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The fight-or-flight-or-freeze or the fight-flight response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event,
attack Attack may refer to: Warfare and combat * Offensive (military) * Charge (warfare) * Attack (fencing) * Strike (attack) * Attack (computing) * Attack aircraft Books and publishing * The Attack (novel), ''The Attack'' (novel), a book * ''Attack No. ...
, or threat to survival. It was first described by
Walter Bradford Cannon Walter Bradford Cannon (October 19, 1871 – October 1, 1945) was an American physiologist, professor and chairman of the Department of Physiology at Harvard Medical School. He coined the term " fight or flight response", and he expanded on Clau ...

Walter Bradford Cannon
. His theory states that animals react to threats with a general discharge of the
sympathetic nervous system The sympathetic nervous system (SNS) is one of two divisions of the autonomic nervous system, along with the parasympathetic nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes co ...
, preparing the animal for fighting or fleeing. More specifically, the
adrenal medulla The adrenal medulla ( la, medulla glandulae suprarenalis) is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of chromaffin cel ...
produces a hormonal cascade that results in the secretion of
catecholamines (noradrenaline) Image:Adrenalin - Adrenaline.svg">140px, epinephrine (adrenaline) A catecholamine (; abbreviated CA) is a monoamine neurotransmitter, an organic compound that has a catechol">organic_compound.html" ;"title="monoamine neurotra ...
, especially
norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all ...

norepinephrine
and
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate p ...

epinephrine
. The hormones
estrogen Estrogens or oestrogens, are a class of natural or synthetic sex hormone Sex hormones, also known as sex steroids, gonadocorticoids and gonadal steroids, are steroid hormone A steroid hormone is a steroid that acts as a hormone. Steroid hor ...

estrogen
,
testosterone Testosterone is the primary sex hormone and anabolic steroid Anabolic steroids, also known more properly as anabolic–androgenic steroids (AAS), are steroidal androgens that include natural androgens like testosterone (medication), testoste ...
, and
cortisol Cortisol is a steroid hormone A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically ma ...
, as well as the neurotransmitters
dopamine Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic compound, organic chemical of the catecholamine and phenethylamine families. Dopamine consti ...

dopamine
and
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vaso ...

serotonin
, also affect how organisms react to stress. The hormone
osteocalcin Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein (BGLAP), is a small (49-amino-acid) noncollagenous protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino a ...
might also play a part. This response is recognised as the first stage of the
general adaptation syndrome
general adaptation syndrome
that regulates stress responses among
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s and other
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
s.


Name

Originally understood as the fight-or-flight response in Cannon's research, the state of hyperarousal results in several responses beyond fighting or fleeing. This has led people to calling it the fight, flight, freeze response (or fight-flight-faint-or-freeze, amongst other variants). The wider array of responses, such as
freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid or ...
, fainting, feeding, or experiencing fright, has led researchers to use more neutral or accommodating terminology such as hyperarousal or the acute stress response.


Physiology


Autonomic nervous system

The autonomic nervous system is a control system that acts largely unconsciously and regulates
heart rateHeart rate is the speed of the heartbeat measured by the number of contractions (beats) of the heart The heart is a muscle, muscular Organ (anatomy), organ in most animals, which pumps blood through the blood vessels of the circulatory syste ...

heart rate
,
digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such as ...
,
respiratory rate The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are ...
,
pupillary response Pupillary response is a physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, includin ...
,
urination Urination is the release of urine from the urinary bladder through the urethra to the outside of the body. It is the urinary system's form of excretion. It is also known medically as micturition, voiding, uresis, or, rarely, emiction, and kno ...
, and
sexual arousal Sexual arousal (also sexual excitement) is typically the arousal Arousal is the physiological and psychological Psychology is the science of mind and behavior. Psychology includes the study of consciousness, conscious and Unconscio ...
. This system is the primary mechanism in control of the fight-or-flight response and its role is mediated by two different components: the sympathetic nervous system and the parasympathetic nervous system.


