Table of Contents
The parasympathetic nervous system, also known as the “rest and digest” system, is a crucial part of the body’s overall physiological response to stress. It is one of the chief divisions of the autonomic nervous system and is concerned with involuntary body processes.
This system works in opposition to the sympathetic nervous system, which is responsible for the “fight or flight” response to stress. While the sympathetic nervous system activates during times of stress, the parasympathetic nervous system helps to return the body to a state of balance and relaxation.
The parasympathetic nervous system is made up of a network of nerves that originate in the brainstem and spinal cord and extend to various parts of the body. These nerves are responsible for controlling and regulating many of the body’s internal functions, including heart rate, blood pressure, digestion, respiration, blood pressure, etc.
The main function of the parasympathetic nervous system is to slow down the body’s physiological response to stress and normalize the body’s processes when the stress is over. When the body is under stress, the sympathetic nervous system is activated, increasing heart rate, blood pressure, and other physiological responses to either fight against stress or flight away. In contrast, the parasympathetic nervous system works to oppose these effects and bring the physiological processes toward a relaxed phase.
Anatomy of the parasympathetic nervous system
The parasynthetic nervous system is also known as the craniosacral division of the autonomic nervous system as its components arise from the brainstem and sacral portion of the spinal cord. It has craniosacral outflow. The cranial outflow is through the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve while sacral outflow is through the pelvic splanchnic nerves arising from the sacral segments of the spinal cord.
The oculomotor nerve gives parasympathetic supply to the iris and ciliary muscles, the facial nerve gives parasympathetic supply to lacrimal, nasal, palatine, pharyngeal, and submandibular glands, the glossopharyngeal nerve gives parasympathetic supply to the parotid gland while vagus nerve is concerned with heart, trachea, bronchi, lungs, liver, gallbladder, stomach, pancreas, kidneys, small intestine and proximal part of the large intestine. Pelvic splanchnic nerves give parasympathetic supply to the distal part of the large intestine including descending colon, sigmoid colon, rectum, as well as bladder, penis, and clitoris.
The parasympathetic nervous system comprises several neuronal pathways that connect the craniosacral component with the target tissues. Each parasympathetic pathway has presynaptic or preganglionic fibers and postsynaptic or postganglionic fibers. Preganglionic fibers have their cell bodies located in the medulla oblongata or sacral segments of the spinal cord. These neurons give long axons that travel toward the postsynaptic neurons and release acetylcholine at their synapses. The postsynaptic neuronal cell bodies are located in the parasympathetic ganglia found near the target organs. As postsynaptic neurons are near the target organs, postganglionic fiber length is significantly shorter than the presynaptic fibers originating from the CNS.
Receptors of the parasympathetic nervous system
The receptors stimulated by acetylcholine are cholinergic receptors. They are distributed in almost every part of the body. There are two main types of cholinergic receptors involved in parasympathetic transmission.
- Muscarinic receptors
- Nicotinic receptors
Muscarinic receptors are G protein-coupled receptors stimulated by acetylcholine. They may increase or decrease a physiological process depending on the signaling cascade involved. Among the subtypes of muscarinic receptors, M1, M2, and M3 are the most important ones regarding parasympathetic transmission. Muscarinic stimulation causes:
- Accommodation to near vision
- Decrease in heart rate
- Decrease in conduction velocity
- GIT cramps, diarrhea
- Increased secretions of glands
- Urinary incontinence etc.
Nicotinic receptors are directly coupled to Na/K ion channels and don’t involve the secondary messenger system. These include NN and NM nicotinic receptors. They are found in the autonomic ganglia, adrenal medulla, and neuromuscular junction. Their stimulation causes:
- Increased secretion of catecholamines
- General ganglionic stimulation
- Contraction of skeletal muscles
How does the parasympathetic system work?
The parasympathetic nervous system accomplishes its role through the release of the neurotransmitter acetylcholine. Acetylcholine is a small, rapidly-acting neurotransmitter that is synthesized in the nerve terminals and stored in the vesicles. Upon stimulation, it is released immediately and produces an instant response by stimulating either muscarinic or nicotinic receptors or both.
Acetylcholine is a chemical messenger that helps to slow down heart rate, lower blood pressure, and stimulate digestion. This neurotransmitter is also responsible for controlling the body’s smooth muscle, which is responsible for regulating the diameter of blood vessels and airways.
Effects of the parasympathetic nervous system
The individual effects of PSNS on different body organs are as under:
Effects on GIT
One of the key functions of the parasympathetic nervous system is to control and regulate the functioning of the body’s digestive system. The parasympathetic nervous system is responsible for stimulating the digestive system and increasing the production of stomach acid and enzymes involved in digestion. The sympathetic nervous system contracts the blood vessels of GIT and diverts the blood flow towards muscles to cope with the emergency situation. It decreases the GIT motility and intestinal absorption of nutrients. The parasympathetic nervous system restores GIT motility after the stress is over and promotes food digestion and absorption.
