Parasympathetic Nervous System (PSNS) | Function & Definition (2023)

There are many processes that are taking place in your body every time. When you are calmly sitting on your resting chair, you are breathing, your heart is pumping, your blood pressure is being controlled, and the cellular metabolic processes are also regulated. You are not aware of any of these processes, but parasympathetic system is there to take care of all these processes for you.

The parasympathetic nervous system is a division of the autonomic nervous system that controls the internal body organs in resting state. It is active all the time and promotes life by regulating the vital body functions, although the person is unconscious about all these processes.

In this article, we will talk about the organization
of the parasympathetic nervous system, its neurotransmitters and receptors, and
the process by which it controls the internal body organs.

In the end, we will talk about some disorders that
change the activity of the parasympathetic nervous system. We will also discuss
some drugs acting on the PSNS.

Anatomy and Organization:

Parasympathetic Nervous System (PSNS) | Function & Definition (1)
(Video) 2-Minute Neuroscience: Parasympathetic Nervous System

The parasympathetic nervous system is also a subdivision of the peripheral nervous system. Like other subdivisions of the PNS, it also consists of nerve fibers leading to or originating from the brain and spinal cord. It also has neuronal cell bodies located in ganglia.

Nerva fibers

The parasympathetic system also has two types of nerve
fibers:

Pre-ganglionic nerve fibers: These nerve fibers originate from the central nervous system and terminate at the ganglia of the parasympathetic nervous system. Contrary to the sympathetic nervous system, the pre-ganglionic fibers in PSNS are long.

Post-ganglionic nerve fibers: They originate from the ganglia of the parasympathetic nervous system and terminate at the target organs. The post-ganglionic fibers are short in case of the parasympathetic system because the ganglia are present near the target organs.

The parasympathetic system has craniosacral outflow.
The pre-ganglionic nerve fibers originate from the nuclei in the brain and the
sacral segments of the spinal cord and terminate in the ganglia.

Ganglia

Parasympathetic Nervous System (PSNS) | Function & Definition (2)

Ganglia are the collections of neuronal cell bodies located outside the CNS. As stated earlier, the parasympathetic system has longer preganglionic fibers because the ganglia are present away from the CNS. They are present near the effector organs.

Examples of the parasympathetic ganglia include
ciliary ganglia, submandibular ganglia, etc. Ciliary ganglion controls the
muscles of the eye and is present just behind the eyeball. The submandibular
ganglia control the secretions of salivary glands and are present in the close
proximity to these glands.

Neurotransmitters

Parasympathetic Nervous System (PSNS) | Function & Definition (3)

These are the chemicals released by the axons at the nerve terminals. They bind to the specific receptors present on the target tissue and initiate chemical responses.

The main neurotransmitter present in the parasympathetic system is acetylcholine. Acetylcholine is released by both preganglionic and postganglionic nerve fibers. Thus, it acts on both the neurons present in the ganglia as well as tissues present in the target organ.

Acetylcholine performs its function by binding to the
specific receptors called cholinergic receptors.

Receptors

Parasympathetic Nervous System (PSNS) | Function & Definition (4)
(Video) Neurology | Parasympathetic Nervous System

The parasympathetic system also contains two types of receptors. The neurotransmitter for both these receptors is the same, i.e. acetylcholine.

Ganglionic receptors: These are the nicotinic receptors present on the neuronal cell bodies in the ganglia of the parasympathetic system.

Target receptors: These are the muscarinic receptors activated by acetylcholine. Nicotine can’t activate these receptors. Depending on their response to activation, they are further divided into three types:

M1 receptors: These are the inhibitory receptors. They are present in heart especially on SA node and AV node. Activation of M1 receptors decrease heart rate. It does not have any effect on the force of contraction.

M2 receptors: These receptors are stimulatory in nature. They are present in the CNS. We have included them in the discussion of the parasympathetic nervous system because they are also activated by acetylcholine. The drugs that modify the activity of PSNS can also act on these receptors.

M3 receptors: These receptors are stimulatory in nature. They are present in smooth muscles of blood vessels, bronchi, gastrointestinal tract, bladder, eyes, etc. The activation of these receptors can cause relaxation or contraction of smooth muscles, depending on their location.

Effects

The parasympathetic nervous system is active all the time. It controls many important functions of the body. It is the most important component of the nervous system involved in regulating body functions in resting state. Below we will give an account of different processes in the body that are under the control of parasympathetic nervous system.

Blood Pressure:

Parasympathetic Nervous System (PSNS) | Function & Definition (5)

The parasympathetic system is important in regulating the blood pressure under resting conditions. It prevents any abnormal increase in blood pressure. If the blood pressure increases due to any reason, it is sensed by the baroreceptor system.

