What is the fight-or-flight response?

Introduction

In his classic book The Wisdom of the Body (1932), American physiologist Walter Bradford Cannon
introduced the term fight-or-flight to describe the physiological (or bodily) reaction that occurs when humans or animals are confronted with something that they see as threatening. He was also the first person to use the word
stress
, a term borrowed from engineering, to describe situations or responses to situations perceived as threatening.

Perhaps the best way to illustrate the fight-or-flight response is to look at common animals. It can be seen in cats or dogs: the hair on the back of their necks stands up, and they may take a position that indicates they are ready to fight. Cats may take the “Halloween cat” stance, with the back arched and hair standing on end, and tend to spit, hiss, and otherwise attempt to look threatening; dogs may growl and keep their heads lowered, eyes narrowed, and feet planted wide, ready to take on the enemy. Alternatively, the animal simply runs. This is fight-or-flight at its most obvious.

In humans, the fight-or-flight response is usually less obvious, but it contains most, if not all, of the same physiological reactions. The heart pounds, the blood rushes to the muscles to prepare to fight or run, sweating begins, and the bowels may actually loosen. Maxims referring to persons being so frightened that they wet themselves actually describe part of the physiological reaction that occurs during the fight-or-flight response. Further, one may feel a strange feeling on the back of one’s neck. This is piloerection, which is the scientific name for the hair on the back of the neck standing up. If one is unaware of what is causing the sensation, it can make what is already stressful even more so.

Basic Biology

To understand the fight-or-flight response, it is necessary to review some of the basic biology of the nervous system. In the nineteenth century, French physiologist Claude Bernard first proposed the idea that there was a need for a stable internal environment even as external conditions change. Cannon further developed this idea in his book The Wisdom of the Body, proposing the concept of negative feedback as the mechanism for maintaining homeostasis
(which means same-staying, or physiological equilibrium) in the body.

The nervous system of all vertebrates is composed of two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal corde —everything encased in bone. The PNS is everything else—all the motor and sensory nerves in the body outside the brain and spinal cord. The PNS is further divided into two parts: the skeletal (also called somatic) system, which controls voluntary muscle movement for the most part, and the autonomic nervous system, which for the most part mediates involuntary activity in the body. The autonomic nervous system is divided further into the parasympathetic division, which is what maintains homeostasis, operating to keep the body, including internal organs, in a mode that allows it to rest and digest, terms introduced by Cannon at the same time as he introduced fight-or-flight. The parasympathetic nervous system
maintains the basal heart rate, respiration, and metabolism under normal conditions. This is the opposite of the sympathetic nervous system, which reacts to anything perceived as stressful or threatening and is responsible for getting the body ready to stand or run, fight or flee.

The body needs to be able to react to changes in the internal or external environment. These changes include blood loss from an accident or injury, running a marathon, changes in temperature, emotional stress, an earthquake, or hearing an unexplained noise in the night. In fact, although people sometimes say that they wish they did not react to stress, it has been demonstrated experimentally that animals that lack stress responses require special care to stay alive. The fight-or-flight response is, therefore, a very adaptive reaction and becomes pathological only when it goes on for long periods of time or when it happens when there is nothing to precipitate it.

Cannon first recognized that the sympathetic and parasympathetic nervous divisions had different functions, although the two divisions actually work together most of the time. It may seem as if they are pulling in opposite directions, but there is a balance between the two systems resulting from an interaction between what is going on inside the body and what is going on in the environment. The body needs a certain amount of tension between the two systems to maintain the correct balance of arousal and relaxation (homeostasis) under normal circumstances.

A good example of how the two systems work together, and how one can be more active than the other at any given time, is the situation of a person sitting quietly in the living room, reading a book after eating dinner. During this time, the parasympathetic nerves have slowed the heart rate and the digestive system has become active. The nerves mediating the sympathetic nervous system are not inactive, but the parasympathetic is “in control.” If the motion sensor–controlled light outside the house were suddenly to come on and the person in the living room were to see a figure standing outside the window, the sympathetic nervous system would react within one or two seconds. The digestion would slow down, the heart rate would increase, and the blood would be diverted from the skin and digestive organs and rush to the muscles and brain. The person would begin to breathe harder to make more oxygen available, the mouth would get dry, the pupils of the eyes would dilate (letting more light in for better sight), and the sweat glands would become more active, getting ready to cool the body during its coming activity—either fighting or running. When the figure turns and the person realizes it is a neighbor retrieving a wayward cat, the parasympathetic system resumes control. Heart rate slows and pupils return to normal size.