Sympathetic nervous system

The sympathetic nervous system originates in the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which co ...

spinal cord
and its main function is to activate the physiological changes that occur during the fight-or-flight response. This component of the autonomic nervous system utilises and activates the release of
norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all ...

norepinephrine
in the reaction.


Parasympathetic nervous system

The parasympathetic nervous system originates in the sacral spinal cord and
medulla Medulla or Medullary may refer to: Science * Medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is ...

medulla
, physically surrounding the sympathetic origin, and works in concert with the sympathetic nervous system. Its main function is to activate the "rest and digest" response and return the body to
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
after the fight or flight response. This system utilises and activates the release of the neurotransmitter
acetylcholine Acetylcholine (ACh) is an organic chemical , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mo ...

acetylcholine
.


Reaction

The reaction begins in the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, kno ...

amygdala
, which triggers a neural response in the
hypothalamus The hypothalamus (from Ancient Greek wikt:ὑπό, ὑπό, "under", and wikt:θάλαμος, θάλαμος, "chamber") is a portion of the brain that contains a number of small Nucleus (neuroanatomy), nuclei with a variety of functions. One of ...

hypothalamus
. The initial reaction is followed by activation of the
pituitary gland upright=1.2, The Hypothalamus-Pituitary Complex. In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingd ...
and secretion of the hormone
ACTH Adrenocorticotropic hormone (ACTH; also adrenocorticotropin, corticotropin) is a polypeptide tropic hormone produced by and secreted by the Anterior pituitary, anterior pituitary gland. It is also used as a Adrenocorticotropic hormone (medicati ...
. The
adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal cortex ...
is activated almost simultaneously, via the sympathetic nervous system, and releases the hormone
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate p ...

epinephrine
. The release of chemical messengers results in the production of the hormone
cortisol Cortisol is a steroid hormone A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically ma ...
, which increases
blood pressure Blood pressure (BP) is the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , ...
,
blood sugar The blood sugar level, blood sugar concentration, or blood glucose level is the concentration of glucose present in the blood of humans and other animals. Glucose is a simple sugar, and approximately 4 g of glucose are present in the blood of a 70 ...
, and suppresses the
immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biolog ...
. The initial response and subsequent reactions are triggered in an effort to create a boost of energy. This boost of energy is activated by epinephrine binding to
liver cells The liver is an organ only found in vertebrates which detoxifies various metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both ...
and the subsequent production of
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
. Additionally, the circulation of cortisol functions to turn
fatty acids fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical co ...
into available energy, which prepares muscles throughout the body for response. Catecholamine hormones, such as
adrenaline Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate p ...

adrenaline
(
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, that are transported to distant organs to regulate p ...

epinephrine
) or
noradrenaline Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as a hormone A hormone (from the Greek participle , "setting in motion") is any m ...

noradrenaline
(norepinephrine), facilitate immediate physical reactions associated with a preparation for violent
muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the Uni ...

muscular
action and: *
Acceleration In mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, more specifically the relationships among force, matter, and motion. Forces applied to objects result in Displacement ( ...
of
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...
and
lung The lungs are the primary organs of the respiratory system in human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowi ...
action * Paling or
flushing Flushing may refer to: Places * Flushing, Cornwall, a village in the United Kingdom * Flushing, Queens, New York City ** Flushing Bay, a bay off the north shore of Queens ** Flushing Chinatown (法拉盛華埠), a community in Queens ** Flushing M ...
, or alternating between both * Inhibition of
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital digestive organ. In the digestive system ...

stomach
and action to the point where
digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such as ...
slows down or stops * General effect on the
sphincters A sphincter is a circular muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochem ...
of the body *
Constriction Constriction is a method used by various snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote Amniotes (from Greek ἀμνίον ''amnion'', ...

