When a person rests, meditates or reads a book, the parasympathetic nervous system is activated.
Also known as the “rest and digest” system, the parasympathetic nervous system is responsible for conserving energy and maintaining vital bodily functions, including digestion, urination, and defecation, especially when the body is at rest. The “rest and digest” system helps the body maintain homeostasis, or a relatively stable internal state, by balancing sympathetic nervous system activity that triggers the “fight or flight” response.
“It can be thought of as the control mechanisms for the body’s housekeeping, maintenance and sustainability functions,” said James Giordano. (opens in new tab)professor of neurology and biochemistry at the Pellegrino Center for Clinical Bioethics at Georgetown University Medical Center in Washington, DC
The parasympathetic nervous system is a part of the autonomic nervous system, which controls bodily functions that occur without conscious control, such as heart rate, breathing, digestion, and blood pressure.
WHAT DOES THE PARASYMPATHETIC NERVOUS SYSTEM DO?
This system controls body processes that occur at rest. It regulates the daily functions of the body’s systems under relatively stable conditions, Giordano told Live Science.
The parasympathetic nervous system performs its functions using the cranial and sacral nerves, according to medical resource StatPearls (opens in new tab).
Cranial nerves originate in the brain and innervate the eyes, salivary glands, lacrimal and parotid glands, mucous membranes of the nasal cavity, and internal organs in the abdomen and chest. Among the cranial nerves are the two vagus nerves. These are considered the main component of the parasympathetic nervous system because they carry 75% of the body’s parasympathetic nerve fibers and have a role in regulating mood, immune response, digestion and heart rate, according to a 2018 review in the Journal of Frontiers in. Psychiatry (opens in new tab).
The sacral nerves, specifically called “pelvic visceral nerves” in the parasympathetic system, originate in the spinal cord and act on organs located in the pelvic area, such as the reproductive organs and bladder.
The parasympathetic nervous system has the following functions in the parts of the body it supplies:
Reproductive system: Increases blood flow to the penis to initiate an erection. It also stimulates arousal in other reproductive organs, including the seminal vesicles and prostate, as well as swelling of the clitoris during sexual arousal.
Heart: Slows the heart rate and reduces the force of contraction.
Lungs: Constricts the bronchioles to reduce the respiratory rate and increases mucus production in the airways.
eyes: Tightens the pupils and enhances vision, meaning nearby objects are seen more clearly. It is also involved in the production of tears.
Blood vessels: Widens blood vessels (known as vasodilation), increasing blood flow and lowering blood pressure.
Salivary glands: Increases the secretion of potassium ions, water and amylase (a digestive enzyme).
Digestive system: Aids digestive motility (the movement of food and other contents through the intestinal tract) and stimulates the secretion of bile and digestive enzymes.
Kidneys and bladder: Relaxes urethral sphincters to allow urination.
HOW IS THE PARASYMPATHETIC NERVOUS SYSTEM ACTIVATED?
Dr. Gurneet Sawhney (opens in new tab), a neurologist based in India, said the parasympathetic nervous system is activated when the body experiences specific stimuli or situations that provide feelings of safety or relaxation. “This includes activities such as deep breathing, gentle stretching, meditation, slow walking and listening to relaxing music,” he told Live Science. Indeed, a study published in the journal PLOS ONE (opens in new tab) suggests that listening to music increased parasympathetic nervous system activity.
When stimulated by these activities, the vagus nerves carry sensory information from internal organs such as the gut, heart, liver and lungs to the brain, according to the Journal of Frontiers in Psychiatry. Other cranial and spinal nerves also carry sensory information from the parts of the body they innervate to the brain, according to a 2007 review in the American Journal of Pharmaceutical Education (opens in new tab). The brain then interprets the information and triggers a response through the parasympathetic nervous system. Parts of the brain called the hypothalamus and brainstem are key players in coordinating this response.
When activated, the parasympathetic nervous system responds by secreting a chemical messenger known as acetylcholine from neurons in the cranial and sacral nerves. (Neurons are nerve cells that receive and send signals). Acetylcholine travels between neurons and binds to receptors located in various parts of the body, where it initiates specific physiological processes, according to medical resource StatPearls (opens in new tab).
“A parasympathetic person is represented by someone who has a normal, baseline heart rate and respiration rate, with mildly constricted pupils, who salivates, digests and metabolizes food, urinates and defecates, and can be sexually aroused,” Giordano said.
Simply put, activation of the parasympathetic nervous system occurs through an exchange of information from the body to the brain and from the brain back to the body, according to a 2016 article from the Institute for Quality and Efficiency in Healthcare. (opens in new tab).
WHAT IS THE DIFFERENCE BETWEEN THE PARASYMPATHETIC AND SYMPATHETIC NERVOUS SYSTEM?
The autonomic nervous system includes two subdivisions: the sympathetic and the parasympathetic nervous system. These have opposite functions. The sympathetic nervous system, or the “fight or flight” system, is activated when there is a threat and prepares the body to react. The parasympathetic nervous system dominates when the body is at rest.
Unlike the parasympathetic nervous system, which supplies the body through the cranial and sacral nerves, the sympathetic nervous system nerves originate in the thoracic and lumbar regions of the spinal cord, according to medical resource StatPearls. (opens in new tab). They also differ in the chemical messengers they use: The parasympathetic nervous system uses only acetylcholine, while the sympathetic nervous system uses both acetylcholine and norepinephrine to carry signals to receptors.
According to StatPearls (opens in new tab), the parasympathetic nervous system is smaller than the sympathetic nervous system. It only supplies structures in the head, internal organs within the pelvis and external genitalia, unlike the sympathetic nervous system which runs through almost all body tissues – meaning its effects are more widespread while the effects of the parasympathetic nervous system are more localized. according to the review in the American Journal of Pharmaceutical Education.
However, both systems complement each other, with the sympathetic nervous system operating even in non-stressful situations. It works with the parasympathetic nervous system to widen the airways when a person breathes, for example, to allow adequate air flow. In addition, the American Journal of Pharmaceutical Education notes that many tissues are supplied by both systems and continuously receive signals from both. This constant push and pull ensures a balance in tissue activity at all times.