Effects on the immune system
The parasympathetic nervous system also plays a role in regulating the body’s immune function. When the body is under stress, the sympathetic nervous system is activated, releasing the hormone cortisol, a glucocorticoid. Cortisol is responsible for suppressing the immune system by interfering with prostaglandins synthesis, thus making the body more susceptible to illness and infection. The parasympathetic nervous system works to counteract these effects and improves the immune response.
Effect on the sleep-wake cycle
The parasympathetic nervous system also plays a role in regulating the body’s sleep-wake cycle. When the body is under stress, the release of adrenaline and norepinephrine is increased. These hormones increase the basal metabolic rate and interfere with the sleep-wake cycle and can lead to insomnia. The parasympathetic nervous system, on the other hand, works to counteract these effects by releasing the hormone acetylcholine, which helps to promote relaxation.
Effects on the sexual system
The parasympathetic nervous system also plays a role in regulating the body’s sexual response. Adrenaline and other stress hormones are responsible for suppressing the sexual response, making it more difficult to achieve an erection or orgasm. The parasympathetic nervous system counteracts these effects and promotes sexual arousal. It is the parasympathetic nervous system that causes vasodilatation of the penile vasculature and increases blood flow to the erectile tissue to cause the erection of the penis.
Effects on blood vessels
The parasympathetic nervous system also plays an important role in regulating vascular tone. This helps to maintain normal blood flow in several body organs. The sympathetic nervous system constricts the blood vessels, especially of GIT mucosa and skin surface, and diverts blood to the muscles for their contractility. This process increases blood pressure due to an increase in peripheral resistance. The parasympathetic nervous system works to oppose these changes and normalizes the vessels’ diameter.
Effects on the eyes
The parasympathetic nervous system plays a crucial role in the functioning of the eyes. It is responsible for controlling the size of the pupils, the amount of light entering the eyes, and the ability to focus on near objects called accommodation.
The pupils are the small circular openings in the center of the eye that allow light to enter the eyes. The size of the pupils is controlled by the iris, a ring of muscle that surrounds the pupils. The parasympathetic nervous system sends signals to the iris to constrict the pupils, which helps to reduce the amount of light entering the eyes and improve visual clarity. This response occurs as a result of muscarinic receptor stimulation by acetylcholine.
The parasympathetic nervous system also plays a role in the ability to focus on near objects. The eye’s lens is responsible for focusing light on the retina which is the part of the eye that detects light and sends signals to the brain. The lens curvature and accommodation are controlled by the ciliary muscle, which is controlled by the parasympathetic nervous system. When the parasympathetic nervous system sends signals to the ciliary muscle, it contracts and causes the lens to change shape and focus on a near object. This is known as accommodation and allows us to see objects clearly at different distances.
In addition to these functions, the parasympathetic nervous system also plays a role in the protection of the eyes. The lacrimal gland, located in the inner corner of the eye, produces tears which act as a lubricant and protect the eyes from dust, dirt, and other foreign particles. The parasympathetic nervous system sends signals to the lacrimal gland to produce tears and keep the eyes moist and healthy. That’s why poisons that increase parasympathetic activity such as organophosphates also cause increased lacrimation.
The parasympathetic nervous system also regulates blood flow to the eyes. The ophthalmic artery, a branch of the internal carotid artery, supplies blood to the eyes. The parasympathetic nervous system sends signals to the ophthalmic artery to decrease the vascular diameter and reduce the flow of blood to the eyes. This helps to reduce the amount of pressure on the eyes and prevents damage to the retina.
Overall, the parasympathetic nervous system plays a vital role in maintaining the health and function of the eyes. It controls the size of the pupils, the ability to focus on objects, and the production of tears, as well as regulating the blood flow to the eyes. When the parasympathetic nervous system is functioning properly, the eyes are able to function normally and effectively. However, if it is disrupted, it may lead to various eye disorders like mydriasis, accommodative dysfunction, and dry eyes. Overstimulation of PSNS also causes disturbing symptoms.
Effects on cardiac muscles
The parasympathetic system affects cardiac muscles greatly. When the body is under the condition of stress, adrenaline, and norepinephrine are released into circulation. These chemicals act on the adrenergic receptors in the heart and increase the heart rate as well as the strength of cardiac contraction. Moreover, they increase peripheral vascular resistance as well by promoting vasoconstriction. All these parameters increase the blood pressure and load on the heart. The parasympathetic system counters these effects and releases acetylcholine which acts on the muscarinic receptors in heart muscles and decreases the activity of the sinoatrial node and conduction across the atrioventricular node. Moreover, it also promotes the relaxation of blood vessels at some locations, decreasing the total peripheral resistance. All these parameters collectively bring blood pressure towards normal and reduce cardiac load.
