How are the two divisions sympathetic and parasympathetic of the autonomic nervous system involved in emotions?

The parasympathetic nervous system (PNS) controls homeostasis and the body at rest and is responsible for the body's "rest and digest" function. The sympathetic nervous system (SNS) controls the body's responses to a perceived threat and is responsible for the "fight or flight" response.

The PNS and SNS are part of the autonomic nervous system (ANS), which is responsible for the involuntary functions of the human body.

	Parasympathetic nervous system	Sympathetic nervous system
Introduction	The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system (ANS). Its general function is to control homeostasis and the body's rest-and-digest response.	The sympathetic nervous system (SNS) is one of two main divisions of the autonomic nervous system (ANS). Its general action is to mobilize the body's fight-or-flight response.
Function	Control the body's response while at rest.	Control the body's response during perceived threat.
Originates in	Sacral region of spinal cord, medulla, cranial nerves 3, 7, 9, and 10	Thoracic and lumbar regions of spinal cord
Activates response of	Rest and digest	Fight-or-flight
Neuron Pathways	Longer pathways, slower system	Very short neurons, faster system
General Body Response	Counterbalance; restores body to state of calm.	Body speeds up, tenses up, becomes more alert. Functions not critical to survival shut down.
Cardiovascular System (heart rate)	Decreases heart rate	Increases contraction, heart rate
Pulmonary System (lungs)	Bronchial tubes constrict	Bronchial tubes dilate
Musculoskeletal System	Muscles relax	Muscles contract
Pupils	Constrict	Dilate
Gastrointestinal System	Increases stomach movement and secretions	Decreases stomach movement and secretions
Salivary Glands	Saliva production increases	Saliva production decreases
Adrenal Gland	No involvement	Releases adrenaline
Glycogen to Glucose Conversion	No involvement	Increases; converts glycogen to glucose for muscle energy
Urinary Response	Increase in urinary output	Decrease in urinary output
Neurotransmitters	neurons are cholinergic: acetylcholine	neurons are mostly adrenergic: epinephrine / norepinephrine (acetylcholine)

The autonomic nervous system (ANS) regulates visceral functions, i.e. functions of the internal organs such as the heart, stomach and intestines. The ANS is part of the peripheral nervous system and also has control over some muscles within the body. The functions of the ANS are involuntary and reflexive, e.g. the beating of the heart, expansion or contraction of blood vessels or pupils, etc. — which is why we are seldom conscious of it. The parasympathetic and sympathetic nervous systems, along with the enteric nervous system make up the ANS.

What is the parasympathetic nervous system?

The parasympathetic nervous system is part of the autonomic nervous system. It originates in the spinal cord and the medulla and controls homeostasis, or the maintenance of the body's systems. The parasympathetic nervous system controls the "rest and digest" functions of the body.

What is the sympathetic nervous system?

The sympathetic nervous system, also part of the autonomic nervous system, originates in the spinal cord; specifically in the thoracic and lumbar regions. It controls the body's "fight or flight" responses, or how the body reacts to perceived danger.

Sympathetic vs Parasympathetic Responses

With sympathetic nervous responses, the body speeds up, tenses up and becomes more alert. Functions that are not essential for survival are shut down. Following are the specific reactions of sympathetic nervous system:

• increase in the rate and constriction of the heart
• dilation of bronchial tubes in the lungs and pupils in the eyes
• contraction of muscles
• release of adrenaline from the adrenal gland
• conversion of glycogen to glucose to provide energy for the muscles.
• shut down of processes not critical for survival
• decrease in saliva production: the stomach does not move for digestion, nor does it release digestive secretions.
• decrease in urinary output
• sphincter contraction.

The parasympathetic nervous system counterbalances the sympathetic nervous system. It restores the body to a state of calm. The specific responses are:

• decrease in heart rate
• constriction of bronchial tubes in the lungs and pupils in the eyes
• relaxation of muscles
• saliva production: the stomach moves and increases secretions for digestion.
• increase in urinary output
• sphincter relaxation.

How it Works

The parasympathetic nervous system is a slower system and moves along longer pathways. Preganglionic fibers from the medulla or spinal cord project ganglia close to the target organ. They create a synapse, which eventually creates the desired response.

The sympathetic nervous system is a faster system as it moves along very short neurons. When the system is activated, it activates the adrenal medulla to release hormones and chemical receptors into the bloodstreams. The target glands and muscles get activated. Once the perceived danger is gone, the parasympathetic nervous system takes over to counterbalance the effects of the sympathetic nervous system's responses.

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A stressful situation — whether something environmental, such as a looming work deadline, or psychological, such as persistent worry about losing a job — can trigger a cascade of stress hormones that produce well-orchestrated physiological changes. A stressful incident can make the heart pound and breathing quicken. Muscles tense and beads of sweat appear.

This combination of reactions to stress is also known as the "fight-or-flight" response because it evolved as a survival mechanism, enabling people and other mammals to react quickly to life-threatening situations. The carefully orchestrated yet near-instantaneous sequence of hormonal changes and physiological responses helps someone to fight the threat off or flee to safety. Unfortunately, the body can also overreact to stressors that are not life-threatening, such as traffic jams, work pressure, and family difficulties.