Constriction
of
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
s in many parts of the body *
Liberation Liberation or liberate may refer to: Concepts *Enlightenment (spiritual) **''Moksha'', the concept of salvation within Indian religions **''Nirvana'', a closely related term *Emancipation *Sexual liberation *Women's liberation or feminism *Libera ...

Liberation
of metabolic energy sources (particularly
fat In nutrition Nutrition is the biochemical and physiological process by which an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies ...
and
glycogen A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysacchar ...

glycogen
) for muscular action * of blood vessels for muscles * Inhibition of the
lacrimal gland The lacrimal glands are paired exocrine gland Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct (anatomy), duct. Examples of exocrine glands include sweat gland, sweat, salivary, mammary, ce ...
(responsible for production) and
salivation Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is 98% water plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), ...
* Dilation of pupil (
mydriasis Mydriasis is the dilation of the pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology'' ...

mydriasis
) * Relaxation of
bladder The urinary bladder, or simply bladder, is a hollow muscular organ in humans and other vertebrates that stores urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ure ...
* Inhibition of
erection An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular, ...

erection
*
Auditory exclusion Auditory means of or relating to the process of hearing: * Auditory system, the neurological structures and pathways of sound perception ** Auditory bulla, part of auditory system found in mammals other than primates ** Auditory nerve, also known a ...
( loss of hearing) *
Tunnel vision Tunnel vision is the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entranc ...

Tunnel vision
(loss of
peripheral vision Peripheral vision, or ''indirect vision'', is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in (or out of) the "corner of one's eye". The vast majority of the area in the ...

peripheral vision
) * Disinhibition of spinal
reflex In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

reflex
es *
Shaking Shake may refer to: * Handshake * Milkshake * Tremor * Shakes (wood), cracks in timber * Shake (shingle), a wooden shingle made from split logs Shake, The Shakes, Shaking, or Shakin' may refer to: Geography * Shake, Zimbabwe * Shake, another nam ...


Function of physiological changes

The physiological changes that occur during the fight or flight response are activated in order to give the body increased strength and speed in anticipation of fighting or running. Some of the specific physiological changes and their functions include: * Increased
blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory system is controlled by homeostasis, homeostatic mechanisms of autoregulation, just as hydraul ...
to the muscles activated by diverting blood flow from other parts of the body. * Increased blood pressure, heart rate, blood sugars, and fats in order to supply the body with extra energy. * The
blood clotting Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechani ...
function of the body speeds up in order to prevent excessive blood loss in the event of an injury sustained during the response. * Increased muscle tension in order to provide the body with extra speed and strength.


Emotional components


Emotion regulation

In the context of the fight or flight response, emotional regulation is used proactively to avoid threats of stress or to control the level of emotional arousal.


Emotional reactivity

During the reaction, the intensity of emotion that is brought on by the stimulus will also determine the nature and intensity of the behavioral response. Individuals with higher levels of emotional reactivity may be prone to
anxiety Anxiety is an emotion Emotions are biological states associated with all of the nerve systems brought on by neurophysiological changes variously associated with thoughts, feelings, behavioural responses, and a degree of pleasure or suffering, ...
and
aggression Aggression is overt or covert, often harmful, social interaction with the intention of inflicting damage or other harm upon another individual. It may occur either reactively or without provocation. In humans, aggression can be caused by various ...
, which illustrates the implications of appropriate emotional reaction in the fight or flight response.


Cognitive components


Content specificity

The specific components of cognitions in the fight or flight response seem to be largely negative. These negative cognitions may be characterised by: attention to negative stimuli, the perception of ambiguous situations as negative, and the recurrence of recalling negative words. There also may be specific negative thoughts associated with emotions commonly seen in the reaction.


Perception of control

Perceived control relates to an individual's thoughts about control over situations and events. Perceived control should be differentiated from actual control because an individual's beliefs about their abilities may not reflect their actual abilities. Therefore, overestimation or underestimation of perceived control can lead to anxiety and aggression.


