Created: October 28, 2009; Last Update: August 19, 2016; Next update: 2022.
The nervous system is made up of all the nerve cells in your body. It is through the nervous system that we communicate with the outside world and, at the same time, many mechanisms inside our body are controlled. The nervous system takes in information through our senses, processes the information and triggers reactions, such as making your muscles move or causing you to feel pain. For example, if you touch a hot plate, you reflexively pull back your hand and your nerves simultaneously send pain signals to your brain. Metabolic processes are also controlled by the nervous system.
There are many billions of nerve cells, also called neurons, in the nervous system. The brain alone has about 100 billion neurons in it. Each neuron has a cell body and various extensions. The shorter extensions (called dendrites) act like antennae: they receive signals from, for example, other neurons and pass them on to the cell body. The signals are then passed on via a long extension (the axon), which can be up to a meter long.
The nervous system has two parts, called the central nervous system and the peripheral nervous system due to their location in the body. The central nervous system (CNS) includes the nerves in the brain and spinal cord. It is safely contained within the skull and vertebral canal of the spine. All of the other nerves in the body are part of the peripheral nervous system (PNS).
Regardless of where they are in the body, a distinction can also be made between voluntary and involuntary nervous system. The voluntary nervous system (somatic nervous system) controls all the things that we are aware of and can consciously influence, such as moving our arms, legs and other parts of the body.
The involuntary nervous system (vegetative or autonomic nervous system) regulates the processes in the body that we cannot consciously influence. It is constantly active, regulating things such as breathing, heart beat and metabolic processes. It does this by receiving signals from the brain and passing them on to the body. It can also send signals in the other direction – from the body to the brain – providing your brain with information about how full your bladder is or how quickly your heart is beating, for example. The involuntary nervous system can react quickly to changes, altering processes in the body to adapt. For instance, if your body gets too hot, your involuntary nervous system increases the blood circulation to your skin and makes you sweat more to cool your body down again.
Both the central and peripheral nervous systems have voluntary and involuntary parts. However, whereas these two parts are closely linked in the central nervous system, they are usually separate in other areas of the body.
The involuntary nervous system is made up of three parts:
The sympathetic nervous system
The parasympathetic nervous system
The enteric (gastrointestinal) nervous system
The sympathetic and parasympathetic nervous systems usually do opposite things in the body. The sympathetic nervous system prepares your body for physical and mental activity. It makes your heart beat faster and stronger, opens your airways so you can breathe more easily, and inhibits digestion.
The parasympathetic nervous system is responsible for bodily functions when we are at rest: it stimulates digestion, activates various metabolic processes and helps us to relax. But the sympathetic and parasympathetic nervous systems do not always work in opposite directions; they sometimes complement each other too.
The enteric nervous system is a separate nervous system for the bowel, which, to a great extent, autonomously regulates bowel motility in digestion.
Sources
Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier; 2012.
Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.
Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.
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Your parasympathetic nervous system is a network of nerves that relaxes your body after periods of stress or danger. It also helps run life-sustaining processes, like digestion, during times when you feel safe and relaxed. The informal descriptions for this system include the rhymes “rest and digest” or “feed and breed.”
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- Overview
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- Anatomy
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Your parasympathetic nervous system is part of your autonomic nervous system. It could be called your “automatic” nervous system, as it’s responsible for many functions that you don’t have to think about to control. This can include control of your heart rate, blood pressure, digestion, urination and sweating, among other functions.
The parasympathetic part of your autonomic nervous system balances your sympathetic nervous system. While your sympathetic nervous system controls your body’s “fight or flight” response, your parasympathetic nervous system helps to control your body’s response during times of rest.
Your parasympathetic nervous system’s job is usually to relax or reduce your body’s activities. Because of the signals it carries, the rhyming phrases “rest and digest” or “feed and breed” are easy ways to remember what your parasympathetic nervous system does.
Your parasympathetic nervous system can have the following effects:
- Eyes: It constricts your pupils to limit how much light enters your eyes. It also makes changes that can help improve your close-up vision, and causes tear production in your eyes.
- Nose and mouth: It makes glands in your mouth produce saliva, and glands in your nose produce mucus. This can be helpful with digestion and breathing during times of rest.
- Lungs: It tightens airway muscles and ultimately reduces the amount of work your lungs do during times of rest.
- Heart: It lowers your heart rate and the pumping force of your heart.
- Digestive tract: It increases your rate of digestion and diverts energy to help you digest food. It also tells your pancreas to make and release insulin, helping your body break down sugars into a form your cells can use.
- Waste removal: It relaxes the muscles that help you control when you pee (urinate) or poop (defecate).
- Reproductive system: It manages some of your body’s sexual functions, including feeling aroused (erections in people with a penis and secreting fluids that provide lubrication during sex in people with a vagina).
What’s the difference between the parasympathetic and sympathetic nervous systems?
Your sympathetic and parasympathetic nervous systems have opposite but complementary roles. Your sympathetic nervous system carries signals that put your body’s systems on alert, and your parasympathetic carries signals that return those systems to their standard activity levels.
Your sympathetic nervous system takes the lead when your safety and survival are at risk, but that system’s actions can strain body systems when it’s active for too long. Because these two systems offset each other, they help maintain balance in your body.
Your parasympathetic nervous system also manages the activity in organs throughout your body when you feel calm and safe. These functions don’t involve risk or danger but are still key in keeping you alive and healthy.
Your parasympathetic nervous system is one of two parts of your autonomic nervous system. Your autonomic nervous system is a subsystem of your peripheral nervous system, which is all the nervous tissue in your body excluding your brain and spinal cord.
