What is the Peripheral Nervous System?

• Cranial Nerves 12 nerve pairs
• Spinal Nerves 31 nerve pairs

Classification of the PNS

• Sensory System
  
  • Soamtic Sensory Fibers
  • Visceral Sensory Fibers
  • Exteroreceptors, Proprioreceptors, Periosteum
• Motor System
  
  • Somatomotor System (Somatic system)
    
    • Voluntary control
  • Autonomic Nervous System / Visceromotor nervous system
    
    • Sympathetic Nervous System (C8-L2 Lateral horn)
    • Parasympahetic Nervous System (S2-S4 Lateral horn)
    • Enteric Nervous System (myenteric and submucosal plexuses in walls of digestive organs)

Clinical Significance

• Lower Motor Neuron damage
  
  • Located in anterior horn of spinal cord and nuclei of cranial nerves
  • Damage to lower motor neurons:
    
    • Flaccid Paralysis
    • Hypotonia
    • Paresis/Plegia
    • Hyporeflexia / areflexia
• Upper Motor Neuron damage
  
  • May rise after a stroke or a perinatal hypoxia
  • Spastic Paralysis
    
    • Paresis / Plegia
    • Hypertonia
    • Hyperreflexia
    • Babinski’s sign

External Scheme of Spinal Nerve

• Rootlets (fila radicularia)
  
  • Anterior root (radix anterior)
  • Posterior root (radic posterior)
    
    • Spinal Ganglion (ganglion sensorium nervi spinalis)
• Trunk of Spinal Nerve (truncus nervi spinalis)
  
  • Posterior Branch (Ramus posterior)
  • Anterior Branch (ramus anterior)
  • Meningeal branch (ramus meningeus)
  • Ganglion of sympathetic trunk (ganglion trunci sympathici)
  • Gray ramus communicans
  • White ramus communicans (ramus communicans albus)

Internal Scheme of Spinal Nerve

• Somatomotor fibers
• Pre-ganglionic sympathetic fibers (Visceromotor fibers)
• Post-ganglionic Fibers
• Pre-ganglionic parasympathetic fibers
• Somatosensory fibers
• Viscerosensory fibers (baroreceptors, chemoreceptors, unspecific organ senstation)

Posterior Branch:

• Suboccipital Nerve (nervus suboccipitalis) C1
• Greater Occipital Nerve (nervus occipitalis major) C2
• Occipital Nerve (nervus occipitalis tertius) C3
• Superior Clunial Nerve (Nervi Clunium Superiores) L1-L3
• Middle Clunial Nerves (Nervi Clunium Medii) S1-S3

Anterior Branch:

• Cervical Plexus (Plexus cervicalis) C1-C4
• Brachial Plexus (Plexus brachialis) C4-T1
• Thoracic Nerves (Nervi thoracici) T1-T12
• Lumbar Plexus (Plexus Lumbalis) T12-L4
• Sacral Plexus (Plexus Sacralis) L4-S4

