The autonomic nervous system (ANS) is the part of the nervous system that regulates many of the body’s internal functions without requiring conscious effort. It helps control heart rate, blood pressure, breathing patterns, digestion, body temperature, sweating, pupil size, and other processes that keep the body’s internal environment stable.
You do not normally have to think about making your heart beat or adjusting blood flow when you stand up. The autonomic nervous system continuously monitors what is happening inside and around your body and adjusts organ activity accordingly.
The ANS is part of the peripheral nervous system, which includes nerves that carry information between the brain, spinal cord, and the rest of the body. It works closely with the brain and spinal cord, especially regions involved in maintaining homeostasis—the body’s ability to keep internal conditions within appropriate ranges.
What does the autonomic nervous system control?
The autonomic nervous system regulates organs and tissues that generally operate outside conscious control. Its functions include:
- Heart and circulation: It changes heart rate and the force of heart contractions and helps regulate blood vessel diameter and blood pressure.
- Breathing: It influences the automatic rhythm of breathing, although breathing can also be consciously controlled for periods of time.
- Digestion: It regulates movement through the digestive tract and influences digestive secretions.
- Eyes: It controls pupil size and helps adjust the eyes for different visual conditions.
- Bladder: It coordinates the storage and release of urine.
- Sweat glands: It regulates sweating, which is important for temperature control.
- Body temperature: It helps coordinate sweating, changes in blood flow to the skin, and other responses involved in heat regulation.
- Sexual function: It contributes to several aspects of sexual arousal and reproductive function.
These activities are not simply switched on or off. The autonomic nervous system continually adjusts them according to the body’s needs.
How does the autonomic nervous system work?
The ANS relies on communication between the brain, spinal cord, peripheral nerves, and organs. Information about the body’s internal state travels to the central nervous system, where it can be integrated with information about the external environment and the body’s current needs. Signals then travel back through autonomic pathways to influence organs.
A typical autonomic pathway involves two neurons in sequence between the central nervous system and the target organ. The first is called a preganglionic neuron; it extends from the brain or spinal cord to an autonomic ganglion, a cluster of nerve-cell bodies outside the central nervous system. A postganglionic neuron then carries the signal from the ganglion to the target tissue.
Autonomic signaling depends heavily on chemical messengers called neurotransmitters. The two most important are acetylcholine and norepinephrine. Different autonomic pathways use different neurotransmitters and receptors, and the same neurotransmitter can produce different effects depending on the receptor and tissue involved.
This arrangement allows the nervous system to produce precise, sometimes opposing effects in different organs at the same time.
The three main divisions of the autonomic nervous system
The autonomic nervous system is traditionally divided into the sympathetic, parasympathetic, and enteric nervous systems. The first two are often discussed as opposing systems, but that description is incomplete: they frequently work together, and their effects depend on the organ and physiological situation.
The sympathetic nervous system
The sympathetic nervous system helps the body respond to situations that require increased alertness or physical readiness. Its activity can increase heart rate and redirect blood flow toward tissues that need greater support during exertion or acute stress.
It also tends to reduce digestive activity during situations in which immediate physical action takes priority. Sympathetic activation contributes to sweating and pupil dilation and works with hormonal responses to stress.
The common phrase “fight or flight” captures part of its role, but the sympathetic system is not activated only during emergencies. It participates in ordinary regulation of circulation, metabolism, temperature, and other functions throughout the day.
The parasympathetic nervous system
The parasympathetic nervous system supports functions associated with conserving energy, digestion, and routine maintenance of the body. It generally slows the heart and promotes digestive activity, although its effects vary by organ.
A major parasympathetic pathway travels through the vagus nerve, which connects the brainstem with organs in the chest and abdomen. The vagus nerve has important roles in regulating the heart, lungs, and digestive tract.
The parasympathetic system is sometimes described as the “rest and digest” system. That phrase is useful as a rough introduction, but it should not be interpreted to mean that the parasympathetic system is active only when a person is completely relaxed.
The enteric nervous system
The enteric nervous system (ENS) is a network of neurons within the wall of the gastrointestinal tract. It coordinates many aspects of digestion, including movement of the intestines and regulation of digestive secretions.
