Parasympathetic Nervous System: Function & Examples

NeurologyNursing/NCLEXUSMLE
Written By Simplico
parasympathetic nervous system function activation definition neurotransmitters examples

Parasympathetic Nervous System: This lecture reviews the parasympathetic nervous system definition, function, rest and digest response, activation, neurotransmitters involved, examples, and more!

Parasympathetic Nervous System

Parasympathetic Nervous System - Quick Notes:

  • Definition:
    The parasympathetic nervous system is a branch of the autonomic nervous system that regulates the body during times of rest, calmness, etc. It counteracts the sympathetic nervous system.

  • Function:
    The parasympathetic nervous system functions to produce physiological responses that promote relaxation, conservation of energy, etc.

  • Response:
    The parasympathetic response is called the “rest and digest” response.

  • Activation:
    The paraympathetic nervous system is activated during times of relaxation or rest, such as meditation, sleep, deep breathing, etc.

  • Neurotransmitters:
    The neurotransmitter involved in producing a parasympathetic response is acetylcholine.

  • Examples:
    Examples of the parasympathetic response include increased digestion, increased salivation, increased urination, miosis (pupil constriction), decreased heart rate, bronchoconstriction, etc.

  • Symptoms:
    Parasympathetic nervous system signs and symptoms include decreased heart rate, decreased respiratory rate, constricted pupils, etc.

  • Disorders:
    Conditions that can affect the parasympathetic nervous system include diabetes mellitus (autonomic neuropathy), autoimmune disorders, Parkinson’s disease, infections, etc.

Simplified:

The parasympathetic nervous system increases responses that are less critical for immediate survival (“digest”) - see below.

The parasympathetic nervous system decreases or relaxes responses that are critical for immediate survival (“rest”) - see below.

Examples:

Activation of the parasympathetic nervous system will increase (upregulate):

  • Salivation

  • Lacrimation

  • Digestion

  • Defecation

  • Urination

Activation of the sympathetic nervous system will decrease/normalize (downregulate):

  • Heart rate (bradycardia)

  • Cardiac output

  • Bronchi (bronchoconstriction)

  • Pupils (miosis)

The parasympathetic and sympathetic nervous system essentially counteract one another.

The parasympathetic nervous system comprises the physiological responses that occur when the body is at rest (rest and digest).

Alternatively, the sympathetic nervous system comprises the physiological responses that occur secondary to a stressful or dangerous situation (fight or flight).

Click here for the sympathetic lecture - Sympathetic Nervous System

Let’s now review the parasympathetic nervous system in more detail.

Parasympathetic Pathway

The parasympathetic pathway can be divided into 3 main parts:

  1. Preganglionic Neurons (presynaptic)

  2. Parasympathetic Ganglia

  3. Postganglionic Neurons (postsynaptic)

The parasympathetic neurons arising from the central nervous system and terminating at parasympathetic ganglia are called preganglionic neurons or presynaptic neurons.

The neurons originating in the parasympathetic ganglia and terminating at their target organs are called postganglionic neurons or postsynaptic neurons.

Preganglionic neurons synapse with postganglionic neurons in parasympathetic ganglia.

Let’s review each step below.

1. Preganglionic Parasympathetic Neurons

Preganglionic Parasympathetic Neurons - Quick Notes:

  • Arise from the central nervous system through the craniosacral region

  • Have long axons as they terminate in ganglia near their target organ

  • Release a neurotransmitter called acetylcholine

  • Are cholinergic neurons, since they release acetylcholine

Preganglionic parasympathetic neurons, also called presynaptic neurons, arise from the central nervous system through cranial nerves and sacral nerves (craniosacral region).

The 4 cranial nerves (CN) that have parasympathetic function include:

  1. Oculomotor Nerve (CN III)

    1. Parasympathetic activation of the oculomotor nerve leads to miosis (pupil constriction)

    2. Preganglionic parasympathetic neurons of the oculomotor nerve originate in the Edinger-Westphal nucleus (midbrain - brainstem)

  2. Facial Nerve (CN VII)

    1. The parasympathetic neurons of the facial nerve innervate the lacrimal, submandibular, sublingual, and nasal cavity glands to stimulate secretion

    2. Preganglionic parasympathetic neurons of the facial nerve originate in the superior salivatory nucleus (pons - brainstem)

  3. Glossopharyngeal Nerve (CN IX)

    1. The parasympathetic neurons of the glossopharyngeal nerve innervate the parotid gland

    2. Preganglionic parasympathetic neurons of the glossopharyngeal nerve originate in the inferior salivatory nucleus (medulla oblongata - brainstem)

  4. Vagus Nerve (CN X)

    1. The vagus nerve provides parasympathetic supply to the heart, lungs, esophagus, stomach, pancreas, gallbladder, liver, kidneys, small intestine, and part of the large intestine

    2. Preganglionic parasympathetic neurons of the vagus nerve originate in the dorsal motor nucleus of the vagus and the nucleus ambiguus (medulla oblongata - brainstem)

The sacral spinal nerves (S) that have parasympathetic function include:

  • S2-S4

  • Sacral spinal nerves S2-S4 give rise to pelvic splanchnic nerves

  • Pelvic splanchnic nerves provide parasympathetic innervation to pelvic organs including the bladder, ureters, prostate, uterus, vagina, and penis

Simplified: The parasympathetic nerve supply and their targets can broadly be remembered:

  1. Oculomotor (CN III) = Eyes

  2. Facial (CN VII) and Glossopharyngeal (CN IX) = Face

  3. Vagus (CN X) = Chest and Abdomen

  4. Pelvic Splanchnic Nerves = Pelvis

After arising from the central nervous system, preganglionic parasympathetic neurons travel to parasympathetic ganglia, where they synapse with postganglionic neurons (see below).