Brain Activity

The fight-or-flight response is mediated by the hypothalamus, which is a very small (about only 4 grams of the approximately 1,400 grams a normal adult brain weighs) but a very important group of nuclei (groups of nerve cells, or neurons, that are functionally related) deep in the brain. The hypothalamus integrates the functions of the autonomic nervous system, both the parasympathetic and the sympathetic divisions, via the pituitary gland and ascending and descending fibers that pass through it, connecting the brain with the rest of the body, including the internal organs.

When the brain perceives a threatening stimulus, whether it is something that one assesses cognitively (such as realizing that one is being attacked verbally) or something that suddenly appears and to which one reacts reflexively (such as being confronted by a bear or a snake in the wild), a cascade of events begins. It may be that by the time the actual physiological reaction takes place, one would have already started to respond, protecting oneself from the bear, for instance, or confronting the person who is attacking. There are hierarchical neural pathways from the sensory receptors that send messages to the brain. Some messages travel through the thalamus—an important area deep within the brain for sending and receiving messages between the brain and the environment—and up to the cortex so appropriate action can be taken. Simultaneously, messages are also sent via the thalamus to the hypothalamus (which contains many nuclei that actually control the autonomic nervous system) and the frontal cortex, passing through the amygdala (a part of the brain important in remembering emotional responses to previous occurrences) and the hippocampus (a part of the brain known to be important in forming memories).

If these brain systems concur that there is a threatening stimulus, the hypothalamus initiates a cascade of events that activates the sympathetic portion of the autonomic nervous system, and the fight-or-flight response begins within one or two seconds. Thus, the hypothalamus sends a chemical signal to the pituitary gland, causing it to release
adrenocorticotropic hormone (ACTH), and also activates the adrenal glands (a set of glands, one of which sits on top of each kidney). Before a person can even begin cognitively processing what has happened, the adrenal medulla, the center of the adrenal gland, has begun to secrete epinephrine (also known as adrenaline), norepinephrine, and steroid stress hormones called glucocorticoids. The substances secreted by the pituitary and adrenal medulla are distributed systemically, which means they are blood-borne. They take longer to reach their destinations than messages traveling along neural pathways, which explains why people can slam on their brakes at the sight of an oncoming car within seconds and then notice that they are shaking and their heart is pounding minutes later.

Pathological Manifestations

Although the fight-or-flight response is an appropriate reaction to many threatening stimuli, there are times when people report all the symptoms of the fight-or-flight response when there are no threatening stimuli. People who describe panic attacks
will often describe all the physiological signs of fight-or-flight, including racing heart, dry mouth, disturbed bowel function, and increased breathing (sometimes even hyperventilation). Panic attacks can be unnerving. People who experience them are usually unaware of what is causing the feelings they are experiencing and may think they are having a heart attack. Evidence suggests that panic attacks have a genetic component, with family history suggesting it is carried on a single dominant gene.

The fight-or-flight response may be considered to correspond to the first, or alarm, phase of the general adaptation syndrome
described by Hans Selye in 1956. When the fight-or-flight response (or alarm phase) is not quickly resolved so that the body is returned to homeostasis, with the parasympathetic division of the autonomic nervous system regaining control, the body then progresses into the second, or resistance, phase and then on to exhaustion. The long-term release of corticosteroids can be harmful to the body because every cell in the body has receptors for them. This is good in the short run, during the fight-or-flight or alarm phase, because these hormones help the body to use glucose for energy more efficiently, among other effects. However, continued production of corticosteroids has been associated with disruption of the immune system, high blood pressure, and other cardiovascular illnesses.

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