Description

Parasympathetic Nervous System Overview

The parasympathetic nervous system is a division of the autonomic nervous system, which is part of the motor division of the peripheral nervous system.

Neuroanatomy: General Structures and Terms

• Group of cell bodies in CNS: Nucleus
• Group of cell bodies outside CNS: Ganglion
• Presynaptic neuron: Cholinergic neuron (Acetylcholine binds to nicotinic receptors, allowing an influx of cations)
• Postganglionic neurons: Cholinergic neurons (Acetylcholine binds to muscarinic receptors of the target organ, activating a G-protein)

Functional Aspects of Sympathetic and Parasympathetic Systems

• Territory:
  • Sympathetic: All areas of the body
  • Parasympathetic: No innervation in body walls and limbs
• Activity:
  • Sympathetic: More generalized (1:15 ratio of pre- to postganglionic neurons)
  • Parasympathetic: More specific (1:2 ratio of pre- to postganglionic neurons)
• Functions:
  • Sympathetic: “Fight-or-Flight” – increased heart rate, bronchodilation, decreased gut motility
  • Parasympathetic: “Rest-and-Digest” – decreased heart rate, bronchoconstriction, increased gut motility

Cranial Outflow

• Oculomotor nerve (nervus oculomotorius)
  • Edinger-Westphal nucleus → Ciliary ganglion → Ciliary body, sphincter pupillae
• Facial nerve (nervus facialis)
  • Superior salivatory nucleus → Pterygopalatine ganglion → Lacrimal, nasal, and palatine glands
  • Chorda tympani → Submandibular ganglion → Submandibular and sublingual glands
• Glossopharyngeal nerve (nervus glossopharyngeus)
  • Inferior salivatory nucleus → Otic ganglion → Parotid gland
• Vagus nerve (nervus vagus)
  • Posterior nucleus of the vagus → Smooth muscle, glands, internal organs

Sacral Outflow

• Sacral parasympathetic nuclei (S2-S4)
• Sacral splanchnic nerves → Inferior hypogastric plexus → Pelvic organs

Sources Used

• Singh, I. (2017). *Human Neuroanatomy (10th ed.)*
• Wineski, L. E. (2019). *Snell’s Clinical Anatomy by Regions (10th ed.)*
• Kozlowski, T. (2017). *Memorix Anatomy: The Complete Study Guide*
• Fryer, A. D., & Jacoby, D. B. (1998). *Muscarinic Receptors and Control of Airway Smooth Muscle*
• University lectures and notes
• Programs: Complete Anatomy, Biorender, PowerPoint, Camtasia

