Parasympathetic Nervous System: Animated Scheme (Craniosacral Outflow)

What’s up, Taim talks med here. In this video 

we’re gonna talk about the parasympathetic nervous system. As you see from this brief diagram, the 

sympathetic and the parasympathetic parts of our nervous system controls more or less all our 

internal organs. Sympathetic being the fight or flight response, and parasympathetic being 

the rest and digest response. And they’re both as you see here a part of the autonomic 

nervous system, which again is the motor division of our peripheral nervous system. I did 

make an introductory video about the peripheral nervous system, so if you guys have absolutely 

no clue what the peripheral nervous system is, I urge you to watch that one first. But all in 

all I’ll try to simplify the parasympathetic nervous system as much as I can so that it’ll 

make sense at a detailed level, within the aspects of anatomy and physiology at least.

So, in this video, we’re going detailed into the parasympathetic nervous system.

And we’re gonna do that by first going through the general structure and terms associated 

with the parasympathetic aspect of the autonomic nervous system. Basically talk a little bit 

about ganglia, the pre and post synaptic neurons and their neurotransmitters, 

and basically how the parasympathetic nervous system is built in general.

Then we’re gonna talk about the cranial outflow, and go quickly through the pathway of the 

cranial nerves involved and what structures they innervate. And then run through the 

sacral outflow, where it originates from and basically what it innervates and its function.

Let’s go ahead and begin with some terms. Now the autonomic nervous system - so both the sympathetic 

and parasympathetic nervous system is made up of a relay that includes two neurons. And when there’s 

a group of nerve cell bodies that are next to each other within the actual central nervous 

system, the whole thing is called a nucleus, while a group of nerve cell bodies that are 

located outside of the central nervous system is called a ganglion. This is a very 

very important thing to remember. Now. The autonomic nervous system has an affect on 

all areas of the body. The Sympathetic outflow is primarily from the thoracolumbar area, right? 

Those are preganglionic cholinergic fibers that go from the spinal cord towards either the 

paravertebral ganglia, or the prevertebral ganglia, from where postganglionic primarily 

adrenergic neurons are gonna go out from. The parasympathetic nervous system has their 

preganglionic fibers coming from the brainstem, which travels towards a peripherally located 

ganglion. As well as from the sacral region, going towards a parasympathetic 

ganglion that lie either near the organ, or within the organs they innervate, to then 

give off postganglionic cholinergic neurons. Now there are three main categories that we 

can see a clear difference between these two systems. And we already know a little bit about 

the sympathetic nervous system since we covered it in our last video, but in terms of territory. You 

know that the sympathetic nervous system is going to innervate all areas of the body, primarily 

because the suprarenal gland is going to spit a lot of epinephrine and norepinephrine within 

the blood. Parasympathetic nervous system is primarily localized to the innervated areas, 

so the distribution is focused on the head, the body cavities, and the external 

genitalia. So the limbs don’t receive parasympathetic innervation for example, 

neither the body walls. In terms of activity, when you activate the sympathetic nervous system, 

you’ll get a more generalized and indirect effect. This is because for one thing you’ll have a large 

amount of catecolamines circulating in the blood, and another reason si because the ratio 

between the pre- and post-ganglionic fibers are approximately 1:15 or more. So 15 or more 

postganglionic fibers are activated just from one preganglionic fiber. If the parasympathetic 

is stimulated, the ratio here is approximately 1 preganglilnic fiber to 2 postganlgionic fibers. 

So you’ll get a more specific and direct response. And functions, again. Sympathetic is 

more associated with increased level of activity and assisting in coping 

with stress and physical exertion. Parasympathetic is associated with things like 

relaxation, homeostatis, restoration and so on. Now I will go through the effect of the 

parasympathetic nervous system to each specific organ later in this video, but as 

you see from just the functional area. The sympathetic and parasympathetic divisions often 

function as antagonistic systems, that is, they produce activities in opposition to one another. 