The baroreceptor reflex stimulates the parasympathetic system. The PSNS causes relaxation of blood vessels, decreasing total peripheral resistance. It also decreases heart rate. As a result, the blood pressure comes back to the normal level.

Heart Rate:

Parasympathetic Nervous System (PSNS) | Function & Definition (6)

Under resting conditions, heart rate is under the control of parasympathetic nervous system. It prevents any abnormal increase in heart rate. A balance between sympathetic and parasympathetic stimulation of the cardiac cells keep the heart rate within normal limits.

The parasympathetic system may be artificially
stimulated to decrease heart rate in conditions like surgery etc. This can be
done by giving cholinergic drugs or by massaging the neck of the person. The
massage of neck stimulates the baroreceptors that increase parasympathetic
stimulation of the heart, thus decreasing the heart rate.

(Video) The Autonomic Nervous System: Sympathetic and Parasympathetic Divisions

Respiration

Parasympathetic Nervous System (PSNS) | Function & Definition (7)

The activation of M3 receptors present in bronchi causes contraction of smooth muscles. As a result, the diameter of the airways decreases, causing bronchiolar constriction.

The parasympathetic stimulation also increases the
secretions of glands present in the respiratory tract. The resultant increase
in mucous further blocks the air passages. The resultant effect of
parasympathetic stimulation is to inhibit the process of breathing.

Digestion

Parasympathetic Nervous System (PSNS) | Function & Definition (8)

The parasympathetic activity increases the contraction of smooth muscles present in the wall of the gut. Thus, it promotes peristalsis and the process of digestion.

The parasympathetic system also promotes the
secretions of different glands associated with the alimentary tract.

The smooth muscles present in sphincters are inhibited
by the parasympathetic system. It causes opening of the sphincters.

The overall effect of parasympathetic system is to
promote the process of digestion.

Urination:

The process of urination is also stimulated by the parasympathetic system. The parasympathetic stimulation contracts the bladder muscles and relaxes the smooth muscles present in sphincters. The combined effect helps in voiding the bladder.

Secretions

The secretions of the sweat glands as well as the
glands associated with the digestive tract, respiratory tract, etc. are under
the control of parasympathetic system. It promotes the secretions of all these
glands.

Sexual Response:

The activation of the parasympathetic system causes
erection.

Pupillary Response:

The muscarinic receptors are present in the ciliary
muscles as well as the circular muscles of the eye. The activation of
muscarinic receptors by parasympathetic activity causes smooth muscle
contraction. The contraction of circular muscles constricts the pupil while
contraction of ciliary muscles cause accommodation for near vision.

(Video) Sympathetic versus Parasympathetic Nervous System | Nervous System

Disorders affecting the activity of Parasympathetic system

Parasympathetic Nervous System (PSNS) | Function & Definition (10)

The activity of the parasympathetic system is either
increased or decreased in the following disorders:

  • Hypertension
  • Heart failure
  • Organophosphate poisoning
  • Orthostatic hypotension
  • Erectile dysfunction
  • Neurogenic bladder

Drugs

Parasympathetic Nervous System (PSNS) | Function & Definition (11)

The drugs acting on the parasympathetic system are
classified into two broad categories:

Cholinomimetic Drugs: These drugs act by binding to the muscarinic receptors and activating them. This category also includes the drugs that increase the levels of acetylcholine in nerve synapses. They are used to treat conditions like Parkinson’s disease, myasthenia gravis, atropine toxicity, etc.

Anti-cholinergic drugs: These drugs either act by blocking the muscarinic receptors or by decreasing the available amount of acetylcholine. They are used in organophosphate poisoning, asthma, heart failure, etc.

Conclusion/Summary

The parasympathetic system is the division of the autonomic
nervous system that regulates important body functions at rest. It is
responsible for the maintenance of vital functions under calm situations.

It consists of long preganglionic fibers and short
postganglionic fibers. The ganglia of the parasympathetic system are present
near the target organ.

Like the sympathetic system, the ganglia of
parasympathetic system also have nicotinic receptors. However, the receptors in
the target tissue are different. These include:

  • M1 receptors (in heart)
  • M2 receptors (in CNS)
  • M3 receptors (in smooth muscles and
    glands)

All the receptors in the parasympathetic system are
activated by acetylcholine, the main neurotransmitter present in the
parasympathetic system.

All the vital functions of the body are regulated
through parasympathetic system in the resting state. These include:

  • Blood pressure
  • Heart rate
  • Breathing
  • Digestion
  • Secretions
  • Urination
  • Sexual responses
  • Pupillary response

The activity of the parasympathetic system may be
increased or decreased in different disorders. The abnormalities of the system
itself can also give rise to a number of disorders.