Over the years, researchers have learned not only how and why these reactions occur, but have also gained insight into the long-term effects chronic stress has on physical and psychological health. Over time, repeated activation of the stress response takes a toll on the body. Research suggests that chronic stress contributes to high blood pressure, promotes the formation of artery-clogging deposits, and causes brain changes that may contribute to anxiety, depression, and addiction.. More preliminary research suggests that chronic stress may also contribute to obesity, both through direct mechanisms (causing people to eat more) or indirectly (decreasing sleep and exercise).

Sounding the alarm

The stress response begins in the brain (see illustration). When someone confronts an oncoming car or other danger, the eyes or ears (or both) send the information to the amygdala, an area of the brain that contributes to emotional processing. The amygdala interprets the images and sounds. When it perceives danger, it instantly sends a distress signal to the hypothalamus.

Command center

When someone experiences a stressful event, the amygdala, an area of the brain that contributes to emotional processing, sends a distress signal to the hypothalamus. This area of the brain functions like a command center, communicating with the rest of the body through the nervous system so that the person has the energy to fight or flee.

The hypothalamus is a bit like a command center. This area of the brain communicates with the rest of the body through the autonomic nervous system, which controls such involuntary body functions as breathing, blood pressure, heartbeat, and the dilation or constriction of key blood vessels and small airways in the lungs called bronchioles. The autonomic nervous system has two components, the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system functions like a gas pedal in a car. It triggers the fight-or-flight response, providing the body with a burst of energy so that it can respond to perceived dangers. The parasympathetic nervous system acts like a brake. It promotes the "rest and digest" response that calms the body down after the danger has passed.

After the amygdala sends a distress signal, the hypothalamus activates the sympathetic nervous system by sending signals through the autonomic nerves to the adrenal glands. These glands respond by pumping the hormone epinephrine (also known as adrenaline) into the bloodstream. As epinephrine circulates through the body, it brings on a number of physiological changes. The heart beats faster than normal, pushing blood to the muscles, heart, and other vital organs. Pulse rate and blood pressure go up. The person undergoing these changes also starts to breathe more rapidly. Small airways in the lungs open wide. This way, the lungs can take in as much oxygen as possible with each breath. Extra oxygen is sent to the brain, increasing alertness. Sight, hearing, and other senses become sharper. Meanwhile, epinephrine triggers the release of blood sugar (glucose) and fats from temporary storage sites in the body. These nutrients flood into the bloodstream, supplying energy to all parts of the body.

All of these changes happen so quickly that people aren't aware of them. In fact, the wiring is so efficient that the amygdala and hypothalamus start this cascade even before the brain's visual centers have had a chance to fully process what is happening. That's why people are able to jump out of the path of an oncoming car even before they think about what they are doing.

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 HPA axis relies on a series of hormonal signals to keep the sympathetic nervous system — the "gas pedal" — pressed down. 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 hormone travels to the adrenal glands, prompting them to release cortisol. The body thus stays revved up and on high alert. When the threat passes, cortisol levels fall. The parasympathetic nervous system — the "brake" — then dampens the stress response.

Techniques to counter chronic stress

Many people are unable to find a way to put the brakes on stress. Chronic low-level stress keeps the HPA axis activated, much like a motor that is idling too high for too long. After a while, this has an effect on the body that contributes to the health problems associated with chronic stress.

Persistent epinephrine surges can damage blood vessels and arteries, increasing blood pressure and raising risk of heart attacks or strokes. Elevated cortisol levels create physiological changes that help to replenish the body's energy stores that are depleted during the stress response. But they inadvertently contribute to the buildup of fat tissue and to weight gain. For example, cortisol increases appetite, so that people will want to eat more to obtain extra energy. It also increases storage of unused nutrients as fat.

Fortunately, people can learn techniques to counter the stress response.

Relaxation response. Dr. Herbert Benson, director emeritus of the Benson-Henry Institute for Mind Body Medicine at Massachusetts General Hospital, has devoted much of his career to learning how people can counter the stress response by using a combination of approaches that elicit the relaxation response. These include deep abdominal breathing, focus on a soothing word (such as peace or calm), visualization of tranquil scenes, repetitive prayer, yoga, and tai chi.

Most of the research using objective measures to evaluate how effective the relaxation response is at countering chronic stress have been conducted in people with hypertension and other forms of heart disease. Those results suggest the technique may be worth trying — although for most people it is not a cure-all. For example, researchers at Massachusetts General Hospital conducted a double-blind, randomized controlled trial of 122 patients with hypertension, ages 55 and older, in which half were assigned to relaxation response training and the other half to a control group that received information about blood pressure control. After eight weeks, 34 of the people who practiced the relaxation response — a little more than half — had achieved a systolic blood pressure reduction of more than 5 mm Hg, and were therefore eligible for the next phase of the study, in which they could reduce levels of blood pressure medication they were taking. During that second phase, 50% were able to eliminate at least one blood pressure medication — significantly more than in the control group, where only 19% eliminated their medication.

Physical activity. People can use exercise to stifle the buildup of stress in several ways. Exercise, such as taking a brisk walk shortly after feeling stressed, not only deepens breathing but also helps relieve muscle tension. Movement therapies such as yoga, tai chi, and qi gong combine fluid movements with deep breathing and mental focus, all of which can induce calm.

Social support. Confidants, friends, acquaintances, co-workers, relatives, spouses, and companions all provide a life-enhancing social net — and may increase longevity. It's not clear why, but the buffering theory holds that people who enjoy close relationships with family and friends receive emotional support that indirectly helps to sustain them at times of chronic stress and crisis.