So for example, sympathetic activity increases heart rate, causes bronchodilation, decreased 

peristalsis in the gut tube, closing of the sphincters, relaxation of the general bladder 

wall, and dilation of the pupils. While the parasympathetic activity results in decelerated 

heart rate, bronchoconstriction, increased gut peristalsis, opening of sphincters, contraction of 

the bladder wall, and constriction of the pupils. However, not always are they antagonistic. The 

two divisions may also be complementary to one another, they can also work as a synergistic 

system. For example, in normal sexual function, parasympathetic activity produces erection, and 

sympathetic activity results in ejaculation. So here these two systems complement each 

other. Another thing is that, one division may function independently of the other, for 

example sympathetic stimulation activates sweat gland secretion, but parasympathetics play 

absolutely no role in sweat gland activity. Now, lastly before we go on an talk about the 

actual outflow of this system. Let’s quickly just go through how all of this function. First 

we have a preganglionic neuron located within the brainstem or the sacral spinal cord. These 

preganglionic neuron release the neurotransmitter acetylcholine which binds to binds to nicotinic 

receptors on the cell membrane of postganglionic neuron cell bodies. Nicotinic receptors are 

ion channels that open when acetylcholine binds to them, and they allow positive ions like 

sodium and potassium to cross the cell membrane, activating the postganglionic neurons.

Postganglionic neurons are also called cholinergic neurons because just 

like the preganglionic neurons, they also release acetylcholine. This time, 

however, the acetylcholine binds to muscarinic receptors on the cells of target organs.

Muscarinic receptors are G-protein-coupled receptors, meaning that when acetylcholine binds, 

they activate the G proteins to ultimately enable cells to change in a number of ways, and 

that’s how the parasympathetic nervous system creates change at the cellular level.

So, sympathetic has adrenergic postganglionic neurons primarily. Parasympathetic has 

cholinergic postganglionic neurons. Awesome. So. Here we see the mesencephalon, Pons, Midbrain 

and the spinal cord. Now the parasympathetic nervous system is primarily going to be 

something called craniosacral outflow. Those in the brain, the cranial part, primarily 

come from specific parasympathetic nuclei located within the brainsteim of certain cranial nerves. 

Those are the Oculomotor nerve, Facial nerve, Glossopharyngeal nerve, and the vagus nerve.

The sacral outflow, or the sacral part has their preganglionic neurons originate in the 

lateral gray matter of the second, third, and fourth sacral segments of the spinal cord. 

However, because the number of cells here is insufficient to form a distinct bulge such as seen 

in the thoracolumbar region, you’ll rarely see a lateral gray horn. It may be absent sometimes. 

Myelinated axons leave the spinal cord in the anterior nerve roots of the corresponding spinal 

nerves, then leave the S2 to S4 spinal nerves, and form the pelvic splanchnic nerves. Because of 

these levels of origin and departure from the CNS, we call the parasympathetic division a what? 

We refer to it as craniosacral outflow. Now let’s start the most proximal nerve of 

the cranial outflow. The oculomotor nerve’s parasympathetic nucleus is located within the 

mesencephalon, specifically at the level of the superior colliculus. Just to refresh your memory 

I’m not going to talk about all this in detail, cuz we already covered all the cranial 

nerves already in the previous videos. But here you see the posterior view of the 

mesencephalon. If we cut the mesencephalon at the level of the superior colliculus and 

look at the cross section, you’ll see this. So, we can see the Superior colliculi, the 

Cerebral peduncles, the Interpeduncular space, and the aqueduct of the midbrain, which connects 

the fourth ventricle to the third ventricle. Within the midbrain, we can find the substantia 

nigra, we can find the superior colluculi. We can see the periaqueductal gray matter, the reticular 

formation, the red nuclei which take in impulses from the brain and the cerebellum, and give 

off rubrospinal tracts for muscle coordination. At this level, we can also find the nucleus 

of the oculomotor nerve, which will give off fibers travelling towards the anterior side, 

and leave through the sulcus of the oculomotor nerve on the anterior side of the midbrain.

