What Is the Autonomic Nervous System?

Autonomic Nervous System

What Is the Autonomic Nervous System?

By Olivia Guy-Evans, published April 28, 2021

The autonomic nervous system (ANS) is part of the peripheral nervous system, and is responsible for the control of vital functions such as heart beat, breathing and digestion.

It is also involved in the acute stress response where it works with the endocrine system to prepare the body to fight or flight. It can be further subdivided into the sympathetic andparasympathetic divisions.

Pupil dilationPupil constriction
Muscle dilationMuscle constriction
Stimulates salivaInhibits saliva
Increased heart rateReduced heart rate
Stimulates digestionInhibits digestion
Stimulates SweatingInhibits Sweating

The ANS transmits information from and to the internal body organs such as the liver and the lungs. It operates automatically, and is generally consideredto be outside the realm of voluntary control.

Examples of the types of functions controlled by the ANS are salivating, sweating, changing pupil size, managing heart rate, crying, and secreting hormones.

The ANS therefore differs from the somatic nervous system (another branch of the peripheral nervous system) as this system is associated with controlling voluntary body movements. Although most of the functions of the ANS are automatic, they can however work in conjunction with the somatic nervous system.

The ANS works by receiving information from either external stimuli or the body. The hypothalamus, which is right above the brain stem, receives autonomic regulatory input from the limbic system (a group of structures deep in the brain which are associated with functions such as memory, emotion, and fear). The hypothalamus uses this input to control much of the activity of the ANS.

There are also three key neurotransmitters involved for successful communication within the ANS:

  1. Acetylcholine – primarily found within the parasympathetic nervous system, which has an inhibiting effect.
  2. Epinephrine – also known as adrenaline, primarily found within the sympathetic nervous system, which has a stimulating effect.
  3. Norepinephrine – also known as noradrenaline, primarily found within the sympathetic nervous system, which has a stimulating effect.


Below is a list of some of the functions of the ANS:

  • Mechanism for the fight-or-flight response
  • Regulating blood pressure
  • Regulating heart rate
  • Secretion of bodily fluids such as saliva, sweat, and urine
  • Breathing
  • Regulating body temperature
  • Pupillary responses
  • Regulating metabolism

The ANS is important for regulating the body, essential for maintain homeostasis. This means the balance of the body’s conditions and functions necessary for living.

More recently, the ANS is believed to be associated with emotions. Activation of the ANS was found when people responded to positive and negative emotions (Shiota et al., 2011).

Divisions of the ANS

There are three branches to the ANS; the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system.

The sympathetic and parasympathetic divisions of the autonomic nervous system have the oppositeeffects on various systems. The two systems have complementary functions, operating intandem to maintain the body’s homeostasis

The nerves in the sympathetic nervous system help to prepare the body for something happening within the environment and expend energy. The nerves in the parasympathetic nervous system mostly work by regulating the body’s functions when at rest, controlling mostly ‘quieter’ activities.

Sympathetic Nervous System

The sympathetic nervous system that is involved in responses which help us deal with emergencies. It slows bodilyprocesses that are less important in emergencies such as digestion.

For instance, if the temperature of a room is hot, the sympathetic system will encourage the body to sweat in response to this change.

The most noticeable function of the sympathetic branch is during the fight-or-flight response.

During conditions that are considered threatening or stressful, the sympathetic system activates, providing an automatic response.

For example, when walking home alone down a dark street, this can be a scary situation for many people. Whilst walking, your pupils may dilate, your heart rate may increase, and you may be sweating.

This response to a stressful situation is caused by the release of large quantities of the neurotransmitter epinephrine from the adrenal gland. Once this stimulating neurotransmitter is released, this triggers the body’s automatic responses. The purpose of stimulating these bodily responses is to prepare the individual to either escape or fight in dangerous situations.

Although the sympathetic nervous system was evolutionarily used in life threatening situations, modern day life and mental health can also trigger this response.

Work-related stress, financial concerns, and relationship problems are examples of when the sympathetic nervous system can produce this stress response.

Similarly, those with anxiety disorders and phobias experience high quantities of epinephrine, resulting in them experiencing the same autonomic responses as if they are in life-threatening situations.

Parasympathetic Nervous System

The parasympathetic nervous system that relaxes the individual once the emergency has passed (eg. slows the heartrate down and reduces blood pressure) and conserves the body’s natural activity by decreasing activity/maintaining it.

The parasympathetic nervous system is associated with returning thebody to resting state functions such as regulating heart rate, relaxing muscles, and controlling the bladder.

This makes the parasympathetic nervous system important in supporting homeostasis.

The parasympathetic nervous system can also come into action once a threatening situation is over. For instance, thinking back to the scenario of walking home alone at night, once returned home and the threatening situation is over, the body relaxes.

The pupils will constrict, the heart rate returns to a resting rhythm, and sweating is reduced or stopped.

