Autonomic Nervous System: Sympathetic and Parasympathetic
Save Time with a Video!
Save time by watching the video first, then supplement it with the lecture below!
Click below to view the EZmed video library. Subscribe to stay in the loop!
Become a Member!
Instant Access to All PDF Lectures, Study Guides, and Flashcards!
Instant access to a members-only page of ALL the flashcards, study guides, and PDF lectures. Cancel anytime.
Sympathetics/Parasympathetics in Action
You are driving home after a long shift at the hospital. Although exhausted, you acknowledge that the shift was a good one. You take in a slow deep breath through the nose and smile as you reflect on the day. Car windows are down and the warm summer breeze gently drifts through the car. Your favorite song plays as you travel down a twisting 2 lane highway adjacent to the river.
Suddenly, another vehicle coming from the opposite direction drifts left of center toward your vehicle. To avoid a head on collision, you swerve toward the shoulder nearly driving off the road into a steep ditch. Your car fishtails back and forth several times before coming to a halt sideways across both lanes.
Fortunately, you and the other driver do not collide and are both safe. Despite knowing you successfully avoided a disaster, you can hardly catch your breath as your heart rapidly pounds against your chest. Your clammy and tremulous hands attempt to grasp the wheel. Your mind is racing, but you know you must move the car from blocking both lanes. Despite having been in a relaxed trance seconds ago, you are now completely awake and situationally aware.
Autonomic Nervous System
The above scenario demonstrates the sympathetic and parasympathetic nervous system in action. They are continuously producing responses, essentially counteracting one another, but external stimuli can tip the scale in one direction or the other.
We will walk through both systems step by step below.
To better understand the sympathetic and parasympathetic nervous system, it is helpful to first organize the nervous system into its various components.
The nervous system can broadly be subdivided into 2 main categories, the central and peripheral nervous system.
The central nervous system comprises the brain and spinal cord. Alternatively, the peripheral nervous system comprises the nerves outside of the brain and spinal cord.
The peripheral nervous system can then be subdivided into the somatic and autonomic nervous system.
The somatic nervous system controls voluntary skeletal muscle movement as well as relays sensory information from the periphery to the brain.
The autonomic nervous system controls involuntary physiological responses, such as controlling heart rate and smooth muscle contraction/relaxation of the blood vessels, stomach, and intestines.
The autonomic system can be subdivided further into the sympathetic and parasympathetic branches.
The sympathetic nervous system is responsible for generating the physiological responses that occur from stressful or dangerous situations, and it is commonly referred to as the fight or flight state.
The parasympathetic nervous system is involved in producing the physiological responses that occur when the body is at rest, and it is often referred to as the rest and digest state.
Lastly, the enteric nervous system is the intrinsic nervous system of the gastrointestinal tract and is involved in gastrointestinal function.
It occasionally gets lumped into the autonomic nervous system, however it has its own independent reflexes that do not rely on the peripheral nervous system.
Sympathetic and parasympathetic signals can influence the enteric system, however the system can still function independently.
The focus of today’s post will be on the autonomic nervous system, mainly the sympathetic and parasympathetic branches. Understanding the sympathetic and parasympathetic nervous system will help to apply it practically.
Many critically ill patients are in stressful states physiologically, and understanding the underlying sympathetic pathophysiology becomes clinically relevant.
Furthermore, there are many medications that act on the sympathetic and parasympathetic nervous system, and knowing the underlying pathophysiology helps to understand the pharmacodynamics and mechanism of action for each of these drugs.
Lastly, the sympathetic and parasympathetic nervous system is important to understand in many toxidromes such as anticholinergic toxicity, cholinergic toxicity, sympathomimetic use, beta blocker or calcium channel blocker overdose, etc.
Sympathetic Nervous System
The functions of the sympathetic and parasympathetic nervous system are essentially the opposite of one another.
Both systems are continuously producing a response, however this balancing act can be tipped in one direction or another based on the current physiological state of that individual or what is occurring around their surroundings.
