apparent danger presented by the strange figure produced a physiological response in your body. Heart rate increases to provide more blood to the limbs, your breathing rate increases to take in more oxygen to aid the increased blood supply, and other physiological changes occurred to prepare you for running away.
Sometimes, a fight or flight response may also occur in less appropriate situations.
You've been asked to give a speech at a huge conference. You find beforehand that your heart is beating fast, your palms are often sweaty, and you feel very stressed.
Before you walk onto the stage, your heart rate increases quite dramatically. The danger is less apparent in this scenario, but the fear-inducing scenario produces a similar response in your body. It goes to show how humans have developed, and more primitive responses have been carried into the modern world.
Fig. 3: The fight-or-flight response helps you run in times of danger.
Adrenaline: Fight or Flight
As discussed above, the stress hormones adrenaline, noradrenaline, and cortisol are the messengers that trigger the physiological fight-or-flight response in the various organs, thanks to the endocrine system. Adrenaline constricts blood vessels, increases blood flow to muscles by reducing intestine movements, and increases heart rate.
Adrenaline is thus important in the fight-or-flight response, allowing the body to prepare to move in cases of emergency.
Fight-or-Flight Response and The Role of Adrenaline - Key takeaways
- The fight or flight response is activated by a release of hormones and activation of the nervous system, preparing the body for action (through fighting or running away).
- The three stages of fight-or-flight are the alarm stage, the resistance stage, and the exhaustion stage.
- The fight-or-flight response, communicated via the sympathetic nervous system, affects many body parts. Heart rate increases, pupils dilate, breathing rate increases, and digestion slows, to name a few responses.
- The sympathetic nervous system is the bells and whistles that ring in alarm when there is a perceived danger.
- The stress hormones adrenaline, noradrenaline, and cortisol are the messengers that trigger the physiological fight-or-flight response in the various organs, thanks to the endocrine system. Adrenaline constricts blood vessels, increases blood flow to muscles by reducing intestine movements, and increases heart rate.
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