The oculomotor nerve is a nerve that consists of somatic fibers and preganglionic parasympathetic 

fibers. The somatic fibers are the fibers you see here, coming from the nucleus of the oculomotor 

nerve, which sipplies the extrinsic muscles of the eyeball. The parasympathetic portion of 

the oculomotor nerve comes from the accessory nucleus of the oculomotor nerve, which is also 

called Edinger Westphal nucleus. They give off parasympathetic fibres that go together with 

the oculomotor nerve, forming the oculomotor nerve complex. And this is the nucleus I’m 

talking about here. This nucleus will give off preganglionic parasympathetic fibers that’ll 

travel through the cavernous sinus, go through the superior orbital fissure and the common tendinous 

ring, to synapse with a postganglionic neuron in the ciliary ganglion, from where postganglionic 

parasympathetic fibers will go as short ciliary nervs towards the cilliaris muscle. The 

cilliaris muscle is then going to contract, and when it contracts, the zonular fibers 

gets relaxed, which allowed the lens to become more globular like. When the lens becomes 

globular, it helps with near vision. So this is an accommodation response. Helps with near vision.

Now, the other muscle that it goes to is called the sphincter pupullae. When the sphincter 

pupillae contracts, it squeezes the pupile hole and makes it really small. So it causes pupile 

constriction. And when you constrict the pupile, it allowed less rays to come into the eye, 

which also has an affect on near vision. So that’s it. That’s the oculomotor 

nerve. Next one is the facial nerve. The facial nerve has several nuclei that give 

off fibers that travel within this actual nerve, but one of the nuclei is called the superior 

salivatory nucleus, which give off preganglionic parasympathetic fibers. These fibers are gonna go 

leave, go through the internal acoustic meatus, and give off an important nerve called the greater 

petrosal nerve. This nerve will synapse with the pterygopalatine ganglion, and then these fibers 

will go innervate several glands. Specifically the lacrimal glands, can be the glands in the nasal 

cavity, sinuses and the palate. So it’s gonna innervate these glands, release acetocholine which 

will stimulate these cells to start increasing the production of these watery secretions, for 

example you’ll have more tear production, nasal secretion, palatine secretion and so on. 

It’s also going to give off the chorda tympani, which will synpase with postganglionic neurons in 

the submandibular ganglion. These postganglionic fibers will innervate the submandibular and the 

sublingual salivary glands. And it will stimulate them into secreting more saliva along with 

digestive enzymes aswell like salivary amylase. So that is that one. Now let’s do the 

glossopharyngeal. The glossopharyngeal nerve has its parasympathetic origin 

within the inferior salivatory nucleus. Which give off preganglionic parasympathetic 

fibers that go through the jugular foramen, then remember they travel through the 

tympanic nerve, through this tympanic plexus, and ultimately leave as the lesser petrosal 

nerve, which will finally end up in the Otic ganglion. This otic ganglion will then 

give off postganglionic parasympathetic fibers to the last salivary gland, one 

of the big ones. Which one is that? It's the parotid gland. So it’s going to 

stimulate the parotid salivary gland to start increasing its secretions, so lots of watery 

secretions and salivary amylase, in order to lubricate out food and digest it chemically 

a little bit aswell through that amylase. So that was this one. Last one is the biggest one. 

The vagus nerve. And this one has the posterior nucleus of vagus nerve, providing parasympathetic 

innervation to the majority of organs within us. I’m not gonna go in detail into each and 

every branch, but I’m gonna keep it very simple and say that it’s going to leave 

the cranium through the jugular foramen, then give off a few fibers towards the upper 

respiratory tract, the larynx and the trachea. To basically release acetylcholine to increase 

the mucus secretion and cause a little bit of contraction of the smooth muscle to constrict the 

airways. Remember airway smooth muscles are extend from the trachea throughout the bronchial tree, 

so it increases in number the further distally you get. And right now we’re activating the 

parasympathetic nervous system, you’re resting. You don’t need all that air in, and you wanna 

produce more mucus to humidify the air and help with protection of certain praticles or foreign 

organisms as way to protect the airways. So that’s what’s primarily is going to happen here. Then 

we got some branches for the cardiac plexus. Now we have to be very careful because the 

parasympathetic nervous system doesn’t really affect the cardiac muscle cells so much like the 

sympathetic nervous system do. The parasympathetic nervous system is primarily going to affect the 

conducting system, nodule cells. So the SA node, and the AV node. Now, you’re resting, you 

wanna decrease the heart rate. So it has a negative chronotropic and dromotropic action.