The sympathetic nervous system comprises all the physiological responses that occur secondary to a stressful or dangerous situation. For this reason, it is commonly called the fight or flight state.
Think of the sympathetic nervous system as revving up all bodily functions that are critically important for immediate survival while downregulating the bodily functions that are less critical.
Therefore, activation of the sympathetic nervous system will increase heart rate, increase cardiac output, dilate the bronchi, dilate the pupils (mydriasis), increase diaphoresis, increase metabolism, increase gluconeogensis, increase blood pressure, and activate the renin-angiotensin-aldosterone system while inhibiting salivation, lacrimation, defecation, digestion, and urination.
Axons of sympathetic neurons arise from the central nervous system at the thoracolumbar region, specifically T1-L2.
Adjacent to the vertebrae lies the sympathetic chain, also referred to as the sympathetic trunk or paravertebral ganglia.
The sympathetic trunk is a collection of neuronal cell bodies, called ganglia, that receive neuronal information and then deliver that information to their target organ.
The neurons leaving the spinal cord and terminating at the sympathetic chain/paravertebral ganglia are called preganglionic neurons, whereas the neurons originating at the sympathetic chain and terminating at their target effector tissue are called postganglionic neurons.
Sympathetic neurons arise from the central nervous system at the thoracolumbar region (T1-L2). Preganglionic neurons are short and terminate at the sympathetic chain. Postganglionic neurons are longer and terminate at their target organ generating a fight or flight sympathetic response.
Preganglionic Sympathetic Neurons
As mentioned above, the preganglionic sympathetic neurons arise from the central nervous system at the thoracolumbar level.
The preganglionic axons are short as they terminate nearby at the paravertebral ganglia of the sympathetic chain.
The preganglionic fibers release acetylcholine that will bind to nicotinic cholinergic receptors on the postganglionic cell body.
The preganglionic neurons are referred to as cholinergic neurons as they release acetylcholine.
An action potential is then generated through the postganglionic neuron once the preganglionic acetylcholine binds to the postganglionic nicotinic cholinergic receptors.
Below zooms in on the synaptic neurotransmission that occurs between the preganglionic and postganglionic neurons.
Preganglionic sympathetic neurons release acetylcholine onto the postganglionic nicotinic receptors.
Postganglionic Sympathetic Neurons
The axons of the postganglionic neurons are longer than those of the preganglionic as they travel much further to their target organ.
Postganglionic neurons release norepinephrine that binds to alpha adrenergic receptors and beta adrenergic receptors on the target organ, causing a sympathetic response.
The postganglionic neurons of the sympathetic nervous system are called adrenergic neurons as they release norepinephrine.
Epinephrine, in addition to norepinephrine, is another key hormone in generating the sympathetic response.
While postganglionic sympathetic neurons release mainly norepinephrine, there are preganglionic sympathetic fibers that terminate on the adrenal medulla which will cause secretion of both epinephrine and norepinephrine into the bloodstream.
Epinephrine and norepinephrine are catecholamines that will circulate in the bloodstream and bind to adrenergic receptors of effector tissues in addition to the norepinephrine that is released from the postganglionic sympathetic fibers.
The binding of these catecholamines to adrenergic receptors will generate a sympathetic response.
Here are some examples:
Activation of the adrenergic receptors in the heart will augment cardiac action potentials resulting in tachycardia, increased cardiac contractility, and increased cardiac output.
Activation of the adrenergic receptors in the lungs will lead to bronchodilation.
Of note, bronchodilation occurs through beta2 adrenergic receptors which have significantly higher affinity for epinephrine.
In fact, there are few postganglionic fibers that terminate on the lungs, and bronchodilation primarily occurs from the circulating epinephrine released by the adrenal medulla.
Activation of adrenergic receptors on blood vessels leading to vasoconstriction, the heart leading to increased cardiac output, and juxtaglomerular cells leading to stimulation of the renin-angiotensin-aldosterone system, will all synergistically work together to increase blood pressure.