Transcript

Introduction
0:06
What’s up!
0:07
Taim Talks Med here.
0:08
Let’s talk about the Peripheral Nervous System!
0:11
This video is going to be an overview of the peripheral nervous system.
0:15
So all I’ve done is gather the information that will help you build a general mind map
0:19
around this topic.
0:21
So if you’re new to the PNS, I highly recommend you watch this video before you start studying
0:27
any plexuses or nerves.
0:29
With that being said, What we’re going to go through is the first
0:32
talk a little bit about what is considered the PNS
0:36
We’re gonna go through the classification of the PNS, where we will be talking a little
0:41
bit about the difference between motor and sensory nerves and the difference between
0:46
autonomic and somatic nerves.
0:48
Then we’ll make a simple outline of a spinal nerve.
0:52
We’re first gonna make an external scheme of the spinal nerve, then an internal scheme
0:57
of the spinal nerve.
What is Considered the PNS?
0:59
Awesome.
1:00
So, what is considered the PNS?
1:03
I guess at this point, since you’re studying the PNS, you already know that the brain and
1:08
the spinal cord are considered the central nervous system.
1:12
That means that all the nerves that exit the brain and the spinal cord are considered the
1:17
PNS.
1:19
What do you call the nerves that exit the brain and the spinal cord?
1:24
Nerves exit the brain within the cranium.
1:26
They’re called cranial nerves.
1:28
Nerves exit the spinal cord; they’re called spinal nerves.
1:32
Easy.
1:33
These Nerves enable bidirectional communication between the central nervous system and the
1:39
rest of the body, which is referred to as the periphery.
1:42
There are 31 pairs of spinal nerves originating from the spinal cord and 12 pairs of cranial
1:49
nerves from the brainstem, giving a total of 43 paired nerves forming the basis of the
1:55
peripheral nervous system.
1:57
Awesome.
1:58
So that Is what is considered the peripheral nervous system.
2:02
Cranial Nerves and Spinal Nerves.
How to Classify the PNS
2:05
But, how do we classify the nerves within the periphery of our body?
2:10
The peripheral nervous system is broken into two systems, right?
2:14
A sensory system and a motor system.
2:17
The sensory system, or the afferent system, are peripheral processes of pseudounipolar
2:24
neurons of the spinal ganglions, and they convey impulses to the central nervous system.
2:30
You know, there can be somatic sensory fibers and visceral sensory fibers.
2:35
Somatic sensory fibers take impulses from the exteroceptors of the skin, sensing cutaneous
2:42
sensations of pain, temperature, touch, vibration, and pressure.
2:47
And they also take impulses from proprioceptors localized in the muscles, joints, ligaments,
2:54
or in the periosteum of bones via spinal nerves and some cranial nerves as well, sensing proprioception.
3:01
The motor system, or the Efferent system, takes impulses from the central nervous system
3:08
to periphery.
3:09
Now we can actually break the motor system into two parts.
3:13
One is called somatomotor, which is a part of your somatic system.
3:18
And this one is under voluntary control.
3:21
So in other words, if I wanna have my biceps brachii contract, I have voluntary control
3:27
over that, right?.
3:28
So we call that somatomotor.
3:31
Whereis, my heart, or my GI tract, or any different part of my actual viscera.
3:37
Those aren’t under voluntary control.
3:39
Thank goodness they’re not.
3:41
They’re under involuntary control.
3:43
So the, involuntary control is a part of the visceromotor.
3:47
They call this the autonomic nervous system.
3:50
Which means it stands by itself.
3:53
Ok.
3:54
Now the autonomic nervous system there’s three traditional branches of it.
3:59
Most people know it as the sympathetic nervous system, the parasympathetic nervous system,
4:04
and the last one most people don’t consider it.
4:07
Aaa they forget about this one, but the enteric nervous system.
4:13
The enteric an example of it are the myenteric and submucosal plexuses located in the walls
4:20
of the digestive organs, which are a part of the enteric nervous system.
4:25
The sympathetic and parasympathetic fibers, come from the lateral part of the spinal cord.
4:31
And they come from specific areas within the spinal cord.
4:34
The segments C8 to L2 are responsible for sympathetic nerve response, while the segments
4:41
S2 to S4 give parasympathetic nerve response.
4:46
And here’s a quick way to remember this.
4:48
S, stands for stress, to remind you that sympathetic neurons are responsible for fight or flight
4:55
response, or stress response.
4:57
What it does is that it causes a sudden increase of hormones that boosts the body’s alertness
5:02
and heart rate, sending extra blood to the muscles.
5:06
So all of this is gonna come from fibers that leave your spinal cord between the C8 and
5:13
L2 segments.
5:14
The P in Parasympathetic stands for Peace, which is rest and digest.
5:20
When the parasympathetic nervous system is activated, it slows our heart and breathing
5:25
rates, lowers blood pressure, and promotes digestion.
5:29
Our body enters a state of relaxation, and this relaxation breeds recovery.
5:35
All of those things come from the fibers that emerge between S2 and S4 spinal segments.
5:42
Alright!
5:43
Now I wanna share some clinical facts regarding motor neurons which’ll help you understand
5:48
the symptoms a person may experience having damage to motor neurons at different levels.
Damage to Motor Neurons
5:55
So from the clinical point of view, there are two types of motor neurones – Upper
