What is the Parasympathetic Nervous System and How Does It Work
Let's go through the parasympathetic nervous system definition. The parasympathetic nervous system is primarily made up of the cranial and sacral spinal nerves. Preganglionic neurons from the brain or sacral spinal cord synapse with only a few postganglionic neurons located in or near the effector organ (muscle or gland). When the body is relaxed, resting, or feeding, the parasympathetic nervous system is in charge of rest and digestion. After a stressful situation, it basically undoes the work of sympathetic division. The parasympathetic nervous system slows respiration and heart rate while speeding up digestion. Stimulation of the parasympathetic nervous system results in the following given below :
Contraction of pupils
Lower heart rate and blood pressure
Improved digestion
Increased saliva and mucus production
Urine secretion increases
Parasympathetic Nervous System Function
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Your PSNS originates in your brain and spreads out through long fibres that connect with special neurons near the organ on which they intend to act. Once PSNS signals reach these neurons, they travel only a short distance to their respective organs. Examples of the areas the PSNS acts on include the following:
eyes
lacrimal glands are responsible for producing tears
parotid glands are responsible for producing produce saliva
salivary glands responsible for producing saliva
nerves in the stomach as well as trunk
nerves that go to the bladder
nerves as well as blood vessels responsible for the male erection
The PSNS is a "business as usual" system that keeps your body's basic functions functioning normally.
Parasympathetic Nerves
The cranial nerves are known as paired nerves and these nerves are responsible for many movements and sensations that take place in your body’s head and neck. The nerves all start in the brain. There are a total of 12 cranial nerves labelled using Roman numerals from I to XII, with the first set of nerves located at the brain’s front.
Major Cranial Nerves
III. Oculomotor nerve is the nerve that aids in constricting the pupil, making it appear smaller.
VII. Facial nerve is the nerve that regulates saliva and mucus secretions in the mouth and nose, respectively.
IX. Glossopharyngeal nerve is the nerve that connects to the parotid salivary glands, which produce extra saliva for the tongue and elsewhere.
X. Vagus nerve. This nerve is responsible for an estimated 75% of all parasympathetic nerve fibres in the body. The stomach, kidneys, liver, pancreas, gallbladder, bladder, anal sphincter, vagina, and penis all have branches from this nerve.
Parasympathetic Stimulation - The Parasympathetic Nervous System and the Vagus Nerve
There is no conversation about the parasympathetic nervous system which is complete without mentioning the vagus nerve.
The vagus nerve is the longest in the autonomic nervous system, reaching nearly every major system in the body. It also serves as the primary component of the parasympathetic nervous system. That is why vagus nerve stimulation (VNS) is known to be an excellent way to activate the parasympathetic nervous system and aid in relaxation.
Examples of Parasympathetic Activity and Responses
The term SLUDD is an easy acronym to remember how and where the PSNS works. This abbreviation stands for the following:
Salivation: As part of its rest-and-digest function, the PSNS stimulates saliva production, which contains enzymes that aid in food digestion.
Lacrimation: Lacrimation is a fancy term for crying. Tears lubricate your eyes, protecting their delicate tissues.
Urination: The PSNS contracts the bladder, causing it to squeeze and urine to escape.
Digestion: The PSNS stimulates saliva production to aid digestion. It also causes peristalsis, or the movement of the stomach and intestines to digest food and release bile to allow the body to digest fats.
Defecation: The PSNS constricts the sphincters in the intestine and moves digested food material down the digestive tract, allowing a person to have proper bowel movements.
Parasympathetic Fibers
The parasympathetic nervous system derives its nerve fibres from the central nervous system. Several cranial nerves, including the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve, are examples of specific nerves. Three sacral spinal nerves (S2-4), also known as the pelvic splanchnic nerves, function as parasympathetic nerves.