The drugs acting on the parasympathetic system act by:

(Video) Autonomic Nervous System: Sympathetic vs Parasympathetic, Animation

  • Stimulating muscarinic receptors
  • Inhibiting receptors
  • Increasing the amount of available
    acetylcholine
  • Decreasing the amount of
    acetylcholine

References

  1. Langley, John Newport (1921).The Autonomic Nervous System. Cambridge: Heffer. p.10.ISBN9781152710191.
  2. Pocock, Gillian (2006).Human Physiology(3rd
    ed.). Oxford University Press. pp.63–64.
    ISBN978-0-19-856878-0.
  3. McCorry, LK (Aug 15, 2007)."Physiology of the autonomic nervous system".American Journal of Pharmaceutical
    Education.71(4): 78.
    doi:10.5688/aj710478.PMC1959222.PMID17786266.
  4. "visceral nerve fibers - definition of visceral nerve
    fibers in the Medical dictionary - by the Free Online Medical Dictionary,
    Thesaurus and Encyclopedia"
    .
    Medical-dictionary.thefreedictionary.com. Retrieved2012-07-06.
  5. "Visceral nerve - RightDiagnosis.com". Wrongdiagnosis.com. 2012-02-01.
    Retrieved2012-07-06.
  6. "The Vertebral Column and Spinal Cord". www.emery.edu. 1997-08-21. Retrieved2013-03-21.
  7. Moore, Keith L.; Agur, A. M. R. (2007).Essential Clinical Anatomy(3rd ed.). Lippincott Williams &
    Wilkins.
    ISBN978-0-7817-6274-8.

FAQs

Which is the best definition of the parasympathetic division? ›

parasympathetic nervous system, division of the nervous system that primarily modulates visceral organs such as glands. The parasympathetic system is one of two antagonistic sets of nerves of the autonomic nervous system; the other set comprises the sympathetic nervous system.

What is parasympathetic nervous system PDF? ›

Description. The parasympathetic nervous system is one of two main branches or subsystems of the. autonomic nervous system, the physical system responsible for nonconsciously maintaining bodily. homeostasis and coordinating bodily responses.

Where is the parasympathetic nervous system? ›

The parasympathetic nervous system is also called the craniosacral division of the ANS, as its central nervous system components are located within the brain and the sacral portion of the spinal cord.

What are the 4 parasympathetic nerves? ›

The cranial nerves involved in the parasympathetic nervous system are the oculomotor, facial, glossopharyngeal, and vagus nerves.

What is the definition of parasympathetic nervous system? ›

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.

What is parasympathetic function? ›

The parasympathetic nervous system predominates in quiet “rest and digest” conditions while the sympathetic nervous system drives the “fight or flight” response in stressful situations. The main purpose of the PNS is to conserve energy to be used later and to regulate bodily functions like digestion and urination.[1]

What is difference between parasympathetic and sympathetic? ›

What is the major difference between the parasympathetic and sympathetic nervous systems? The parasympathetic nervous system restores the body to a calm and composed state and prevents it from overworking. The sympathetic nervous system, on the other hand, prepares the body for fight and flight response.

What activates the parasympathetic nervous system? ›

There are many ways to practice using your parasympathetic nervous system. These include mild exercise, meditation, yoga, deep breathing from your diaphragm, even nature walks. For some people, traditional meditation isn't their thing. It's about finding your body's way of meditating, what helps you to decompress.

Which nerve has parasympathetic control of the heart? ›

Parasympathetic innervation to the heart and lungs is provided by the vagus nerve (CN X). Cardiac preganglionic fibers originate in the brain stem medulla.

Is parasympathetic voluntary? ›

The parasympathetic nervous system is a subdivision of the autonomic nervous system (ANS) which regulates bodily functions which are outside of voluntary control, therefore being automatic.

What hormones does the parasympathetic nervous system release? ›

The parasympathetic nervous system (PNS) releases the hormone acetylcholine to slow the heart rate. Such factors as stress, caffeine, and excitement may temporarily accelerate your heart rate, while meditating or taking slow, deep breaths may help to slow your heart rate.

Does parasympathetic increase heart rate? ›

The sympathetic nervous system increases heart rate, whereas the parasympathetic nervous system suppresses it.