6:00
and Lower motor neurons.
6:03
The upper motor neurones is located in the cerebral cortex and the lower motor neuron
6:09
is located in the anterior horns of the spinal cord and in the nuclei of the cranial nerves.
6:16
If there’s, you know, damage to the lower motor neuron.
6:19
How do you think it’s gonna look like?
6:22
Well if there’s a damage to the lower motor neurons, which is from the anterior horn of
6:27
the spinal cord, the patient’s gonna have a flaccid paralysis.
6:32
Meaning the muscle tone is gonna be decreased called hypotonia.
6:36
There’s gonna be a partial or complete impairment of voluntary movement, called paresis or plegia.
6:44
And the tendon reflexes are weakened or completely absent, called hyporeflexia or areflexia.
6:51
Ok so flaccid paralysis is a LMN damage, what happens if there’s a damage to the upper
6:59
motor neurons?
7:00
Damage to the motor neurons in the actual brain?
7:03
If there is a damage to the upper motor neurons, the patient will most probably experience
7:08
a condition called Spastic paralysis.
7:11
It occurs as a result of lesions in the cerebral motor cortex or from damage to the axons of
7:18
UMN that descend from the cortex to the spinal cord.
7:22
What are some conditions that may cause this?
7:26
This may arise after ischaemic insults seen in strokes, or fi there’s a perinatal hypoxia
7:32
that can occur from complicated labour.
7:35
Spastic paralysis manifests as partial or complete impairment of voluntary movements.
7:42
So we also get paresis or plegia here aswell.
7:45
But this time, we get an increased muscle tone, so hypertonia.
7:50
And signs of spasticity, so hyperreflexia and irritative pyramidal signs including the
7:57
Babinski sign.
7:59
Cerebral palsy is a classic example of an upper motor neurone disorder.
Spinal Nerve
8:03
To suture nerves a special microsurgical technique is used to apply stitches to the perineurium
8:10
of individual fascicles.
8:12
Alright!
8:13
I hope that made sense!
8:15
Now that we know a little bit about what nerves are considered the PNS, and a little bit about
8:21
the classification system of the PNS, basically, what type of neurons and systems are involved,
8:28
Let’s now go ahead and make a scheme of the spinal nerve because this is very, very important
8:33
to know when you wanna study the peripheral nervous system.
8:37
Ok.
8:38
So here we got a model of the brain and the spinal cord, with spinal nerves leaving from
8:43
it.
8:44
If we now take a section of one spinal nerve, together with the spinal cord.
8:48
We’ll see this.
External Scheme of the Spinal Nerve
8:50
So what we can see here from this scheme is a spinal cord and the spinal nerve.
8:56
Now, if we go ahead and zoom in a little bit on the spinal nerve.
9:00
We can see that the spinal nerve consists of rootlets, or roots, that connect the peripheral
9:06
nervous system to the spinal cord.
9:08
These rootlets meet and form the trunk of the spinal nerve.
9:13
And notice here that we can see the intervertebral foramen between the trunk and the roots of
9:18
the spinal nerve.
9:20
That means that the roots of the spinal nerve are located within the vertebral canal, and
9:25
the trunk is just outside of the vertebra, formed just as it passes the intervertebral
9:31
foramen, leaving the vertebral canal.
9:34
Alright.
9:35
Let’s go ahead and talk a little bit about the spinal rootlets first.
9:39
In order to do that, though.
9:40
We need to remind ourselves that the gray matter of the spinal cord is composed of an
9:46
anterior Anterior horn containing cell bodies of the somatomotor neurons or motor fibers.
9:52
There’s a Posterior horn containing cell bodies of the somatosensory neurons.
9:57
We got a lateral horn on specific areas of the spinal cord, containing cell bodies of
10:02
the visceromotor or the autonomic nervous system.
10:06
And then, we got the Intermediate zone, containing cell bodies of the visceral sensory neurons.
10:12
The roots of the spinal nerves are divided as the anterior spinal root, containing fibers
10:18
that exit the spinal cord and a posterior spinal root, containing fibers that primarily
10:23
enter the spinal cord.
10:26
Notice that the posterior spinal root has a ganglion called the spinal ganglion.
10:31
What is a ganglion?
10:33
A ganglion is just a group of cell bodies – In the peripheral nervous system, which
10:38
is outside the CNS.
10:40
So outside of the brain and the spinal cord, you gonna have different types of cell bodies
10:45
clustered together, forming a ganglion.
10:49
And if it’s located within the vertebral canal, like this one is, it’s then called the spinal
10:55
root ganglia, which has cell bodies of the afferent nerves entering the spinal cord
11:00
So that’s the external scheme of the rootlets.
11:04
Let’s now talk a little bit about the trunk of the spinal nerve and its branches.
11:08
When the spinal nerve leaves the spinal canal, it divides into a posterior branch, which
11:15
innervates the back.
11:16
It divides into an anterior branch, which innervates the anterior part of the trunk
11:21
and limbs.
11:22
There’s a meningeal branch that goes back into the vertebral canal to innervate the
11:27
spinal meninges.
11:30
Another thing we can see here is a Ganglion on the side here, called ganglia of the sympathetic
11:35
trunk, or simply paravertebral ganglia, so we got another ganglion now.
11:41
Remember, if the ganglia are located within the spinal cord, it’s called spinal ganglia.
11:47
If it’s located on the side of the spinal cord, it’s called chain ganglia or paravertebral
11:53
ganglia.
11:54
