When we laugh or run for sport which autonomic system takes place in this time?
While I am not sure what happens in heart while laughing (tachycardia or bradycardia), but for sport running I'm sure that the heart has tachycardia. Does it mean that the sympathetic system works there (or there isn't relation between them)? If it does, then I really don't understand why being anger cause damage to the cardiovascular while sport running said to be healthy for the cardiovascular system, here are both activate the sympathetic system. Isn't it?
How Your Lungs Work
You don't have to think about breathing because your body's autonomic nervous system controls it, as it does many other functions in your body. If you try to hold your breath, your body will override your action and force you to let out that breath and start breathing again. The respiratory centers that control your rate of breathing are in the brainstem or medulla. The nerve cells that live within these centers automatically send signals to the diaphragm and intercostal muscles to contract and relax at regular intervals. However, the activity of the respiratory centers can be influenced by these factors:
- Oxygen: Specialized nerve cells within the aorta and carotid arteries called peripheral chemoreceptors monitor the oxygen concentration of the blood and feed back on the respiratory centers. If the oxygen concentration in the blood decreases, they tell the respiratory centers to increase the rate and depth of breathing.
- Carbon dioxide: Peripheral chemoreceptors also monitor the carbon dioxide concentration in the blood. In addition, a central chemoreceptor in the medulla monitors the carbon dioxide concentration in the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord carbon dioxide diffuses easily into the CSF from the blood. If the carbon dioxide concentration gets too high, then both types of chemoreceptors signal the respiratory centers to increase the rate and depth of breathing. The increased rate of breathing returns the carbon dioxide concentration to normal and the breathing rate then slows down.
- Hydrogen ion (pH): The peripheral and central chemoreceptors are also sensitive to the pH of the blood and CSF. If the hydrogen ion concentration increases (that is, if the fluid becomes more acidic), then the chemoreceptors tell the respiratory centers to speed up. Hydrogen ion concentration is heavily influenced by the carbon dioxide concentration and bicarbonate concentration in the blood and CSF.
- Stretch: Stretch receptors in the lungs and chest wall monitor the amount of stretch in these organs. If the lungs become over-inflated (stretch too much), they signal the respiratory centers to exhale and inhibit inspiration. This mechanism prevents damage to the lungs that would be caused by over-inflation.
- Signals from higher brain centers: Nerve cells in the hypothalamus and cortex also influence the activity of the respiratory centers. During pain or strong emotions, the hypothalamus will tell the respiratory centers to speed up. Nerve centers in the cortex can voluntarily tell the respiratory center to speed up, slow down or even stop (holding your breath). Their influence, however, can be overridden by chemical factors (oxygen, carbon dioxide, pH).
- Chemical irritants: Nerve cells in the airways sense the presence of unwanted substances in the airways such as pollen, dust, noxious fumes, water, or cigarette smoke. These cells then signal the respiratory centers to contract the respiratory muscles, causing you to sneeze or cough. Coughing and sneezing cause air to be rapidly and violently exhaled from the lungs and airways, removing the offending substance.
Of these factors, the strongest influence is the carbon dioxide concentration in your blood and CSF followed by the oxygen concentration.
Sometimes the respiratory centers go temporarily awry and send extra impulses to the diaphragm. These impulses cause unwanted contractions (hiccups). The same thing happens in unborn children many pregnant women often feel their babies hiccup. This happens because the respiratory centers of the developing child's brain are working just like those of an adult even though they are not yet breathing air.
Straight Talk From the Performance Director
Your autonomic nervous system works in two states, sympathetic and parasympathetic. When performing intense physical activity, the sympathetic nervous system takes over, this is known as a fight or flight response. When relaxed, the parasympathetic nervous system takes over. This is known as rest and digest.