Your parasympathetic nervous system uses four of your 12 cranial nerves. These are nerves that connect directly to your brain. Three of those four only involve your senses and glands connected to your eyes, nose and mouth.
The fourth, your vagus nerve, connects to part of your mouth and also extends down through your neck to your chest and abdomen (belly). Your vagus nerve makes up about 75% of your parasympathetic nervous system overall, connecting to your heart, lungs and other vital internal organs.
Farther down, 31 spinal nerves connect directly to your spinal cord, but your parasympathetic nervous system only uses some of them in the lower part of your spine. This sends signals to your bladder and bowels to relax so you can use the bathroom.
What is it made of?
Your parasympathetic nervous system’s components are similar to those found in other parts of your nervous system. Neurons are the main type of cell — they can generate and receive signals.
Many conditions and problems can affect your autonomic nervous system, including your parasympathetic nervous system. Potential problems include:
- Type 2 diabetes. Uncontrolled Type 2 diabetes can damage your autonomic nervous system, including your parasympathetic nervous system.
- Congenital and genetic conditions. These are disorders or conditions you have at birth. You have genetic conditions because you inherit them from one or both parents. Inherited forms of amyloidosis can cause parasympathetic nervous system problems.
- Parasympathetic nervous system problems can cause incontinence when there’s damage to the nerves that control your bladder and bowels.
- Multiple system atrophy. This severe condition is similar to Parkinson’s disease, damaging autonomic nerves over time.
- Sexual dysfunction. People with parasympathetic nervous system damage may have erectile dysfunction.
- Trauma. Nerve damage from injuries is potentially long-term or even permanent. This is especially the case when you have injuries to your spinal cord or main nerve structures that impair or cut off parasympathetic connections farther down.
What are some common signs and symptoms of parasympathetic nervous system problems?
Your parasympathetic nervous system controls processes in your body that should happen automatically. That means these problems usually get noticed when something doesn’t happen as expected. Possible symptoms include:
What are some common tests to check parasympathetic nervous system functions?
Potential tests include:
What are the common treatments for parasympathetic nervous system conditions?
Treating conditions that affect your parasympathetic nervous system is often challenging. That’s because the treatments can change dramatically, depending on what’s behind the problem. The treatments can also include many different approaches, ranging from medication to surgery.
Sometimes, treatment of a parasympathetic nervous system problem requires treating or curing an underlying problem. In other cases, a condition isn’t curable, and the goal will be to treat and minimize the impact of symptoms.
Prevention can make all the difference in protecting and maintaining your parasympathetic nervous system. The best preventive measures include:
- Eat a balanced diet. Vitamin deficiencies, especially vitamin B12, can damage your nervous system. You should also avoid overusing vitamins (especially B6, which has toxic effects on your nervous systems at high levels).
- Avoid abusing drugs and alcohol. Substance use, including frequent heavy drinking, can have toxic effects and damage your sympathetic nervous system.
- Stay physically active and maintain a healthy weight. Type 2 diabetes damages your autonomic nerves over time. That’s why preventing it, or at least delaying when it starts, can make a big difference in your overall well-being.
- Wear safety equipment as needed. Safety equipment can be a big help if you want to prevent nerve damage from injuries, regardless of whether you use the equipment during work or play activities.
- Manage chronic conditions as recommended. If you have a chronic condition that can damage your nervous system, you should take steps to manage this condition. Your healthcare provider can help guide you on how to do that. That guidance can make a big difference in helping you limit the condition's progress or how it affects your life.
A note from Cleveland Clinic
Your parasympathetic nervous system is a key part of your body’s long-term survival. This system constantly works without you thinking about it, whether you’re asleep or awake. It helps relax you in times of calm and provides a balance with your body’s short-term survival responses. While conditions that affect it are rare, protecting your parasympathetic nervous system is still important to your overall health and well-being.
Last reviewed by a Cleveland Clinic medical professional on 06/06/2022.
References
- Amthor FR. Autonomic Nervous System: Sympathetic, Parasympathetic, & Enteric. In: Amthor FR, Theibert AB, Standaert DG, Roberson ED. eds. Essentials of Modern Neuroscience. McGraw Hill; 2020. Accessed 6/6/2022.
- Beck-Yoo K. The Nervous System. In: Janson LW, Tischler ME. eds. The Big Picture: Medical Biochemistry. McGraw Hill; 2018. Accessed 6/6/2022.
- LeBouef T, Yaker Z, Whited L. Physiology, Autonomic Nervous System. (//www.ncbi.nlm.nih.gov/books/NBK538516/) [Updated 2021 May 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Accessed 6/6/2022.
- The Autonomic Nervous System. In: Nestler EJ, Kenny PJ, Russo SJ, Schaefer A. eds. Nestler, Hyman & Malenka’s Molecular Neuropharmacology: A Foundation for Clinical Neuroscience, 4e. McGraw Hill; 2020. Accessed 6/6/2022.
- The Autonomic Nervous System. In: Waxman SG. eds. Clinical Neuroanatomy, 29e. McGraw Hill; 2020. Accessed 6/6/2022.
- Tindle J, Tadi P. Neuroanatomy, Parasympathetic Nervous System. (//www.ncbi.nlm.nih.gov/books/NBK553141/) [Updated 2021 Nov 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Accessed 6/6/2022.
- Waxenbaum JA, Reddy V, Varacallo M. Anatomy, Autonomic Nervous System. (//www.ncbi.nlm.nih.gov/books/NBK539845/) [Updated 2021 Jul 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Accessed 6/6/2022.
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