The autonomic nervous system (ANS) is part of the peripheral nervous system (PNS) responsible for regulating involuntary bodily functions, such as heart rate, digestion, respiratory rate, and pupillary response.
It consists of two main divisions: the sympathetic and parasympathetic systems, which often work in opposition to maintaining the body’s internal balance or homeostasis. It plays a key role in the fight-or-flight response.
Key Takeaways
- The autonomic nervous system (ANS) controls involuntary body functions like heart rate, breathing, digestion, and pupil response.
- It is divided into three branches: the sympathetic (fight-or-flight), parasympathetic (rest-and-digest), and enteric (gut-based) systems.
- The ANS maintains homeostasis by balancing responses to stress and recovery through its different branches.
- Emotional states like anxiety and trauma can dysregulate the ANS, sometimes leading to chronic health or psychological issues.
- ANS dysfunction can occur due to aging, disease, or injury, with symptoms ranging from fainting and abnormal heart rate to digestive problems.
The autonomic nervous system differs from the somatic nervous system (another branch of the peripheral nervous system), which 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.
Functions of the ANS
The ANS is important for regulating the body, and is essential for maintaining homeostasis. This means balancing the body’s conditions and functions necessary for living.
Below is a list of some of the functions of the ANS:
- Regulating blood pressure
- Regulating heart rate
- Secretion of bodily fluids such as saliva, sweat, and urine
- Breathing
- Regulating body temperature
- Pupillary responses
- Regulating metabolism
After eating lunch, Jamal feels drowsy and relaxed. His heart rate slows slightly, and his digestive system activates to process food. This is his parasympathetic system in action, prioritizing digestion and energy storage.
Research shows the ANS responds to emotional states like fear and happiness. Activation of the ANS was found when people responded to positive and negative emotions (Shiota et al., 2011).
Neurotransmitters of the autonomic 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 that is associated with functions such as memory, emotion, and fear). The hypothalamus controls ANS activity using signals from the limbic system.
There are also three critical neurotransmitters involved in successful communication within the ANS:
- Acetylcholine – is primarily found within the parasympathetic nervous system, which has an inhibiting effect.
- Epinephrine – also known as adrenaline, is primarily found within the sympathetic nervous system, which has a stimulating effect.
- Norepinephrine – also known as noradrenaline, is primarily found within the sympathetic nervous system and has a stimulating effect.
Divisions of the autonomic Nervous system
There are three branches to the ANS; the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system:
Sympathetic Nervous System
The sympathetic nervous system is involved in responses that help us deal with stress. It slows bodily processes that are less important in emergencies, such as digestion.
For instance, if the temperature of a room is too 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.
The sympathetic system activates and releases epinephrine (adrenaline) during threatening or stressful conditions, providing an automatic response.
Sarah walks home alone at night. She hears footsteps behind her. Her heart races, pupils dilate, and she starts to sweat. This is her sympathetic nervous system preparing her to react—either to flee or defend herself.
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, such as work stressors and relationship problems, can also trigger this response.
Similarly, those with anxiety disorders and phobias experience high quantities of epinephrine, resulting in them experiencing the same autonomic responses as in life-threatening situations.
Parasympathetic Nervous System
The parasympathetic nervous system relaxes the individual once the stress has passed (e.g., slows the heart rate down and reduces blood pressure) and conserves the body’s natural activity by decreasing activity/maintaining it.
Later, Sarah gets home and locks the door. She exhales, her heart rate slows, and her muscles relax. Her parasympathetic system takes over, helping her body return to a resting state.
The pupils will constrict, the heart rate will return to a resting rhythm, and sweating will be 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 this can lead to chronic stress. This shows how important the parasympathetic is in maintaining homeostasis (a balance in the body).
Sympathetic vs. Parasympathetic Nervous System
| Organ/System | Sympathetic Nervous System (“Fight or Flight”) | Parasympathetic Nervous System (“Rest and Digest”) |
|---|---|---|
| Heart | Increases heart rate | Decreases heart rate |
| Lungs | Dilates bronchi (more air in) | Constricts bronchi (returns to normal) |
| Pupils | Dilates pupils (better vision in danger) | Constricts pupils (normal vision) |
| Digestive System | Slows digestion | Stimulates digestion |
| Bladder | Relaxes bladder (inhibits urination) | Contracts bladder (promotes urination) |
| Salivary Glands | Inhibits saliva production | Stimulates saliva production |
| Sweat Glands | Activates sweating | No significant effect |
| Liver | Stimulates glucose release | Promotes glucose storage |
| Adrenal Glands | Stimulates adrenaline release | No direct stimulation |
| Reproductive Organs | Decreases function | Stimulates arousal |
Enteric Nervous System
The enteric nervous system (ENS) is a branch of the ANS that operates independently of the central nervous system. This system consists of neurons confined to the gastrointestinal tract (also known as the gut).