How many parasympathetic nerves are there? ›

Three spinal nerves in the sacrum (S2-4), commonly referred to as the pelvic splanchnic nerves, also act as parasympathetic nerves.
...
Parasympathetic nervous system
MeSHD010275
TA98A14.3.02.001
TA26661
FMA9907
6 more rows

What is sympathetic and parasympathetic examples? ›

The sympathetic system controls “fight-or-flight” responses. In other words, this system prepares the body for strenuous physical activity. The events that we would expect to occur within the body to allow this to happen do, in fact, occur. The parasympathetic system regulates “rest and digest” functions.

What are the functions of sympathetic nervous system? ›

Your sympathetic nervous system is best known for its role in responding to dangerous or stressful situations. In these situations, your sympathetic nervous system activates to speed up your heart rate, deliver more blood to areas of your body that need more oxygen or other responses to help your get out of danger.

What do sympathetic and parasympathetic divisions refer to? ›

The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic system is associated with the fight-or-flight response, and parasympathetic activity is referred to by the epithet of rest and digest. Homeostasis is the balance between the two systems.

Which are effects of the parasympathetic division on the digestive system? ›

The parasympathetic nervous system is responsible for the body's rest and digestion response when the body is relaxed, resting, or feeding. It basically undoes the work of sympathetic division after a stressful situation. The parasympathetic nervous system decreases respiration and heart rate and increases digestion.

What is sympathetic division? ›

sympathetic nervous system, division of the nervous system that functions to produce localized adjustments (such as sweating as a response to an increase in temperature) and reflex adjustments of the cardiovascular system.

Which anatomical description is true of the parasympathetic division of the autonomic nervous system quizlet? ›

Which anatomical description is true of the parasympathetic division of the autonomic nervous system? The preganglionic fibers and postganglionic fibers synapse in the ganglia, which are located in the tissues of the target organs.

Is parasympathetic voluntary? ›

The parasympathetic nervous system is a subdivision of the autonomic nervous system (ANS) which regulates bodily functions which are outside of voluntary control, therefore being automatic.

How do you activate the parasympathetic nervous system? ›

Activate your parasympathetic nervous system with these simple techniques
  1. Reduce stress. Stress can seem unavoidable for the most of us. ...
  2. Meditation. ...
  3. Massage. ...
  4. Yoga. ...
  5. Nutrition. ...
  6. Exercise. ...
  7. Osteopathy. ...
  8. Get enough sleep.
19 Jan 2021

What is the meaning of PNS? ›

Your peripheral nervous system (PNS) is that part of your nervous system that lies outside your brain and spinal cord. It plays key role in both sending information from different areas of your body back to your brain, as well as carrying out commands from your brain to various parts of your body.

What is an example of a parasympathetic response? ›

Examples of parasympathetic responses

Salivation: As part of its rest-and-digest function, the PSNS stimulates production of saliva, which contains enzymes to help your food digest. Lacrimation: Lacrimation is a fancy word for making tears. Tears keep your eyes lubricated, preserving their delicate tissues.

What hormones are released by the parasympathetic nervous system? ›

The parasympathetic nervous system (PNS) releases the hormone acetylcholine to slow the heart rate. Such factors as stress, caffeine, and excitement may temporarily accelerate your heart rate, while meditating or taking slow, deep breaths may help to slow your heart rate.

What happens if the parasympathetic nervous system is damaged? ›

It can affect blood pressure, temperature control, digestion, bladder function and even sexual function. The nerve damage affects the messages sent between the brain and other organs and areas of the autonomic nervous system. These areas include the heart, blood vessels and sweat glands.

What are 3 autonomic nervous system? ›

The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric.

What is a sympathetic nerve? ›

Your sympathetic nervous system is a network of nerves that helps your body activate its “fight-or-flight” response. This system's activity increases when you're stressed, in danger or physically active.

What are the receptors of the parasympathetic nervous system? ›

Receptors. The parasympathetic nervous system uses chiefly acetylcholine (ACh) as its neurotransmitter, although peptides (such as cholecystokinin) can be used. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors.

Which neurotransmitter is released from the parasympathetic ganglion? ›

Acetylcholine released from all parasympathetic postganglionic neurons and some sympathetic postganglionic neurons traveling to sweat glands binds to these receptors.

Where are ganglia of the parasympathetic division located? ›

Parasympathetic ganglia

Those ganglia can be found both in head and neck (and they are part of the cranial nerves) and in the trunk, close to the thoracic and abdominal/pelvic organs. Their preganglionic neurons are located in the cranial nuclei of the brainstem, and in the lateral horn of the sacral spinal cord.

Which of the following is not controlled by the ANS? ›

Answer and Explanation: The organs that are not controlled by the autonomic nervous system are the skeletal muscles. Unlike the organs of the autonomic nervous system, such as the heart, kidneys, and liver, the skeletal muscles are under conscious control.

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