What is the Parasympathetic Nervous System?
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
1. What is the primary function of the Parasympathetic Nervous System (PNS)?
The primary function of the Parasympathetic Nervous System is to regulate the body's unconscious actions, often referred to as the 'rest and digest' state. It works to conserve energy by slowing the heart rate, increasing intestinal and glandular activity, and relaxing sphincter muscles in the gastrointestinal tract. It essentially counteracts the 'fight or flight' response of the sympathetic nervous system to maintain homeostasis.
2. How does the parasympathetic nervous system differ from the sympathetic nervous system?
The key difference lies in their opposing roles. The parasympathetic nervous system promotes 'rest and digest' functions to conserve energy, active during calm states. In contrast, the sympathetic nervous system triggers the 'fight or flight' response during stressful situations, preparing the body for intense physical activity by increasing heart rate and releasing adrenaline. They work in tandem to maintain the body's internal balance.
3. What are some common examples of parasympathetic responses in the body?
Common parasympathetic responses are focused on conserving energy and aiding in bodily maintenance. Examples include:
- Slowing of the heartbeat (bradycardia).
- Constriction of the pupils (miosis) to limit light intake.
- Stimulation of saliva and tear production.
- Increased digestion through enhanced stomach motility and enzyme secretion.
- Contraction of the bladder to facilitate urination.
4. What is the role of acetylcholine in the parasympathetic nervous system?
Acetylcholine (ACh) is the chief neurotransmitter of the parasympathetic nervous system. It is released by both preganglionic and postganglionic parasympathetic neurons. When ACh binds to its receptors on target organs (like the heart, stomach, or bladder), it transmits the signal that initiates the 'rest and digest' response, such as slowing the heart or stimulating digestion.
5. Which major nerves are associated with the parasympathetic nervous system?
The parasympathetic nervous system's fibres originate in the brainstem and the sacral region of the spinal cord. Four key cranial nerves carry these fibres:
- Oculomotor Nerve (CN III): Controls pupil constriction.
- Facial Nerve (CN VII): Stimulates salivary and lacrimal (tear) glands.
- Glossopharyngeal Nerve (CN IX): Also involved in stimulating salivary glands.
- Vagus Nerve (CN X): The most significant, innervating the heart, lungs, and most of the digestive tract.
6. Why is the parasympathetic system's response often slower and more localised than the sympathetic system's?
The parasympathetic response is more targeted and slower because its postganglionic neurons are very short and located near or within the target organ. This allows for precise, organ-specific actions (e.g., only affecting the bladder). In contrast, the sympathetic system triggers a widespread, rapid response because a single preganglionic neuron can activate multiple postganglionic neurons simultaneously, preparing the entire body for immediate action.
7. How can activities like deep breathing help activate the parasympathetic nervous system?
Activities like deep, diaphragmatic breathing and meditation directly stimulate the vagus nerve, the main highway of the parasympathetic nervous system. This stimulation sends signals to the brain and heart to slow down, reduce blood pressure, and decrease the production of stress hormones like cortisol. By consciously controlling your breath, you can actively shift your body from a sympathetic (stressed) state to a parasympathetic (relaxed) state.