This all seems pretty basic—something that just happens—and for the most part it does. However, there are ways we can control it to get better results. Getting the adrenaline rushing by playing your favorite song, consuming caffeine, or getting a back slap my help stimulate the sympathetic nervous system to help get an extra few pounds on a squat or get some emotion going for a big game makes sense. Being at a heightened level of alertness when peak performance is needed is somewhat of a given. So why do we care about getting into a sympathetic state when we are “chillin”? The reason is recovery. Our bodies do not recover when in a sympathetic state. We may be able to lift more, run faster and make quicker in-game decisions while we are in a fight or flight mode, but we do not get good rest or properly digest nutrients when we are in this state.
After competition or training it is ideal to quickly return to a relaxed state. This will help you recover and be prepared for the next practice, lift or game. This will help your body adapt to the stimulus it just went through and perform even better the next time around. Below is a list of tricks to return to a relaxed state after training.
Heart Rate Response to Exercise
The primary purpose of the cardiorespiratory system is to deliver adequate amounts of oxygen and remove waste from body tissues . The purpose of cardiovascular regulation is maintaining adequate blood flow to all body tissues. In addition, the circulatory system transports nutrients and aids in temperature regulation. During exercise, the demand for oxygen to the muscles is 15 to 25 times greater than at rest. The heart cannot accomplish this by itself, and does not work in isolation. The respiratory system and the circulatory system function together as a “coupled unit” delivering the body’s oxygen and nutrients and taking away carbon dioxide and wastes to maintain homeostasis.
During exercise, there is an increase in oxygen demand on body tissues and many things happen in the body such as an increase in blood pressure, heart rate, and respiratory rate. To meet the demand for oxygen, two major adjustments of blood flow are made, and increase in the amount of blood being pumped per minute by the heart or the cardiac output, and a redistribution of blood flow from inactive organs to the active skeletal muscle.
The heart has an electrical conduction system makes of two nodes (special conduction cells) and a series of conduction pathways. The heart begins beating with an electrical impulse from the sinoatrial (SA) node. The SA node is the pacemaker of the heart, responsible for setting rate and rhythm and is located in the wall of the right atrium. The impulse spreads through the walls of the atria, causing them to contract. Then, the impulse moves through the atrioventricular (AV) node (a relay station) located at the junction between the atria and ventricles. As the impulse travels down the bundles, the ventricles contract and the cycles repeats itself, this cycle of atrial and ventricular contractions pumps blood of the heart to the rest of the body.
Resting and exercise heart rate are controlled by the sympathetic and parasympathetic nervous system. The sympathetic division of the autonomic nervous system prepares the body for physical activity by increasing heart rate, blood pressure and respiration. The sympathetic division also stimulates the release of glucose from the liver for energy. Once exercise begins, the sympathetic nervous system is activated and the heart rate rises quickly. Heart rate also rises by simply thinking about exercise, which is referred to as anticipatory heart rate response.
The parasympathetic division helps to slow down heart rate and respiration. At rest, the heart is controlled by the parasympathetic division, which is why the average resting heart rate is 60 beats per minute or less. One of the explanations of why endurance athletes have such a low resting heart rate following training is due to increased parasympathetic response. During exercise, the release of epinephrine and norepinephrine stimulate receptors in the heart which causes heart rate to increase.
98% of People Can't Ace This Human Biology Quiz. Can You?
You know that the toe bone's connected to the foot bone. And the foot bone's connected to the -- which? Right, the heel bone. And so on..
Here's something about your body you may not know: ever wonder why you can't sneeze with your eyes open? (Admit it we've all tried.) Well, it turns out you can, it's just that your body automatically blinks for you -- blinking keeps the germs from your sneeze from infecting your eyes. (And no, your eyeballs won't pop out if your eyelids are open.)
Here's another you may not know, as well: that your human body isn't just made up of human cells. What else are you made of? Good question. Bacteria? Stardust? Alien cells? When a Dutch merchant named Antony van Leeuwenhoek looked through a homemade microscope and saw, for the first time, the microorganisms that share the world with us, he changed what humans had believed for centuries. Before van Leeuwenhoek's discovery, what you saw in the world around you was what you believed the world was -- and that didn't include microscopic single-celled organisms, such as the bacteria that causes the common cold or the virus responsible for influenza, or the millions of bacteria that make us who we are.