Preganglionic parasympathetic axons are generally long as they terminate in ganglia near their target.

Preganglionic neurons release acetylcholine onto postganglionic neurons.

The acetylcholine released by preganglionic neurons binds to nicotinic cholinergic receptors on postganglionic cell bodies.

The binding of acetylcholine to nicotinic receptors generates an action potential through the postganglionic neuron (see below).

2. Parasympathetic Ganglia

Preganglionic neurons synapse with postganglionic neurons in parasympathetic ganglia.

The parasympathetic ganglia are collections of postganglionic cell bodies that receive neuronal information from the preganglionic neurons.

Parasympathetic ganglia are typically located near or within their target organs.

Main parasympathetic ganglia include:

  1. Ciliary Ganglion - oculomotor nerve

    1. Preganglionic parasympathetic neurons of the oculomotor nerve synapse with postganglionic neurons in the ciliary ganglion

  2. Pterygopalatine and Submandibular Ganglia - facial nerve

    1. Preganglionic parasympathetic neurons of the facial nerve synapse with postganglionic neurons in the pterygopalatine and submandibular ganglia

  3. Otic Ganglion - glossopharyngeal nerve

    1. Preganglionic parasympathetic neurons of the glossopharyngeal nerve synapse with postganglionic neurons in the otic ganglion

  4. Terminal Ganglia - vagus nerve

    1. Preganglionic parasympathetic neurons of the vagus nerve synapse with postganglionic neurons in terminal ganglia near/within thoracic and abdominal target organs

  5. Intramural Ganglia - pelvic splanchnic nerves

    1. Preganglionic parasympathetic neurons of the pelvic splanchnic nerves synapse with postganglionic neurons in intramural ganglia located within the walls of pelvic target organs

    2. Intramural ganglia and pelvic splanchnic nerves travel through and are a part of plexuses, such as the inferior hypogastric plexus

Preganglionic neurons release acetylcholine onto postganglionic neurons in the parasympathetic ganglia.

This initiates an action potential (neuronal signal) through the postganglionic neurons to target organs (see below).

3. Postganglionic Parasympathetic Neurons

Postganglionic Parasympathetic Neurons - Quick Notes:

  • Originate in the parasympathetic ganglia (see above)

  • Have shorter axons compared to the preganglionic neurons, as they travel short distances to their target organs

  • Release a neurotransmitter called acetylcholine

  • Are cholinergic neurons, since they release norepinephrine

Postganglionic parasympathetic neurons originate in parasympathetic ganglia (see above).

They travel short distances to target organs.

Postganglionic neurons release acetylcholine onto target tissues and organs.

The acetylcholine released by postganglionic neurons binds to muscarinic cholinergic receptors on target organs.

When acetylcholine binds to a target organ, it causes a parasympathetic response in that particular organ.

Let’s review example parasympathetic responses below.

Parasympathetic Nervous System - Function & Examples

Postganglionic parasympathetic neurons release acetylcholine onto muscarinic receptors on target cells/organs (see above).

Activation of muscarinic receptors generates a parasympathetic nervous system response.

Example Parasympathetic Nervous System Responses:

  • Heart

    • Activation of muscarinic receptors in the heart (primarily M2 receptors) leads to a decrease in heart rate (bradycardia) and a decrease in the force of cardiac contraction

    • This allows the heart and body to counteract the sympathetic nervous system, conserve energy, and focus on other responses, such as digestion, during the parasympathetic rest and digest state

  • Lungs

    • Activation of muscarinic receptors in the lungs (primarily M3 receptors) leads to bronchoconstriction and mucus secretion

    • This provides a protective mechanism for the lungs and prevents harmful substances or pollutants to enter the lungs during the rest and digest state, a state in which the airways do not necessarily need to be dilated (as in a sympathetic response)

  • GI Tract

    • Activation of muscarinic receptors in the gastrointestinal tract (primarily M2 and M3 receptors) leads to GI smooth muscle contraction

    • This allows for increased digestion during the rest and digest state

  • Bladder

    • Activation of muscarinic receptors in the bladder (primarily M2 and M3 receptors) leads to detrusor smooth muscle contraction in the bladder

    • This allows for urination during the rest and digest state

  • Pupils

    • Activation of muscarinic receptors on the iris sphincter muscle (primarily M3 receptors) leads to pupil constriction, called miosis

    • Pupil constriction allows for less light to enter the eye, protecting the retina and enhancing near vision during a rest and digest state

  • Salivary and Lacrimal Glands

    • Activation of muscarinic receptors on salivary and lacrimal glands (primarily M3 receptors) leads to salivary and tear production, respectively.

    • Production of saliva aids in digestion during the rest and digest state, and tear secretion aids in protecting the eyes

  • Blood Vessels

    • Activation of muscarinic receptors on blood vessels primarily leads to vasodilation in certain regions, such as the cranial and sacral areas

    • Note: The parasympathetic nervous system has less direct innervation on blood vessels compared to the sympathetic nervous system

  • Liver

    • Activation of muscarinic receptors in the liver increases bile production and release

    • This aids in digestion during the rest and digest state

Visit the muscarinic receptor lecture (coming soon) for more information about:

  • The different types of muscarinic receptors

  • Where they are located

  • The different effects they produce, etc.

Resources

https://www.kenhub.com/en/library/anatomy/the-parasympathetic-nervous-system

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Sympathetic Nervous System: Function & Response