Social information processing

The social information processing model proposes a variety of factors that determine behavior in the context of social situations and preexisting thoughts. The attribution of hostility, especially in ambiguous situations, seems to be one of the most important cognitive factors associated with the fight or flight response because of its implications towards aggression.


Other animals


Evolutionary perspective

An
evolutionary psychology Evolutionary psychology is a theoretical approach in the social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchang ...
explanation is that early animals had to react to threatening stimuli quickly and did not have time to psychologically and physically prepare themselves. The fight or flight response provided them with the mechanisms to rapidly respond to threats against survival.


Examples

A typical example of the stress response is a grazing
zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three extant species: the Grévy's zebra The Grévy's zebra (''Equus grevyi''), also known as the imperial zebra, is the ...

zebra
. If the zebra sees a
lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, deep-chested body, short, rounded head, round ears, and a hairy tuft at the end of its tail. It is sexually dimorphic; ...

lion
closing in for the kill, the stress response is activated as a means to escape its
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (which ...
. The escape requires intense muscular effort, supported by all of the body's systems. The
sympathetic nervous system The sympathetic nervous system (SNS) is one of two divisions of the autonomic nervous system, along with the parasympathetic nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes co ...
’s activation provides for these needs. A similar example involving fight is of a cat about to be attacked by a dog. The cat shows accelerated heartbeat, piloerection (hair standing on end), and pupil dilation, all signs of sympathetic arousal. Note that the zebra and cat still maintain
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
in all states. In July 1992, ''
Behavioral Ecology Behavioral ecology, also spelled behavioural ecology, is the study of the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristic ...
'' published experimental research conducted by biologist Lee A. Dugatkin where
guppies The guppy (), also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish Tropical fish are generally those fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Lim ...

guppies
were sorted into "bold", "ordinary", and "timid" groups based upon their reactions when confronted by a
smallmouth bass The smallmouth bass (''Micropterus dolomieu'') is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often define ...

smallmouth bass
(i.e. inspecting the predator, hiding, or swimming away) after which the guppies were left in a tank with the bass. After 60 hours, 40 percent of the timid guppies and 15 percent of the ordinary guppies survived while none of the bold guppies did.


Varieties of responses

Animals respond to threats in many complex ways. Rats, for instance, try to escape when threatened but will fight when cornered. Some animals stand perfectly still so that predators will not see them. Many animals freeze or play dead when touched in the hope that the predator will lose interest. Other animals have alternative self-protection methods. Some species of cold-blooded animals change color swiftly to camouflage themselves. These responses are triggered by the
sympathetic nervous system The sympathetic nervous system (SNS) is one of two divisions of the autonomic nervous system, along with the parasympathetic nervous system. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes co ...
, but, in order to fit the model of fight or flight, the idea of flight must be broadened to include escaping capture either in a physical or sensory way. Thus, flight can be disappearing to another location or just disappearing in place, and fight and flight are often combined in a given situation. The fight or flight actions also have polarity – the individual can either fight against or flee from something that is threatening, such as a hungry lion, or fight for or fly towards something that is needed, such as the safety of the shore from a raging river. A threat from another animal does not always result in immediate fight or flight. There may be a period of heightened awareness, during which each animal interprets behavioral signals from the other. Signs such as paling, piloerection, immobility, sounds, and body language communicate the status and intentions of each animal. There may be a sort of negotiation, after which fight or flight may ensue, but which might also result in playing, mating, or nothing at all. An example of this is kittens playing: each kitten shows the signs of sympathetic arousal, but they never inflict real damage.


See also


Notes


References


Further reading

* Sapolsky, Robert M., 1994. Why Zebras Don't Get Ulcers. W.H. Freeman and Company. *


External links

* {{DEFAULTSORT:Fight-Or-Flight Response Sympathetic nervous system Aggression Fear Dichotomies Psychological theories