Biology is, to put it most basically, the study of life and its basic principles help us understand our human physiology -- and the molecular basis for human life as well as human disease. That's everything that makes up you -- and that includes your vital organs, which are essential (vital) for your survival as well as your your biological systems, which carry out all the functions that need to be done to keep you alive. It's called homeostasis, the complex balance and regulation of all the systems of the body, and it's needed for your human machine to run properly. But how much do you know about those organs and systems? Let's find out how much you know about how your body works.
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Norepinephrine, also called noradrenaline, substance that is released predominantly from the ends of sympathetic nerve fibres and that acts to increase the force of skeletal muscle contraction and the rate and force of contraction of the heart. The actions of norepinephrine are vital to the fight-or-flight response, whereby the body prepares to react to or retreat from an acute threat.
Norepinephrine is classified structurally as a catecholamine—it contains a catechol group (a benzene ring with two hydroxyl groups) bound to an amine (nitrogen-containing) group. The addition of a methyl group to the amine group of norepinephrine results in the formation of epinephrine, the other major mediator of the flight-or-flight response. Relative to epinephrine, which is produced and stored primarily in the adrenal glands, norepinephrine is stored in small amounts in adrenal tissue. Its major site of storage and release are the neurons of the sympathetic nervous system (a branch of the autonomic nervous system). Thus, norepinephrine functions mainly as a neurotransmitter with some function as a hormone (being released into the bloodstream from the adrenal glands).
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Fight-or-flight response, response to an acute threat to survival that is marked by physical changes, including nervous and endocrine changes, that prepare a human or an animal to react or to retreat. The functions of this response were first described in the early 1900s by American neurologist and physiologist Walter Bradford Cannon.
When a threat is perceived, the sympathetic nerve fibres of the autonomic nervous system are activated. This leads to the release of certain hormones from the endocrine system. In physiological terms, a major action of these hormones is to initiate a rapid, generalized response. This response may be triggered by a fall in blood pressure or by pain, physical injury, abrupt emotional upset, or decreased blood glucose levels (hypoglycemia). The fight-or-flight response is characterized by an increased heart rate (tachycardia), anxiety, increased perspiration, tremour, and increased blood glucose concentrations (due to glycogenolysis, or breakdown of liver glycogen). These actions occur in concert with other neural or hormonal responses to stress, such as increases in corticotropin and cortisol secretion, and they are observed in some humans and animals affected by chronic stress, which causes long-term stimulation of the fight-or-flight response.
In addition to increased secretion of cortisol by the adrenal cortex, activation of the fight-or-flight response causes increased secretion of glucagon by the islet cells of the pancreas and increased secretion of catecholamines (i.e., epinephrine and norepinephrine) by the adrenal medulla. The tissue responses to different catecholamines depend on the fact that there are two major types of adrenergic receptors (adrenoceptors) on the surface of target organs and tissues. The receptors are known as alpha-adrenergic and beta-adrenergic receptors, or alpha receptors and beta receptors, respectively (see human nervous system: Anatomy of the human nervous system). In general, activation of alpha-adrenergic receptors results in the constriction of blood vessels, contraction of uterine muscles, relaxation of intestinal muscles, and dilation of the pupils. Activation of beta-receptors increases heart rate and stimulates cardiac contraction (thereby increasing cardiac output), dilates the bronchi (thereby increasing air flow into and out of the lungs), dilates the blood vessels, and relaxes the uterus.
The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.
Reticular Formation Pathways
Reticular formation pathways are split according to sensory and motor pathways (ARAS and DRS) and according to whether a nerve fiber or group of fibers enters or exits this part of the brainstem – in other words, whether the RF receives or transmits information. Connections bring messages to the reticular formation from the spinal cord and brain. Efferent pathways bring messages from the reticular formation directly or indirectly to other structures. Complex and simpler networks use the reticular formation as a central control or relay base.
Reticular Formation Afferent Pathways
When the reticular formation receives information from other regions, the routes these messages follow are afferent pathways. Messages travel via synapses from the spinal cord to the RF. These multiple, sensory pathways send us information about pain, temperature, crude touch, fine touch, vibration, and proprioception – the position and movement of our body.