The vagus nerve is going to contribute to the pulmonary plexus, causing bronchoconstriction and 

a little bit of increased mucus secretion aswell, right? Same as what it did in the 

the upper respiratory tract. Then the nerve is going to continue along the 

esophagus and form an esophageal plexus. Now when you’re resting and digesting, what do you 

want the GI tract to do? To increase motility. So it primarily increases the peristalsis. Now 

the vagus nerve is going to continue along the esophagus, go through the diaphragmatic 

hiatus and then split into a left and a right vagus nerve. Going anterior and posterior to the 

stomach. The left one will give a nerve towards the liver and the biliary tree, going within the 

hepatic plexus. Basically helping the liver to be able to stimulate glycogenesis, storing the 

glucose. Remember you’re resting and digesting, you wanna digest, so it also helps contract your 

gallbladder to a little degree, the anterior vagal trunk primarily stimulate the liver. The stomach 

is also going to be innervated to increase in motility and increase in gastric secretion.

Now one of the major branches of this nerve are the celiac branches, going towards many 

different plexuses within the abdominal cavity. Primarily the celiac plexus, but it can 

also go to the splenic plexus, hepatic plexus, renal plexus, suprarenal plexus and the superior 

mesenteric plexus. The inferior mesenteric plexus is more for the sacral outflow. But it’s going 

to basically help innervate a lot of different organs within us, to help with the rest and digest 

state. Like increase in urine production, increase motility and secretion and absorption from the GI 

tract, stimulate the pancreas in the exocrine and endocrine secretion, like releasing insulin. When 

it comes to the large intestine however it really only innervates the proximal parts up until 

the proximal 2/3s of the transverse colon. The rest of it being innervates by the sacral outflow.

Alright, what else. From the actual posterior vagal trunk, you might have branches that instead 

of going through the celiac branches, they may go to hepatic plexus directly, or branches 

that go towards the renal plexus directly. So that was mainly the cranial outflow.

Let’s now quickly do the sacral outflow. The sacral preganglionic neurons originate from 

the segments S2, S3 and S4. These axons leave the spinal cord in the anterior nerve roots of 

the corresponding spinal nerves, then leave the S2 to S4 spinal nerves, and form the pelvic 

splanchnic nerves. The pelvis splanchnic nerves will run into the inferior hypogastric plexus, 

and then innervate the descending colon, remaining transverse colon, sigmoid colon and the rectum. 

Increasing the motility, increase the secretion and increasing absorption. Also controlling the 

internal anal sphincter which relaxes to offer the faeces to move forward if the rectum is full. If 

you don’t have time for that at that moment, you contract the external anal sphincter consciously 

to prevent the poop from exiting at that moment. It’s also going to innervate the bladder wall, 

to cause bladder wall contraction, and internal urethral sphincter relaxation to basically help 

you urinate. And lastly, it’s also going to innervate the male and female genitals. For the 

male reproductive system, remember it’s going to help engorging the penis with blood, helping 

with erection by basically releasing acetylcholine to stimulate the cavernosal endothelial cells to 

produce nitric oxide, which is going to act on the corpora cavernosa. Causing smooth muscle 

cell relaxation, vasodilation and erection. For female, it’s going to increase 

the blood flow to the clitoris, engorging the clitoris with blood.

So that was everything I had for the parasympathetic nervous system. And we now covered 

both the sympathetic and the parasympathetic nervous system in the last two videos.

Thank you so much for watching another one of my videos. If you enjoyed, learned 

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