The parasympathetic system is therefore important for ensuring we return to normal after a stressful situation. Without this system, the body will be constantly alert, draining all energy, and can lead to chronic stress.

Enteric Nervous System

The enteric nervous system (ENS) is a branch of the ANS, which operates independently of the central nervous system.This system consists of neurons which are confined to the gastrointestinal tract (also known as the gut).

It can also function autonomously of the sympathetic and parasympathetic nervous systems, although it may be influenced by them.

The neurons which comprise the enteric system are responsible for controlling the motor functions of the system as well as secreting enzymes within the gastrointestinal tract. The types of neurons within the enteric system as sensory, motor, and interneurons.

The neurons within this system communicate through many neurotransmitters, such as dopamine, serotonin, and acetylcholine. The circuits of neurons within this system as also able to control local blood flow and modulate immune functions.

Autonomic Dysfunction

Dautonomic dysfunction, or dysautonomia, is a condition in which the autonomic nervous system (ANS) does not function properly.

In developed countries, the most common cause of issues with the ANS result from diabetes (Bishop, 2010). However, other reasons could be due to hereditary reasons, aging, Parkinson’s disease, cancer, or chronic fatigue syndrome. Other reasons could be inflicted onto someone via damage to the head, damage to the neck nerves, alcohol and drug abuse or infections.

If someone believes they may have an issue with their ANS, they could be displaying one or more of the following symptoms:

  • Abnormally high or low blood pressure
  • Lack of pupillary response
  • Severe anxiety or depression
  • Digestive issues
  • Breathing
  • Lack of sweating or too much sweating
  • Tachycardia (abnormally fast heart rate)
  • Incontinence issues
  • Feeling achy, or experiencing pains
  • Light-headedness
  • Feeling faint or actually fainting

Autonomic neuropathy refers to the damage of autonomic nerves. These are disorders which can affect the sympathetic nerves, parasympathetic nerves, or both.

The features of autonomic neuropathy include having a fixed heart rate, constipation, abnormal sweating, decreased pupil size, and absent or delayed light reflexes (Bankenahally & Krovvidi, 2016).

There are a number of other disorders which can be the result of ANS dysfunction:

  • Acute autonomic paralysis – associated with spinal cord injury, resulting in acute and uncontrolled hypertension.
  • Multiple system atrophy – a rare condition which causes gradual damage to the nerve cells.Pure autonomic failure – dysfunction of many processes controlled by the ANS.
  • Familial dysautonomia – also known as Riley-Day syndrome. This is an inherited condition where the nerve fibers do not function properly so these individuals have trouble feeling pain, temperature, pressure, and positioning their arms and legs.

Autonomic Dysfunction Diagnosis and Treatment

If experiencing the aforementioned symptoms and an individual wants to know whether this is related to their ANS being dysfunctional, there are many tests that can be carried out, depending on the symptom that is being experienced.

For instance, if experiencing abnormal heart rhythms, a doctor may use an electrocardiogram to measure electrical activity within the heart.

Blood pressure monitors can also be used to test whether blood pressure is abnormally high or low. Sweat tests can be used to assess whether the sweat glands are functioning properly.

This involves the use of electrodes to stimulate the glands and measure the sweat volumes produced when presented with a stimulus. Pupillary light reflex tests can also be used to determine how sensitive pupils are to changes in light and whether they respond appropriately or not.

These types of physical examinations are required if someone believes they may have an issue with their ANS. Typically, if there is an issue, this may require a lot of trial and error of many tests to be able to diagnose a condition.

To be able to treat a dysfunctional ANS, again depends on the type of diagnosis given. For example, if the cause of dysfunction is due to diabetes, controlling blood sugars will be the primary treatment. In many cases, treatment of the underlying disease (if applicable) can allow damaged nerves within the ANS to repair and regenerate.

It could be that lifestyle changes are recommended by a doctor in order to treat ANS dysfunction. This could be to exercise more frequently or to modify eating habits or diet, cut down on caffeine, or take vitamin supplements.

Drug therapies have also shown to be effective in helping to treat or manage ANS dysfunction. This can include medications that are used to lower blood pressure and non-steroid anti-inflammatories to help control pain (especially for fibromyalgia).

Similarly, antidepressants and anti-anxiety medications can assist with the symptoms of anxiety and have been shown to help re-balance the ANS.

Olivia Guy-Evans obtained her undergraduate degree in Educational Psychology at Edge Hill University in 2015. She then received her master’s degree in Psychology of Education from the University of Bristol in 2019. Olivia has been working as a support worker for adults with learning disabilities in Bristol for the last four years.

How to reference this article:

Guy-Evans, O. (2021, April 28).Autonomic nervous system: definition, division and function. Simply Psychology. www.simplypsychology.org/autonomic-nervous-system.html

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Источник: https://www.simplypsychology.org/autonomic-nervous-system.html

The Autonomic Nervous System and Hypertension

What Is the Autonomic Nervous System?