Afferent pathways also arrive from the brain and cranial nerves. These bring information to the RF corresponding with eye movement, sounds, proprioception, and the presence of darkness and light that will, after being relayed through the RF, synchronize our sleep and wake patterns. A rather cruel study on cats in the late 1960s showed that the reticular formation has a lot of influence on how visual information accesses the brain.
Other cranial nerve and brain to RF pathways connect sounds to arousal, regulate hormone secretion, and adjust our levels of consciousness. When your alarm clock wakes you up in the morning, your ARAS is quickly stimulated through sound and your DRS opens your eyes and helps you to show that clock exactly what you think of it.
Reticular Formation Efferent Pathways
Efferent connections send information to other structures rather than receive it. In this case, efferent reticular tracts run out of the RF to the spinal cord or other regions of the brain – the cranial nerves, cerebellum, thalamus, and hypothalamus, for example. This information can be used to cause a response. Responses regulated via the RF are cognitive, sleep-wake, endocrine, emotional, and motor responses. The psychology definition of reticular formation function speaks of it being a regulatory center for sleep, alertness, fatigue, reward, and even different personality traits. The majority of responses to our inner and outer environments travel through the RF.
The autonomic nervous system is divided into the sympathetic nervous system and parasympathetic nervous system. The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3. The parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves (specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve and vagus nerve) and sacral (S2-S4) spinal cord.
The autonomic nervous system is unique in that it requires a sequential two-neuron efferent pathway the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will begin at the “outflow” and will synapse at the postganglionic, or second, neuron's cell body. The postganglionic neuron will then synapse at the target organ.
Sympathetic division Edit
The sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" (general visceral efferent) neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:
- (3) of the sympathetic chain (these run on either side of the vertebral bodies)
- (3) (12) and rostral lumbar ganglia (2 or 3)
- caudal lumbar ganglia and sacral ganglia
- (celiac ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior mesenteric ganglion) of the adrenal medulla (this is the one exception to the two-neuron pathway rule: the synapse is directly efferent onto the target cell bodies)
These ganglia provide the postganglionic neurons from which innervation of target organs follows. Examples of splanchnic (visceral) nerves are:
- Cervical cardiac nerves and thoracic visceral nerves, which synapse in the sympathetic chain (greater, lesser, least), which synapse in the prevertebral ganglia , which synapse in the prevertebral ganglia , which synapse in the inferior hypogastric plexus
These all contain afferent (sensory) nerves as well, known as GVA (general visceral afferent) neurons.
Parasympathetic division Edit
The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem (Cranial Nerves III, VII, IX, X) or the sacral spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:
- of the head: Ciliary (Cranial nerve III), Submandibular (Cranial nerve VII), Pterygopalatine (Cranial nerve VII), and Otic (Cranial nerve IX)
- In or near the wall of an organ innervated by the Vagus (Cranial nerve X) or Sacral nerves (S2, S3, S4)
These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are:
- The postganglionic parasympathetic splanchnic (visceral) nerves
- The vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach
Sensory neurons Edit
The sensory arm is composed of primary visceral sensory neurons found in the peripheral nervous system (PNS), in cranial sensory ganglia: the geniculate, petrosal and nodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach and gut content. They also convey the sense of taste and smell, which, unlike most functions of the ANS, is a conscious perception. Blood oxygen and carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal (IXth) ganglion. Primary sensory neurons project (synapse) onto “second order” visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract (nTS), that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion (the memory that ensures that an animal that has been poisoned by a food never touches it again). All this visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the ANS.
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the vagus nerve and sympathetic supply by splanchnic nerves. The sensory part of the latter reaches the spinal column at certain spinal segments. Pain in any internal organ is perceived as referred pain, more specifically as pain from the dermatome corresponding to the spinal segment. 