The enteric nervous system can carry out complex local activity without direct moment-to-moment instructions from the brain. It nevertheless communicates extensively with the central nervous system and is influenced by both sympathetic and parasympathetic pathways.
Because of its size and complexity, the enteric nervous system is sometimes called the “brain of the gut,” although it is not a second brain in the same sense as the brain in the skull.
Are the sympathetic and parasympathetic systems opposites?
Not exactly. They often have opposing effects, but the relationship is more nuanced.
For example, sympathetic activity generally accelerates the heart, while parasympathetic activity generally slows it. In other organs, however, the two systems may have different targets, different strengths of influence, or effects that are not simply opposite.
Some organs also receive relatively little direct input from one of the two divisions. Sweat glands, for example, are controlled by sympathetic neurons, even though those neurons use acetylcholine rather than the norepinephrine commonly associated with sympathetic signaling.
The ANS therefore works less like a simple two-way switch and more like a coordinated control system whose signals vary according to the organ and the body’s circumstances.
How does the autonomic nervous system maintain homeostasis?
Homeostasis means maintaining relatively stable internal conditions despite changing demands. The autonomic nervous system is central to this process.
Consider what happens when you stand up. Gravity causes blood to shift toward the lower part of the body. Without compensation, less blood would return to the heart and blood pressure could fall. Sensors in the cardiovascular system detect changes in pressure and send information to the brainstem. Autonomic responses then adjust heart activity and blood vessel tone to help maintain adequate blood flow to the brain and other tissues.
Similar feedback mechanisms operate continuously. Changes in body temperature, blood pressure, blood chemistry, digestion, and other internal conditions can trigger adjustments in autonomic activity.
The hypothalamus, brainstem, and spinal cord are particularly important in coordinating these responses. The hypothalamus, a small region deep within the brain, integrates autonomic regulation with functions such as temperature control, fluid balance, appetite, endocrine activity, and responses to stress.
How is the autonomic nervous system different from the somatic nervous system?
The somatic nervous system is the part of the peripheral nervous system most directly associated with conscious sensation and voluntary control of skeletal muscles. It allows you to intentionally move your arm, for example, and carries sensory information such as touch and pain to the central nervous system.
The autonomic nervous system primarily regulates internal organs and glands.
The distinction is useful, but it is not absolute. Some processes traditionally described as autonomic can be influenced consciously. Breathing is a good example: its basic rhythm is automatically regulated, but you can deliberately change your breathing for a time.
Likewise, autonomic activity can be influenced indirectly by conscious behavior, emotions, thoughts, and learned responses. The nervous system is an integrated network rather than a collection of completely independent circuits.
What happens when the autonomic nervous system is disrupted?
Problems affecting autonomic nerves or the brain regions that control them are broadly described as autonomic dysfunction or dysautonomia. Depending on which pathways are affected, symptoms can involve several different body systems.
Possible manifestations include abnormal changes in heart rate or blood pressure, dizziness or fainting when standing, unusual sweating, difficulty regulating body temperature, digestive problems, bladder dysfunction, or certain sexual difficulties.
Autonomic dysfunction can occur for many different reasons, including disorders that damage nerves, certain medications, metabolic conditions, infections, and other neurological or systemic diseases. The specific symptoms and their significance depend heavily on the underlying cause.
Because the autonomic nervous system controls so many functions, symptoms that seem unrelated—such as changes in blood pressure and digestive problems—can sometimes arise from the same underlying autonomic disorder.
The autonomic nervous system is always working
The autonomic nervous system is not simply a system that turns on during stress and turns off during relaxation. It is active continuously, adjusting organ function from moment to moment.
Its importance becomes especially apparent when circumstances change: when you exercise, eat, become overheated, stand up, fall asleep, experience pain, or face an acute threat. In each case, autonomic circuits help coordinate the cardiovascular, respiratory, digestive, and other systems so that the body’s internal environment remains compatible with normal function.
Rather than operating as a separate “automatic” brain, the ANS is an integrated part of the nervous system. Its constant communication with the brain, spinal cord, organs, and endocrine system allows the body to respond rapidly to changing demands while maintaining the basic physiological stability required for life.