Transcript

Introduction
0:06
What’s up, Taim talks med here. In this video we’re gonna talk about the parasympathetic nervous
0:10
system. As you see from this brief diagram, the sympathetic and the parasympathetic parts of our
0:16
nervous system controls more or less all our internal organs. Sympathetic being the fight
0:21
or flight response, and parasympathetic being the rest and digest response. And they’re both
0:27
as you see here a part of the autonomic nervous system, which again is the motor
0:32
division of our peripheral nervous system. I did make an introductory video about the peripheral
0:37
nervous system, so if you guys have absolutely no clue what the peripheral nervous system is,
0:42
I urge you to watch that one first. But all in all I’ll try to simplify the parasympathetic
0:48
nervous system as much as I can so that it’ll make sense at a detailed level, within the
0:54
aspects of anatomy and physiology at least. So, in this video, we’re going detailed into
0:59
the parasympathetic nervous system. And we’re gonna do that by first going
1:04
through the general structure and terms associated with the parasympathetic aspect of the autonomic
1:09
nervous system. Basically talk a little bit about ganglia, the pre and post synaptic
1:14
neurons and their neurotransmitters, and basically how the parasympathetic
1:18
nervous system is built in general. Then we’re gonna talk about the cranial outflow,
1:23
and go quickly through the pathway of the cranial nerves involved and what structures
1:27
they innervate. And then run through the sacral outflow, where it originates from
1:32
and basically what it innervates and its function. Let’s go ahead and begin with some terms. Now the
Ganglion and Nucleus
1:39
autonomic nervous system – so both the sympathetic and parasympathetic nervous system is made up of a
1:45
relay that includes two neurons. And when there’s a group of nerve cell bodies that are next to
1:51
each other within the actual central nervous system, the whole thing is called a nucleus,
1:57
while a group of nerve cell bodies that are located outside of the central nervous system
2:02
is called a ganglion. This is a very very important thing to remember. Now.
General Structure
2:08
The autonomic nervous system has an affect on all areas of the body. The Sympathetic outflow
2:14
is primarily from the thoracolumbar area, right? Those are preganglionic cholinergic fibers that
2:20
go from the spinal cord towards either the paravertebral ganglia, or the prevertebral
2:26
ganglia, from where postganglionic primarily adrenergic neurons are gonna go out from.
2:32
The parasympathetic nervous system has their preganglionic fibers coming from the brainstem,
2:38
which travels towards a peripherally located ganglion. As well as from the sacral region,
2:44
going towards a parasympathetic ganglion that lie either near the organ,
2:49
or within the organs they innervate, to then give off postganglionic cholinergic neurons.
Sympathetic vs Parasympathetic
2:56
Now there are three main categories that we can see a clear difference between these two
3:01
systems. And we already know a little bit about the sympathetic nervous system since we covered it
3:06
in our last video, but in terms of territory. You know that the sympathetic nervous system is going
3:12
to innervate all areas of the body, primarily because the suprarenal gland is going to spit
3:18
a lot of epinephrine and norepinephrine within the blood. Parasympathetic nervous system is
3:24
primarily localized to the innervated areas, so the distribution is focused on the head,
3:30
the body cavities, and the external genitalia. So the limbs don’t receive
3:35
parasympathetic innervation for example, neither the body walls. In terms of activity,
3:41
when you activate the sympathetic nervous system, you’ll get a more generalized and indirect effect.
3:48
This is because for one thing you’ll have a large amount of catecolamines circulating in the blood,
3:54
and another reason si because the ratio between the pre- and post-ganglionic fibers
3:59
are approximately 1:15 or more. So 15 or more postganglionic fibers are activated just from
4:08
one preganglionic fiber. If the parasympathetic is stimulated, the ratio here is approximately
4:15
1 preganglilnic fiber to 2 postganlgionic fibers. So you’ll get a more specific and direct response.
4:22
And functions, again. Sympathetic is more associated with increased level
4:27
of activity and assisting in coping with stress and physical exertion.
4:32
Parasympathetic is associated with things like relaxation, homeostatis, restoration and so on.
4:39
Now I will go through the effect of the parasympathetic nervous system to each
4:44
specific organ later in this video, but as you see from just the functional area. The
4:49
sympathetic and parasympathetic divisions often function as antagonistic systems, that is, they
4:55