- and posterior vagal trunks
- PS: vagus nerves
- S: greater splanchnic nerves
- PS: posterior vagal trunks
- S: greater splanchnic nerves
- S: greater splanchnic nerves
- PS: vagus nerve
- S: celiac plexus
- right phrenic nerve
- PS: vagus nerves and pelvic splanchnic nerves
- S: lesser and least splanchnic nerves
- , T11, T12 (proximal colon) , L2, L3, (distal colon)
- PS: vagus nerves
- S: thoracic splanchnic nerves
- nerves to superior mesenteric plexus
- PS: vagus nerve
- S: thoracic and lumbar splanchnic nerves
Motor neurons Edit
Motor neurons of the autonomic nervous system are found in ‘’autonomic ganglia’’. Those of the parasympathetic branch are located close to the target organ whilst the ganglia of the sympathetic branch are located close to the spinal cord.
The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains. The activity of autonomic ganglionic neurons is modulated by “preganglionic neurons” located in the central nervous system. Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels. Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei the dorsal motor nucleus of the vagus nerve the nucleus ambiguus, the salivatory nuclei, and in the sacral region of the spinal cord.
Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic system is often considered the "fight or flight" system, while the parasympathetic system is often considered the "rest and digest" or "feed and breed" system.
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second-to-second, modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. In general, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. Higher organisms maintain their integrity via homeostasis which relies on negative feedback regulation which, in turn, typically depends on the autonomic nervous system.  Some typical actions of the sympathetic and parasympathetic nervous systems are listed below. 
|Digestive system||Increase peristalsis and amount of secretion by digestive glands||Decrease activity of digestive system|
|Liver||No effect||Causes glucose to be released to blood|
|Lungs||Constricts bronchioles||Dilates bronchioles|
|Urinary bladder/ Urethra||Relaxes sphincter||Constricts sphincter|
|Kidneys||No effects||Decrease urine output|
|Heart||Decreases rate||Increase rate|
|Blood vessels||No effect on most blood vessels||Constricts blood vessels in viscera increase BP|
|Salivary and Lacrimal glands||Stimulates increases production of saliva and tears||Inhibits result in dry mouth and dry eyes|
|Eye (iris)||Stimulates constrictor muscles constrict pupils||Stimulate dilator muscle dilates pupils|
|Eye (ciliary muscles)||Stimulates to increase bulging of lens for close vision||Inhibits decrease bulging of lens prepares for distant vision|
|Adrenal Medulla||No effect||Stimulate medulla cells to secrete epinephrine and norepinephrine|
|Sweat gland of skin||No effect||Stimulate to produce perspiration|
Sympathetic nervous system Edit
Promotes a fight-or-flight response, corresponds with arousal and energy generation, and inhibits digestion
- Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction
- Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1200% in the case of skeletal muscles)
- Dilates bronchioles of the lung through circulating epinephrine, which allows for greater alveolar oxygen exchange
- Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for enhanced blood flow to skeletal muscles
- Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye and enhances far vision
- Provides vasodilation for the coronary vessels of the heart
- Constricts all the intestinal sphincters and the urinary sphincter
- Inhibits peristalsis
- Stimulates orgasm
Parasympathetic nervous system Edit
The parasympathetic nervous system has been said to promote a "rest and digest" response, promotes calming of the nerves return to regular function, and enhancing digestion. Functions of nerves within the parasympathetic nervous system include: [ citation needed ]
- Dilating blood vessels leading to the GI tract, increasing the blood flow.
- Constricting the bronchiolar diameter when the need for oxygen has diminished
- Dedicated cardiac branches of the vagus and thoracic spinal accessory nerves impart parasympathetic control of the heart (myocardium)
- Constriction of the pupil and contraction of the ciliary muscles, facilitating accommodation and allowing for closer vision
- Stimulating salivary gland secretion, and accelerates peristalsis, mediating digestion of food and, indirectly, the absorption of nutrients
- Sexual. Nerves of the peripheral nervous system are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal.
