The parasympathetic nervous system (PNS) is an essential part of the autonomic nervous system (ANS), which regulates the body's involuntary functions, such as breathing, heart rate, and digestion. The PNS is responsible for regulating the body's "rest and digest" response, which is the opposite of the "fight or flight" response activated by the sympathetic nervous system (SNS) during stress. In this article, we will explore the anatomy, function, and dysfunction of the parasympathetic nervous system.

Anatomy of the Parasympathetic Nervous System

The parasympathetic nervous system originates in the brainstem and the sacral region of the spinal cord. It is made up of two main components: cranial nerves (head and nes) and pelvic (lower abdomen) nerves.

Cranial nerves are responsible for the parasympathetic control of the head, neck, thorax, and upper abdomen. The ten cranial nerves that have parasympathetic fibers are:

  • Olfactory nerve (CN I)
  • Optic nerve (CN II)
  • Oculomotor nerve (CN III)
  • Trochlear nerve (CN IV)
  • Trigeminal nerve (CN V)
  • Facial nerve (CN VII)
  • Glossopharyngeal nerve (CN IX)
  • Vagus nerve (CN X)
  • Spinal accessory nerve (CN XI)
  • Hypoglossal nerve (CN XII)
The parasympathetic nervous system sends instructions from the brain to the organs in the body. Also, information from the organs may be transmitted back to the brain. The parasympathetic nervous system sends fibers to the adrenal gland, which is the yellowish structure sitting on top of the kidney, to reduce stress. Image: Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436., CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

The vagus nerve (CN X) is the most important cranial nerve for parasympathetic function, as it provides parasympathetic innervation to most of the body's organs. It originates in the medulla oblongata and travels through the neck, thorax, and abdomen, providing parasympathetic fibers to the heart, lungs, liver, spleen, pancreas, stomach, small intestine, and large intestine. It also sends fibers to the adrenal gland to switch off the stress response by adrenaline (or epinephrine).

The pelvic nerves are responsible for the parasympathetic control of the lower abdomen, pelvis, and genitals. They arise from the sacral (lower) region of the spinal cord and provide parasympathetic fibers to the bladder, rectum, and reproductive organs.

Function of the Parasympathetic Nervous System

The parasympathetic nervous system is responsible for regulating the body's "rest and digest" response, which is the opposite of the "fight or flight" response activated by the sympathetic nervous system. When the body is at rest, the parasympathetic nervous system dominates, and it slows down many bodily functions to conserve energy and promote healing.

The parasympathetic nervous system has several important functions, including:

  1. Regulating heart rate: The parasympathetic nervous system slows down the heart rate by releasing acetylcholine, a messenger molecule which acts on the heart's pacemaker cells to reduce their activity.
  2. Regulating breathing: Slow and deep breathing are signs you are calm. Both are promoted by the parasympathetic nervous system. Breathing exercises to reduce stress are based on activating the parasympathetic nervous system to get you back into the "rest and digest" mode.
  3. Stimulating digestion: The parasympathetic nervous system promotes digestion by stimulating the release of saliva, digestive enzymes, and stomach acid. It also increases blood flow to the digestive organs, which helps to absorb nutrients from food.
  4. Constricting the pupils: The parasympathetic nervous system constricts the pupils of the eyes, which helps to improve near vision.
  5. Stimulating urination and defecation: The parasympathetic nervous system stimulates the bladder to contract and the rectum to relax, which promotes urination and defecation.
  6. Promoting relaxation and sleep: The parasympathetic nervous system promotes relaxation and sleep by slowing down bodily functions and reducing stress hormones.

Dysfunction of the Parasympathetic Nervous System

Dysfunction of the parasympathetic nervous system can result in a wide range of symptoms and conditions , including:

  1. Bradycardia: Bradycardia is a condition in which the heart rate is too slow, usually less than 60 beats per minute. This can be caused by dysfunction of the parasympathetic nervous system, which normally slows down the heart rate.
  2. Gastrointestinal disorders: Dysautonomia, a condition characterized by dysfunction of the autonomic nervous system, can cause a range of gastrointestinal symptoms, including nausea, vomiting, constipation, and diarrhea. These symptoms are related to a dysfunction of the parasympathetic regulation of digestion.
  3. Bladder dysfunction: Dysfunction of the parasympathetic nerves that control the bladder can lead to urinary retention, urinary incontinence, or overactive bladder syndrome.
  4. Sexual dysfunction: Dysfunction of the parasympathetic nerves that control the genitals can lead to sexual dysfunction, including erectile dysfunction in men and decreased libido in both men and women.
  5. Sleep disorders: Dysfunction of the parasympathetic nervous system can interfere with the body's ability to relax and fall asleep, leading to insomnia and other sleep disorders. The sympathetic nervous system will dominate, keeping you alert and awake.
  6. Pupillary abnormalities: Dysfunction of the parasympathetic nervous system can cause abnormalities in the size of the pupils, such as miosis (constriction of the pupils) or anisocoria (unequal pupil size).
  7. Dry mouth and eyes: Dysfunction of the parasympathetic nervous system can cause dry mouth and dry eyes, as the parasympathetic nervous system normally stimulates the production of saliva and tears. You can also have a dry mouth during stress. This is caused by the stronger activity of the sympathetic nervous system as compared to the parasympathetic nervous system.

Be aware that the parasympathetic nervous system is hardly active during stress, and you may have all of these symptoms due to the overpowering activity of the sympathetic nervous system. You have therefore to ask yourself whether you are facing stressful events that may be at the origin of these symptoms. If so, they will pass when the stress passes, and there is no need to see a doctor. You can read more about stress and the autonomous nervous system here.

Treatment of Parasympathetic Dysfunction

The treatment of parasympathetic dysfunction depends on the underlying cause and the specific symptoms present. Some possible treatment options include:

  1. Medications: Medications that stimulate the parasympathetic nervous system, such as cholinesterase inhibitors or muscarinic agonists, may be used to treat certain types of parasympathetic dysfunction, such as dry mouth or urinary retention.
  2. Lifestyle modifications: Lifestyle modifications, such as stress reduction techniques and dietary changes, may be recommended to help regulate the parasympathetic nervous system and improve symptoms.
  3. Physical therapy: Physical therapy may be recommended to improve bladder function and other symptoms of parasympathetic dysfunction.
  4. Surgery: In some cases, surgery may be necessary to correct the underlying cause of parasympathetic dysfunction, such as a tumor or structural abnormality.