Activation of the adrenergic receptors on the gastrointestinal tract or bladder will decrease digestion and urination.
The sweat glands are one exception in that the postganglionic sympathetic fibers release acetylcholine onto muscarinic cholonergic receptors on the sweat glands rather than releasing norepinephrine onto adrenergic receptors.
For more detail about the different types of adrenergic receptors, where they are located, and the different effects they produce, I highly recommend checking out the alpha receptor post and the beta receptor post as they serve as continuations to this sympathetic section.
Below is a closer look at the neurotransmission between postganglionic sympathetic neurons and their target organs.
Postganglionic sympathetic neurons release norepinephrine onto target organ adrenergic receptors.
Practical Application
There will be future posts that practically apply the sympathetic nervous system.
However, you can see how sympathomimetics and medications (such as vasopressors) that activate adrenergic receptors will increase sympathetic activity.
Alternatively, blocking adrenergic receptors (such as beta blockers) will inhibit sympathetic activity.
Lastly, if an individual is experiencing inadequate perfusion from a type of shock, the sympathetic nervous system will be activated to improve perfusion to vital organs.
Parasympathetic Nervous System
As mentioned above, the sympathetic and parasympathetic nervous system essentially counteract one another.
The sympathetic nervous system comprises all the physiological responses that occur secondary to a stressful or dangerous situation.
Alternatively, the parasympathetic nervous system comprises all the responses that occur when the body is at rest and is commonly referred to as the rest and digest state.
Think of the parasympathetic nervous system as ramping up all the bodily functions that are not necessary for immediate survival while downregulating or normalizing all the actions that are critical for survival.
Therefore, activation of the parasympathetic nervous system results in increased salivation, lacrimation, digestion, defecation, urination while decreasing or normalizing heart rate, constricting pupils (miosis), and constricting bronchi.
Unlike the axons of the sympathetic nervous system that arise from the central nervous system at the thoracolumbar region, parasympathetic neurons arise from the central nervous system through cranial and sacral nerves or the craniosacral region.
As preganglionic parasympathetic neurons arise from cranial and sacral neurons, they travel long distances and terminate on ganglia near or in their target effector tissue.
The postganglionic neurons then travel a short distance to the target organ.
Parasympathetic neurons arise from the central nervous system at the craniosacral region. Preganglionic neurons terminate near their target. Postganglionic neurons terminate at their target organ generating a rest and digest parasympathetic response.
Preganglionic Parasympathetic Neurons
The preganglionic parasympathetic neurons arise from the central nervous system through cranial nerves and sacral nerves (craniosacral region).
The 4 cranial nerves that have parasympathetic function are the oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X).
Parasympathetic activation of the oculomotor nerve leads to miosis.
The parasympathetic neurons of the facial nerve innervate the lacrimal, submandibular, sublingual, and nasal cavity glands to stimulate secretion.
The parasympathetic neurons of the glossopharyngeal nerve innervate the parotid gland.
The vagus nerve provides parasympathetic supply to the heart, lungs, esophagus, stomach, pancreas, gallbladder, liver, kidneys, small intestine, and part of the large intestine.
Lastly, the pelvic splanchnic parasympathetic nerves that arise from the sacral region innervate much of the pelvis including bladder, ureters, prostate, uterus, vagina, and penis.
You can broadly remember where each parasympathetic nerve supply targets with the following:
Parasympathetic Nerve Supply
Oculomotor (CN III) = Eyes
Facial (CN VII) and Glossopharyngeal (CN IX) = Face
Vagus (CN X) = Chest and Abdomen
Pelvic Splanchnic Nerves = Pelvis
The preganglionic parasympathetic axons are generally long as they terminate at ganglia near their target.
This is in contrast to the short preganglionic sympathetic axons that terminate quickly at the paravertebral ganglia of the sympathetic chain.
Neurotransmission in the synapse between the preganglionic and postganglionic parasympathetic fibers is essentially the same as the neurotransmission between the preganglionic and postganglionic fibers of the sympathetic nervous system.