produce activities in opposition to one another. So for example, sympathetic activity increases
5:02
heart rate, causes bronchodilation, decreased peristalsis in the gut tube, closing of the
5:08
sphincters, relaxation of the general bladder wall, and dilation of the pupils. While the
5:14
parasympathetic activity results in decelerated heart rate, bronchoconstriction, increased gut
5:20
peristalsis, opening of sphincters, contraction of the bladder wall, and constriction of the pupils.
5:27
However, not always are they antagonistic. The two divisions may also be complementary to one
5:37
another, they can also work as a synergistic system. For example, in normal sexual function,
5:44
parasympathetic activity produces erection, and sympathetic activity results in ejaculation.
5:51
So here these two systems complement each other. Another thing is that, one division
5:57
may function independently of the other, for example sympathetic stimulation activates sweat
6:03
gland secretion, but parasympathetics play absolutely no role in sweat gland activity.
Neurotransmitters
6:10
Now, lastly before we go on an talk about the actual outflow of this system. Let’s quickly
6:16
just go through how all of this function. First we have a preganglionic neuron located within
6:22
the brainstem or the sacral spinal cord. These preganglionic neuron release the neurotransmitter
6:28
acetylcholine which binds to binds to nicotinic receptors on the cell membrane of postganglionic
6:34
neuron cell bodies. Nicotinic receptors are ion channels that open when acetylcholine
6:41
binds to them, and they allow positive ions like sodium and potassium to cross the cell membrane,
6:46
activating the postganglionic neurons. Postganglionic neurons are also called
6:52
cholinergic neurons because just like the preganglionic neurons,
6:56
they also release acetylcholine. This time, however, the acetylcholine binds to muscarinic
7:02
receptors on the cells of target organs. Muscarinic receptors are G-protein-coupled
7:08
receptors, meaning that when acetylcholine binds, they activate the G proteins to ultimately enable
7:16
cells to change in a number of ways, and that’s how the parasympathetic nervous
7:20
system creates change at the cellular level. So, sympathetic has adrenergic postganglionic
7:27
neurons primarily. Parasympathetic has cholinergic postganglionic neurons. Awesome.
7:34
So. Here we see the mesencephalon, Pons, Midbrain and the spinal cord. Now the parasympathetic
Craniosacral Outflow
7:41
nervous system is primarily going to be something called craniosacral outflow.
7:46
Those in the brain, the cranial part, primarily come from specific parasympathetic nuclei located
7:53
within the brainsteim of certain cranial nerves. Those are the Oculomotor nerve, Facial nerve,
7:59
Glossopharyngeal nerve, and the vagus nerve. The sacral outflow, or the sacral part has
8:05
their preganglionic neurons originate in the lateral gray matter of the second, third,
8:10
and fourth sacral segments of the spinal cord. However, because the number of cells here is
8:16
insufficient to form a distinct bulge such as seen in the thoracolumbar region, you’ll rarely see a
8:22
lateral gray horn. It may be absent sometimes. Myelinated axons leave the spinal cord in the
8:36
anterior nerve roots of the corresponding spinal nerves, then leave the S2 to S4 spinal nerves,
8:42
and form the pelvic splanchnic nerves. Because of these levels of origin and departure from the CNS,
8:50
we call the parasympathetic division a what? We refer to it as craniosacral outflow.
Cranial Part
8:57
Now let’s start the most proximal nerve of the cranial outflow. The oculomotor nerve’s
9:02
parasympathetic nucleus is located within the mesencephalon, specifically at the level of the
9:08
superior colliculus. Just to refresh your memory I’m not going to talk about all this in detail,
9:13
cuz we already covered all the cranial nerves already in the previous videos.
9:17
But here you see the posterior view of the mesencephalon. If we cut the mesencephalon
9:21
at the level of the superior colliculus and look at the cross section, you’ll see this.
9:27
So, we can see the Superior colliculi, the Cerebral peduncles, the Interpeduncular space,
9:33
and the aqueduct of the midbrain, which connects the fourth ventricle to the third ventricle.
9:38
Within the midbrain, we can find the substantia nigra, we can find the superior colluculi. We can
9:41
see the periaqueductal gray matter, the reticular formation, the red nuclei which take in impulses
9:47
from the brain and the cerebellum, and give off rubrospinal tracts for muscle coordination.
9:52
At this level, we can also find the nucleus of the oculomotor nerve, which will give off
9:58
fibers travelling towards the anterior side, and leave through the sulcus of the oculomotor
10:03
nerve on the anterior side of the midbrain. The oculomotor nerve is a nerve that consists of
10:09