Effects on trachea and bronchi
The parasympathetic nervous system causes bronchospasm and increased secretions in the trachea & bronchi by stimulation of muscarinic receptors.
When the PSNS is activated, it sends signals to the trachea and bronchi to constrict. This constriction reduces the diameter of airways, making it more difficult for air to pass through. It is termed bronchospasm and increases the respiratory effort.
The parasympathetic nervous system also promotes the production of mucus in the trachea and bronchi. Mucus is a thick, sticky fluid that lines the airways and helps to trap particles, such as dust and bacteria that could otherwise cause irritation or infection. In this way, PSNS plays a protective role in the trachea and bronchi. However, excessive PSNS stimulation leads to increased production of mucus in the respiratory tract which compromises breathing capability.
Due to these effects of PSNS on the respiratory tract, excessive PSNS stimulation is distressing to patients with asthma, COPD, emphysema, and other respiratory disorders.
Under activity and overactivity of the parasympathetic nervous system
Underactivity or overactivity of PSNS causes several abnormalities in the functioning of the body organs.
Underactivity of the parasympathetic nervous system causes decreased lacrimation, decreased salivation, mydriasis, tachycardia, constipation, and urinary retention, etc. while overactivity of PSNS leads to bradycardia, excessive salivation, increased lacrimation, excessive sweating, urinary incontinence, diarrhea, blood pressure drop, abdominal cramps, skeletal muscle stimulation followed by receptor desensitization and paralysis.
Drugs affecting the parasympathetic nervous system
Two classes of drugs alter the activity of the parasympathetic nervous system. These are parasympathomimetic drugs which increase the activity of PSNS and anticholinergic drugs which decrease the activity of PSNS.
Parasympathomimetics are used in the treatment of ileus, glaucoma, xerostomia, Alzheimer’s disease, myasthenia gravis, etc. Anticholinergic drugs are useful in organophosphate poisoning, asthma, COPD, motion sickness, overactive bladder, etc.
- The parasympathetic nervous system is the chief division of the autonomic nervous system concerned with the resting stage of the body. It controls the autonomic processes of the body and brings them toward the resting phase.
- When our body experiences a stressful state, the sympathetic system is activated which prepares the body to cope with the emergency situation. SNS increases heart rate, inhibits digestion, dilates pupils, increases blood flow to muscles, causes peripheral vasoconstriction, and raises blood pressure. PSNS works opposite to the SNS and normalizes all the physiological responses of the body when stress is over. It decreases heart rate, promotes digestion, increases GIT motility, and causes constriction of pupils.
- PSNS is also known as the craniosacral division of the autonomic nervous system due to its craniosacral outflow. The oculomotor nerve, facial nerve, glossopharyngeal nerve, vagus nerve, and sacral segments of the spinal cord transmit the preganglionic fibers. Postganglionic fibers of PSNS are shorter than preganglionic fibers as ganglia are located near the target tissues.
- The neurotransmitter involved in parasympathetic transmission is acetylcholine, a small rapidly acting neurotransmitter that activates postsynaptic muscarinic or nicotinic receptors. Muscarinic receptors are G protein-coupled receptors while nicotinic receptors are linked to Na/K ion channels. They principally affect the GIT tract, cardiac muscles, eyes, lacrimal glands, salivary glands, sweat glands, and urinary bladder.
- Muscarinic activation causes meiosis, accommodation to near vision, bradycardia, reduced secretions of salivary glands, sweat glands & lacrimal glands, urinary incontinence, diarrhea, etc.
- Nicotinic receptors are present in the autonomic ganglia; at neuromuscular junctions and in the adrenal medulla.
- Activation of nicotinic receptors causes general ganglionic stimulation, catecholamine release, and skeletal muscle contraction.
- Underactivity or overactivity of PSNS disrupts the autonomic processes of the body causing a variety of symptoms.
- These symptoms can be treated by parasympathomimetic drugs or anticholinergic drugs.
- Parasympathomimetics increase the activity of PSNA and are used o treat myasthenia gravis, ileus, urinary retention, Alzheimer’s disease, etc.
- Anticholinergic drugs decrease the activity of PSNS and are useful in organophosphate poisoning, diarrhea, motion sickness, asthma, COPD, etc.
LeBouef T, Yaker Z, Whited L. Physiology, Autonomic Nervous System. [Updated 2022 May 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538516/
Tindle J, Tadi P. Neuroanatomy, Parasympathetic Nervous System. [Updated 2022 Oct 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK553141/