Preganglionic acetylcholine is released and binds to postganglionic nicotinic receptors. For this reason, preganglionic parasympathetic fibers are also cholinergic neurons.
Below zooms in on the synaptic neurotransmission that occurs between the preganglionic and postganglionic neurons.
Preganglionic parasympathetic neurons release acetylcholine onto the postganglionic nicotinic receptors.
Postganglionic Parasympathetic Neurons
Similar to the sympathetic nervous system, an action potential is generated through the postganglionic parasympathetic neuron once preganglionic acetylcholine binds to postganglionic nicotinic cholinergic receptors.
However, the neurotransmission between the postganglionic parasympathetic neuron and the target organ is different than what occurs in the sympathetic nervous system.
Postganglionic parasympathetic neurons travel short distances to their target and release acetylcholine onto their targets, rather than norepinephrine as seen in the sympathetic nervous system.
Postganglionic acetylcholine will bind to muscarinic cholinergic receptors on target organs. This will generate a parasympathetic response.
If the muscarinic receptors on the heart are activated then bradycardia or decreased heart rate will occur.
Activation of the muscarinic receptors in the lungs leads to bronchoconstriction.
Activation of the muscarinic receptors in the gastrointestinal tract and bladder will increase digestion and urination.
Below takes a closer look at the neurotransmission that takes between the postganglionic parasympathetic neurons and their target organs.
Postganglionic parasympathetic neurons release acetylcholine onto the target organ muscarinic receptors.
Practical Application
There will be future posts that practically apply the parasympathetic nervous system.
However, you can see how anticholinergic toxicity or medications that have anticholinergic properties will block parasympathetic activity tipping the balance to sympathetic activity only.
Alternatively, cholinergic toxicity will lead to overactivity of the parasympathetic nervous system.
Conclusion
The autonomic nervous system mainly comprises the sympathetic and parasympathetic nervous system.
The function of these systems is to autoregulate involuntary bodily functions, and generate physiological responses during stress (sympathetic nervous system) and during rest (parasympathetic nervous system).
The 2 systems counteract one another and are essentially the opposite.
The sympathetic nervous system is involved in the fight or flight response, and the parasympathetic nervous system is involved in the rest and digest response.
Preganglionic sympathetic neurons arise from the central nervous system in the thoracolumbar region and travel short distances to the postganglionic fibers in the sympathetic chain.
The preganglionic sympathetic fibers release acetylcholine onto the postganglionic nicotinic receptors.
The postganglionic neurons travel further distances to their target organ and release norepinephrine onto target organ adrenergic receptors.
Preganglionic sympathetic neurons also terminate on the adrenal medulla which will secrete both norepinephrine and epinephrine into the bloodstream to act on adrenergic receptors as well.
This will cause tachycardia, mydriasis, bronchodilation, diaphoresis while decreasing salivation, lacrimation, digestion, defecation, and urination.
Preganglionic parasympathetic neurons arise from the central nervous system in the craniosacral region and travel long distances to the postganglionic fibers near the target organs.
The preganglionic parasympathetic fibers release acetylcholine onto the postganglionic nicotinic receptors.
The postganglionic neurons travel short distances to their target organ and release acetylcholine onto target organ muscarinic receptors.
This will cause increased salivation, lacrimation, digestion, defecation, and urination while normalizing or decreasing heart rate, constricting pupils, and constricting bronchi.
Below is a picture and chart to summarize it all up.
Before You Go, Make Your Medical Experience Easier!
If you enjoyed the content in this post, don’t forget to join the EZmed community for free on the bottom of the page or in the navigation bar so you don’t miss out on future medical topics made easy.
Boost your medical knowledge, perform well on exams, and keep up with your medical education throughout your career using:
High yield EZmed content on Instagram: @ezmedlearning
EZmed animations and videos on YouTube: Ezmed
EZmed Illustrations and flashcards on Pinterest: ezmedlearning
Feel free to use the contact button to reach out with any feedback or suggestions you may have for future topics. Thank you for using EZmed!
References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1959222/