somatic fibers and preganglionic parasympathetic fibers. The somatic fibers are the fibers you see
10:16
here, coming from the nucleus of the oculomotor nerve, which sipplies the extrinsic muscles of
10:21
the eyeball. The parasympathetic portion of the oculomotor nerve comes from the accessory
10:27
nucleus of the oculomotor nerve, which is also called Edinger Westphal nucleus. They give off
10:33
parasympathetic fibres that go together with the oculomotor nerve, forming the oculomotor
10:38
nerve complex. And this is the nucleus I’m talking about here. This nucleus will give
10:45
off preganglionic parasympathetic fibers that’ll travel through the cavernous sinus, go through the
10:51
superior orbital fissure and the common tendinous ring, to synapse with a postganglionic neuron in
10:58
the ciliary ganglion, from where postganglionic parasympathetic fibers will go as short ciliary
11:05
nervs towards the cilliaris muscle. The cilliaris muscle is then going to contract,
11:10
and when it contracts, the zonular fibers gets relaxed, which allowed the lens to
11:16
become more globular like. When the lens becomes globular, it helps with near vision. So this is an
11:22
accommodation response. Helps with near vision. Now, the other muscle that it goes to is called
11:27
the sphincter pupullae. When the sphincter pupillae contracts, it squeezes the pupile hole
11:33
and makes it really small. So it causes pupile constriction. And when you constrict the pupile,
11:38
it allowed less rays to come into the eye, which also has an affect on near vision.
11:44
So that’s it. That’s the oculomotor nerve. Next one is the facial nerve.
11:49
The facial nerve has several nuclei that give off fibers that travel within this actual nerve,
11:55
but one of the nuclei is called the superior salivatory nucleus, which give off preganglionic
12:01
parasympathetic fibers. These fibers are gonna go leave, go through the internal acoustic meatus,
12:08
and give off an important nerve called the greater petrosal nerve. This nerve will synapse with the
12:14
pterygopalatine ganglion, and then these fibers will go innervate several glands. Specifically the
12:21
lacrimal glands, can be the glands in the nasal cavity, sinuses and the palate. So it’s gonna
12:27
innervate these glands, release acetocholine which will stimulate these cells to start increasing the
12:32
production of these watery secretions, for example you’ll have more tear production,
12:37
nasal secretion, palatine secretion and so on. It’s also going to give off the chorda tympani,
12:44
which will synpase with postganglionic neurons in the submandibular ganglion. These postganglionic
12:50
fibers will innervate the submandibular and the sublingual salivary glands. And it will stimulate
12:56
them into secreting more saliva along with digestive enzymes aswell like salivary amylase.
13:02
So that is that one. Now let’s do the glossopharyngeal. The glossopharyngeal
13:06
nerve has its parasympathetic origin within the inferior salivatory nucleus.
13:12
Which give off preganglionic parasympathetic fibers that go through the jugular foramen,
13:17
then remember they travel through the tympanic nerve, through this tympanic plexus,
13:23
and ultimately leave as the lesser petrosal nerve, which will finally end up in the Otic
13:28
ganglion. This otic ganglion will then give off postganglionic parasympathetic
13:33
fibers to the last salivary gland, one of the big ones. Which one is that?
13:39
It’s the parotid gland. So it’s going to stimulate the parotid salivary gland to start
13:44
increasing its secretions, so lots of watery secretions and salivary amylase, in order to
13:50
lubricate out food and digest it chemically a little bit aswell through that amylase.
13:56
So that was this one. Last one is the biggest one. The vagus nerve. And this one has the posterior
14:03
nucleus of vagus nerve, providing parasympathetic innervation to the majority of organs within us.
14:09
I’m not gonna go in detail into each and every branch, but I’m gonna keep it very
14:14
simple and say that it’s going to leave the cranium through the jugular foramen,
14:19
then give off a few fibers towards the upper respiratory tract, the larynx and the trachea.
14:24
To basically release acetylcholine to increase the mucus secretion and cause a little bit of
14:29
contraction of the smooth muscle to constrict the airways. Remember airway smooth muscles are extend
14:35
from the trachea throughout the bronchial tree, so it increases in number the further distally
14:41
you get. And right now we’re activating the parasympathetic nervous system, you’re resting.
14:47
You don’t need all that air in, and you wanna produce more mucus to humidify the air and help
14:53
with protection of certain praticles or foreign organisms as way to protect the airways. So that’s
15:00
what’s primarily is going to happen here. Then we got some branches for the cardiac plexus.