Physiological studies have long documented the key role played by the autonomic nervous system in modulating cardiovascular functions and in controlling blood pressure values, both at rest and in response to environmental stimuli.

Experimental and clinical investigations have tested the hypothesis that the origin, progression, and outcome of human hypertension are related to dysfunctional autonomic cardiovascular control and especially to abnormal activation of the sympathetic division.

Here, we review the recent literature on the adrenergic and vagal abnormalities that have been reported in essential hypertension, with emphasis on their role as promoters and as amplifiers of the high blood pressure state.

We also discuss the possible mechanisms underlying these abnormalities and their importance in the development and progression of the structural and functional cardiovascular damage that characterizes hypertension. Finally, we examine the modifications of sympathetic and vagal cardiovascular influences induced by current nonpharmacological and pharmacological interventions aimed at correcting elevations in blood pressure and restoring the normotensive state.

That hypertension is due to a derangement of sympathetic and parasympathetic cardiovascular regulation is one of the most widely accredited and tested hypotheses in cardiovascular research.

Its proposal followed from the demonstration that autonomic cardiovascular influences play a fundamental role in homeostatic control of the cardiovascular system.

In animal models of hypertension, both an increased sympathetic nerve activity and a reduction of vagal cardiac tone are associated with and responsible for the appearance and maintenance of high blood pressure, with their role expanding to include hypertension-related sequelae.

1–3 Albeit through a longer and more difficult journey, evidence is now available that similar autonomic alterations may have a causative or cocausative role also in the generation and maintenance of human hypertension.4–7

We begin this review by describing the alterations in autonomic cardiovascular control that characterize human hypertension.

We then discuss the possible mechanisms underlying these alterations and the evidence that they contribute to the functional and structural changes of the heart and systemic circulation that accompany a chronic hypertensive state and lead to its clinical complications.

Finally, we consider the effects of nonpharmacological and pharmacological treatment of hypertension on autonomic cardiovascular control.

Our focus is limited to the condition defined as essential or primary hypertension, with no reference to the secondary forms of high blood pressure because secondary hypertension accounts for only a small fraction of the overall prevalence of hypertension8 and its causes do not include alterations of central or reflex autonomic drive. In addition, the concomitant alterations of sympathetic and parasympathetic cardiovascular control that develop in secondary hypertension have been less extensively documented and remain somewhat controversial.1,4,6

Autonomic Dysfunction in Prehypertension

Abnormal increases in circulating plasma levels of the adrenergic neurotransmitters norepinephrine and epinephrine have repeatedly been demonstrated in normotensive individuals with a family history of hypertension. Moreover, these abnormalities are detectable when measured during maneuvers that activate autonomic cardiovascular control.

9–13 Pressor responses to a variety of laboratory stressors have also been examined and found to predict the subsequent development of hypertension.

14,15 Furthermore, in a more refined experimental approach (measurement of the clearance of norepinephrine after the infusion of small amounts of its radiolabeled form), it was shown that the increase in norepinephrine is not due to its reduced tissue disposal but rather to an enhanced spillover rate from neuroeffective junctions and thus to augmented norepinephrine secretion from sympathetic nerve terminals.16 Finally, in microneurographic studies aimed at quantifying postganglionic sympathetic nerve traffic to the skeletal muscle circulation and including normotensive controls, both the number and amplitude of sympathetic bursts were shown to be higher not only in individuals with a family history of hypertension17 but also in those with white-coat and masked hypertension (Figure 1),18–20 that is, conditions in which patients have a markedly greater risk of progressing to true hypertension.21 Thus, there is little doubt that a central sympathetic overdrive is present in individuals predisposed to developing high blood pressure because of either a genetic background or a specific blood pressure phenotype. Interestingly, this sympathetic hyperactivity is ly to be accompanied by an impaired vagal influence on the heart. Evidence for this impairment comes from studies of the normotensive offspring of hypertensive parents. In this group, spectral analysis of the R–R interval showed a reduction of low-frequency fluctuations in heart rate,22,23 that is, fluctuations that are known to be a component of heart rate variability that reflects vagal modulation of the sinus node.24 Thus, not just 1 but both divisions of the autonomic nervous system may be altered in individuals who have a greater risk of developing hypertension, even when an overt blood pressure abnormality is not yet detectable. This points to the causative role of the autonomic nervous system in the development of high blood pressure condition.

Figure 1.Muscle sympathetic nerve activity (MSNA), expressed as burst frequency over time (bs/min, left) and as burst frequency corrected for heart rate (bs/100 hb), measured by microneurography in the peroneal nerve in normotensive subjects (NT) and in age-matched patients with white-coat hypertension (WCHT) and masked hypertension (MHT). *P

Источник: https://www.ahajournals.org/doi/10.1161/circresaha.114.302524

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