The enteric nervous system can function independently, although it interacts with both the sympathetic and parasympathetic systems.
The neurons that comprise the enteric system are responsible for controlling the motor functions of the system as well as secreting enzymes within the gastrointestinal tract.
The neurons within this system communicate through many neurotransmitters, such as dopamine, serotonin, and acetylcholine. The circuits of neurons within this system are also able to control local blood flow and modulate immune functions.
Autonomic Dysfunction
Signs of dysfunction associated with the autonomic nervous system include:
If someone believes they may have an issue with their ANS, they could be displaying one or more of the following symptoms:
- Lack of pupillary response
- Abnormally high or low blood pressure
- 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
The ANS and Emotional Stress
Chronic stress, anxiety, and trauma can disrupt normal autonomic regulation by overactivating the sympathetic nervous system.
This keeps the body in a prolonged “fight or flight” state—raising heart rate, increasing cortisol, and impairing digestion and sleep. Over time, this imbalance can lead to exhaustion and health issues.
Polyvagal theory, proposed by Stephen Porges, suggests the vagus nerve plays a central role in emotional regulation, safety perception, and social behavior.
It explains how traumatic experiences can trigger immobilization or dissociation by activating a primitive “shutdown” response—highlighting the ANS’s role in both physiological and psychological well-being.
ANS Development and Aging
The autonomic nervous system develops gradually after birth and becomes more stable into later childhood and into adulthood.
However, with aging, the ANS may become less responsive. Older adults often show reduced heart rate variability, slower pupillary responses, and impaired thermoregulation, making them more vulnerable to fainting, temperature extremes, and stress-related health issues.
These changes reflect a natural decline in autonomic flexibility, which can also affect resilience to illness and recovery.
Autonomic Neuropathy/ Disorders Caused by ANS Dysfunction
Autonomic neuropathy refers to the damage of autonomic nerves. These are disorders that 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 that 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.
Taking Care of the Autonomic Nervous System
While many autonomic processes happen automatically, lifestyle choices can influence how well the system functions.
Managing stress is key—chronic stress can overactivate the sympathetic system and disrupt balance. Practices like deep breathing, mindfulness meditation, and yoga help activate the parasympathetic system and promote calm.
Regular physical activity improves heart rate variability and supports overall autonomic tone.
Good sleep hygiene, staying hydrated, and eating a balanced diet rich in fiber and nutrients also support gut health and the enteric nervous system.
Reducing stimulants like caffeine and alcohol may help people who experience sensitivity or dysregulation.
In some cases, people benefit from biofeedback, vagal nerve stimulation, or working with a therapist to manage anxiety or trauma-related ANS imbalance.
Taking care of the autonomic nervous system means maintaining habits that support physical and emotional resilience.
Which division of the autonomic nervous system returns the body to a relaxed condition after an emergency?
The parasympathetic division of the autonomic nervous system is responsible for returning the body to a relaxed and restorative state after an emergency or stress.
It counteracts the effects of the sympathetic division, which initiates the “fight or flight” response during emergencies. The parasympathetic system promotes “rest and digest” functions, restoring balance and conserving energy.
Which division of the ans can function independently without being stimulated by the central nervous system?
The enteric division of the autonomic nervous system (ANS) can function independently without being stimulated by the central nervous system. It primarily manages the functions of the gastrointestinal tract, including digestion and motility, and can operate autonomously but also communicates with the central nervous system.
Which division of the autonomic nervous system prepares the body for action in a stressful situation?
The sympathetic division of the autonomic nervous system prepares the body for action in stressful situations, often referred to as the “fight or flight” response. It increases heart rate, dilates airways, and redirects blood flow to muscles, among other responses, to ready the body for immediate action.
Related articles:
Neurotransmitters: Types, Function and Examples
Peripheral Nervous System (PNS): Parts and Function
Sympathetic Nervous System: Functions & Examples
Parasympathetic Nervous System (PSNS) Functions & Division