15:06
Now we have to be very careful because the parasympathetic nervous system doesn’t really
15:10
affect the cardiac muscle cells so much like the sympathetic nervous system do. The parasympathetic
15:15
nervous system is primarily going to affect the conducting system, nodule cells. So the SA node,
15:21
and the AV node. Now, you’re resting, you wanna decrease the heart rate. So it has a
15:26
negative chronotropic and dromotropic action. The vagus nerve is going to contribute to the
15:31
pulmonary plexus, causing bronchoconstriction and a little bit of increased mucus secretion aswell,
15:37
right? Same as what it did in the the upper respiratory tract. Then
15:40
the nerve is going to continue along the esophagus and form an esophageal plexus.
15:45
Now when you’re resting and digesting, what do you want the GI tract to do? To increase motility. So
15:52
it primarily increases the peristalsis. Now the vagus nerve is going to continue along
15:59
the esophagus, go through the diaphragmatic hiatus and then split into a left and a right
16:05
vagus nerve. Going anterior and posterior to the stomach. The left one will give a nerve towards
16:11
the liver and the biliary tree, going within the hepatic plexus. Basically helping the liver to
16:17
be able to stimulate glycogenesis, storing the glucose. Remember you’re resting and digesting,
16:23
you wanna digest, so it also helps contract your gallbladder to a little degree, the anterior vagal
16:29
trunk primarily stimulate the liver. The stomach is also going to be innervated to increase in
16:35
motility and increase in gastric secretion. Now one of the major branches of this nerve
16:41
are the celiac branches, going towards many different plexuses within the abdominal
16:46
cavity. Primarily the celiac plexus, but it can also go to the splenic plexus, hepatic plexus,
16:53
renal plexus, suprarenal plexus and the superior mesenteric plexus. The inferior mesenteric plexus
16:59
is more for the sacral outflow. But it’s going to basically help innervate a lot of different
17:05
organs within us, to help with the rest and digest state. Like increase in urine production, increase
17:10
motility and secretion and absorption from the GI tract, stimulate the pancreas in the exocrine and
17:16
endocrine secretion, like releasing insulin. When it comes to the large intestine however it really
17:21
only innervates the proximal parts up until the proximal 2/3s of the transverse colon. The
17:27
rest of it being innervates by the sacral outflow. Alright, what else. From the actual posterior
17:34
vagal trunk, you might have branches that instead of going through the celiac branches, they may go
17:37
to hepatic plexus directly, or branches that go towards the renal plexus directly.
Sacral Part
17:43
So that was mainly the cranial outflow. Let’s now quickly do the sacral outflow.
17:49
The sacral preganglionic neurons originate from the segments S2, S3 and S4. These axons leave the
17:57
spinal cord in the anterior nerve roots of the corresponding spinal nerves, then leave
18:02
the S2 to S4 spinal nerves, and form the pelvic splanchnic nerves. The pelvis splanchnic nerves
18:09
will run into the inferior hypogastric plexus, and then innervate the descending colon, remaining
18:15
transverse colon, sigmoid colon and the rectum. Increasing the motility, increase the secretion
18:21
and increasing absorption. Also controlling the internal anal sphincter which relaxes to offer the
18:28
faeces to move forward if the rectum is full. If you don’t have time for that at that moment, you
18:33
contract the external anal sphincter consciously to prevent the poop from exiting at that moment.
18:39
It’s also going to innervate the bladder wall, to cause bladder wall contraction, and internal
18:44
urethral sphincter relaxation to basically help you urinate. And lastly, it’s also going to
18:50
innervate the male and female genitals. For the male reproductive system, remember it’s going
18:55
to help engorging the penis with blood, helping with erection by basically releasing acetylcholine
19:00
to stimulate the cavernosal endothelial cells to produce nitric oxide, which is going to act on
19:06
the corpora cavernosa. Causing smooth muscle cell relaxation, vasodilation and erection.
19:12
For female, it’s going to increase the blood flow to the clitoris,
19:16
engorging the clitoris with blood. So that was everything I had for the
Ending
19:21
parasympathetic nervous system. And we now covered both the sympathetic and the parasympathetic
19:25
nervous system in the last two videos. Thank you so much for watching another
19:29
one of my videos. If you enjoyed, learned something from it, please remember to like,
19:33
comment your favourite moment, subscribe. Turn on those notifications. If you looking for other
19:37
ways to support, go ahead and check out the link in the description box. Have fun ya’ll. Peace.