The parasympathetic nervous system exits the central nervous system via cranial nerves (CN) III, VII, IX, and X. The vagus nerves (CN-X) contain approximately 80% or more of all parasympathetic nerve fibres, which pass to the heart, lungs, oesophagus, stomach, and small intestine, the proximal half of the colon, the liver, gallbladder, pancreas, as well as the upper portions of the colon and ureters.
The vagus nerves (CN-X) contain approximately 80% or more of all parasympathetic nerve fibres, which pass to the heart, lungs, oesophagus, stomach, and small intestine, the proximal half of the colon, the liver, gallbladder, pancreas, as well as the upper portions of the ureters.
FAQs on Parasympathetic Nervous System Structure and Role in the Body
1. What is the parasympathetic nervous system?
The parasympathetic nervous system is a division of the autonomic nervous system (ANS) that promotes the body's “rest and digest” activities. It conserves energy and maintains normal body functions during relaxed states.
- Slows the heart rate
- Stimulates digestion
- Promotes glandular secretion
- Supports energy storage
2. What is the main function of the parasympathetic nervous system?
The main function of the parasympathetic nervous system is to conserve energy and restore the body to a calm, resting state. It counteracts the effects of the sympathetic nervous system.
- Decreases heart rate
- Increases intestinal movement and digestion
- Stimulates saliva and digestive enzyme secretion
- Promotes urination and defecation
3. How does the parasympathetic nervous system work?
The parasympathetic nervous system works by sending nerve impulses from the brainstem and sacral spinal cord to target organs using specific neurotransmitters. It follows a two-neuron pathway:
- Preganglionic neurons originate in the brainstem or sacral spinal cord
- They synapse in parasympathetic ganglia near or within the target organ
- Postganglionic neurons release acetylcholine to produce effects
4. What neurotransmitter does the parasympathetic nervous system use?
The primary neurotransmitter of the parasympathetic nervous system is acetylcholine. It is released at both synapses in the parasympathetic pathway.
- Preganglionic neurons release acetylcholine onto nicotinic receptors
- Postganglionic neurons release acetylcholine onto muscarinic receptors in target organs
5. What is the difference between the sympathetic and parasympathetic nervous systems?
The sympathetic nervous system prepares the body for “fight or flight,” while the parasympathetic nervous system supports “rest and digest” functions. Key differences include:
- Sympathetic increases heart rate; parasympathetic decreases it
- Sympathetic inhibits digestion; parasympathetic stimulates digestion
- Sympathetic uses mainly norepinephrine; parasympathetic uses acetylcholine
6. Where does the parasympathetic nervous system originate?
The parasympathetic nervous system originates from the craniosacral region of the central nervous system. Its outflow includes:
- Cranial nerves III, VII, IX, and especially X (vagus nerve)
- Sacral spinal cord segments S2–S4
7. What organs are controlled by the parasympathetic nervous system?
The parasympathetic nervous system controls several vital organs involved in rest and maintenance functions. Major target organs include:
- Heart – slows heart rate
- Lungs – constricts bronchi
- Stomach and intestines – increases digestion
- Salivary glands – increases saliva production
- Urinary bladder – promotes urination
8. Why is the parasympathetic nervous system called the "rest and digest" system?
The parasympathetic nervous system is called the “rest and digest” system because it is most active during calm states and promotes digestion and recovery. Its effects include:
- Reduced heart rate and blood pressure
- Enhanced digestive enzyme secretion
- Increased intestinal peristalsis
- Energy storage through nutrient absorption
9. What is an example of parasympathetic response?
An example of a parasympathetic response is a decrease in heart rate after a stressful event has passed. Other examples include:
- Increased salivation while eating
- Stimulation of gastric juice secretion after a meal
- Constriction of pupils in bright light
10. How does the vagus nerve relate to the parasympathetic nervous system?
The vagus nerve (cranial nerve X) is the main nerve of the parasympathetic nervous system, supplying most thoracic and abdominal organs. It carries parasympathetic fibers to:
- Heart – slows heart rate
- Lungs – regulates bronchial activity
- Digestive organs – stimulates digestion and secretion