Enteric nervous system Edit
The enteric nervous system is the intrinsic nervous system of the gastrointestinal system. It has been described as "the Second Brain of the Human Body".  Its functions include:
- Sensing chemical and mechanical changes in the gut
- Regulating secretions in the gut
- Controlling peristalsis and some other movements
At the effector organs, sympathetic ganglionic neurons release noradrenaline (norepinephrine), along with other cotransmitters such as ATP, to act on adrenergic receptors, with the exception of the sweat glands and the adrenal medulla:
- is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons. Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors.
- At the adrenal medulla, there is no postsynaptic neuron. Instead the presynaptic neuron releases acetylcholine to act on nicotinic receptors. Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which acts on adrenoceptors, thereby indirectly mediating or mimicking sympathetic activity.
The specialised system of the autonomic nervous system was recognised by Galen. In 1665, Willis used the terminology, and in 1900, Langley used the term, defining the two divisions as the sympathetic and parasympathetic nervous systems. 
Caffeine is a bioactive ingredient found in commonly consumed beverages such as coffee, tea, and sodas. Short-term physiological effects of caffeine include increased blood pressure and sympathetic nerve outflow. Habitual consumption of caffeine may inhibit physiological short-term effects. Consumption of caffeinated espresso increases parasympathetic activity in habitual caffeine consumers however, decaffeinated espresso inhibits parasympathetic activity in habitual caffeine consumers. It is possible that other bioactive ingredients in decaffeinated espresso may also contribute to the inhibition of parasympathetic activity in habitual caffeine consumers. 
Caffeine is capable of increasing work capacity while individuals perform strenuous tasks. In one study, caffeine provoked a greater maximum heart rate while a strenuous task was being performed compared to a placebo. This tendency is likely due to caffeine's ability to increase sympathetic nerve outflow. Furthermore, this study found that recovery after intense exercise was slower when caffeine was consumed prior to exercise. This finding is indicative of caffeine's tendency to inhibit parasympathetic activity in non-habitual consumers. The caffeine-stimulated increase in nerve activity is likely to evoke other physiological effects as the body attempts to maintain homeostasis. 
The effects of caffeine on parasympathetic activity may vary depending on the position of the individual when autonomic responses are measured. One study found that the seated position inhibited autonomic activity after caffeine consumption (75 mg) however, parasympathetic activity increased in the supine position. This finding may explain why some habitual caffeine consumers (75 mg or less) do not experience short-term effects of caffeine if their routine requires many hours in a seated position. It is important to note that the data supporting increased parasympathetic activity in the supine position was derived from an experiment involving participants between the ages of 25 and 30 who were considered healthy and sedentary. Caffeine may influence autonomic activity differently for individuals who are more active or elderly. 
Why It's Important
The fight-or-flight response plays a critical role in how we deal with stress and danger in our environment. When we are under threat, the response prepares the body to either fight or flee.
The fight-or-flight response can be triggered by both real and imaginary threats.
By priming your body for action, you are better prepared to perform under pressure. The stress created by the situation can actually be helpful, making it more likely that you will cope effectively with the threat.
This type of stress can help you perform better in situations where you are under pressure to do well, such as at work or school. And in cases where the threat is life-threatening, the fight-or-flight response plays a critical role in your survival. By gearing you up to fight or flee, the fight-or-flight response makes it more likely that you will survive the danger.
While the fight-or-flight response happens automatically, that doesn't mean that it is always accurate. Sometimes we respond in this way even when there is no real threat. Phobias are good examples of how the fight-or-flight response might be falsely triggered in the face of a perceived threat.
A person who is terrified of heights might experience an acute stress response if they have to go to the top floor of a skyscraper to attend a meeting. Their body might go on high alert, with their heartbeat and respiration rate increasing. If the response is severe, it can lead to a panic attack.
Understanding the body's natural fight-or-flight response is one way to help cope with such situations. When you notice that you are becoming tense, you can start looking for ways to calm down and relax your body.
The stress response is one of the major topics studied in the rapidly-growing field of health psychology. Health psychologists are interested in helping people find ways to combat stress and live healthier, more productive lives. By learning more about the fight-or-flight response, psychologists can help people explore new ways to deal with their natural reaction to stress.