If they’re located in the front, they’re called prevertebral ganglia.
11:59
But this one highlighted in blue is a chain ganglia, or paravertebral ganglia, or ganglia
12:04
of the sympathetic trunk.
12:06
Many names for this one.
12:08
The chain ganglia will connect to each other, forming a sympathetic chain.
12:14
These are a part of the autonomic nervous system, and we’ll make our own dedicated video
12:18
for the autonomic nervous system.
12:20
But just keep in mind.
12:22
On the sides of the vertebral column, there will be a chain of neurons for the sympathetic
12:27
nerve connections, formed by paravertebral ganglions connected to each other, as well
12:33
as connected to the spinal nerve through the white rami communicans, containing myelinated
12:39
axons going to the sympathetic ganglion, we call these preganglionic fibers.
12:44
And the gray rami communicans.
12:47
Now the question is, why is it grey?
12:50
Because it contains fibers that are not myelinated.
12:53
It contains fibers that originate from the sympathetic ganglion and goes into the spinal
12:59
nerve containing postganglionic fibers that are unmyelinated.
13:04
Ok, so that we’ve got the general idea of the different parts of the actual spinal nerve.
Internal Scheme of Spinal Nerve
13:10
Let’s try to schematically draw all the types of neurons that go within the spinal nerve.
13:17
One type of fiber that’s going to go within the spinal nerve is the somatomotor fiber,
13:22
or motor fiber, that originates from the anterior horn of the spinal cord and go out to the
13:29
different nerves within the periphery of our body to regulate movements and tension in
13:34
skeletal muscles.
13:35
This one is efferent.
13:37
It leaves the spinal cord, so it goes through the anterior root of the spinal nerve.
13:43
So that was the somatomotor fibers.
13:46
Another efferent nerve that leaves the spinal cord is the Visceromotor fibers, or the Preganglionic
13:53
fibers, that leave the spinal cord from C8–L2.
13:58
What they do is that they join the ganglia of the sympathetic trunk via the white rami
14:03
communicantes.
14:04
So the white rami communicantes exist only at the levels of the spinal cord where the
14:10
intermediolateral cell column is present, so between C8-L2.
14:15
Remember, intermediolateral nuclei are responsible for the sympathetic outflow of neurons.
14:23
So they’re responsible for carrying preganglionic nerve fibers from the spinal cord to the paravertebral
14:29
ganglia.
14:31
The preganglionic nerve fibers will synapse with post ganglionic sympathetic fibers within
14:37
the paravertebral ganglia, which will go back into the spinal cord through the grey rami
14:43
communicans.
14:44
These postganglionic nerve fibers don’t necessarily have to go out from the gray ramus communicans
14:50
tho, though some fibers can leave the ganglia independently as well.
14:54
We also have the preganglionic parasympathetic fibers as well.
14:58
They’re not related to the sympathetic trunk, so they don’t synapse in the ganglia of the
15:04
sympathetic trunk.
15:05
But again, we’ll talk more about this in detail when we talk about the autonomic nervous system.
15:10
Alright!
15:12
So two types of fibers that leave the spinal cord through the anterior root are the somatomotor
15:18
and the visceromotor fibers.
15:20
Then we got Somatosensory fibers, which transfer information from mechanoreceptors, exteroceptors,
15:28
proprioceptors, and thermoreceptors on the body surface.
15:32
So, in other words, these fibers take in information like touch, pressure, vibration, temperature,
15:39
itch, tickle, and pain.
15:42
And since we have somatosensory fibers, we also got viscerosensory fibers, which transfer
15:49
information about baroreceptors and chemoreceptors and receptors side the internal organs, sensing
15:55
wall tension, special receptors for visceral pain and chemical stimuli.
16:02
So these fibers will go together and enter the posterior root of the spinal nerve, where
16:08
they’re gonna have their neural body within the spinal ganglion.
16:12
Then they’ll go into the spinal cord to synapse with nuclei, primarily in the posterior horn.
16:20
So these are primarily the fibers that go within the spinal nerve.
16:24
Now within the posterior branch of the spinal nerve, you’ll find postganglionic fibers,
Posterior Branch of Spinal Nerve
16:30
you’ll find somatomotor fibers, and somatosensory fibers as well.
16:35
The posterior branches are thinner and shorter than the anterior branches.
16:41
They’re going to innervate the muscles of the back and neck and provide sensory innervation
16:47
for the skin of the neck, back, and the butt.
16:51
Some important examples of these nerve branches, specifically the posterior branch of C1, are
16:57
called suboccipital nerves.
17:00
It’s a purely motor nerve for the suboccipital muscles and the semispinalis capitis.
17:06
The posterior branch of C2 is called the greater occipital nerve, which innervates the skin
17:13
of the occipital region and deep muscles of the back in the neck region
17:19
The posterior branch of C3 is called the occipital nerve, which is a purely somatosensory nerve
17:28
innervating a narrow area of the skin medial to the greater occipital nerve
17:33
Posterior branches of L1 to L3 are called the superior clunial nerves, which provide
17:39
somatosensory innervation of the superior gluteal region.
17:44
And the posterior branches of S1 to S3 are called middle clinical nerves, providing somatosensory
17:50
innervation of part of the gluteal region.
17:54
So that’s the posterior branch.
Anterior Branch of Spinal Nerve
17:57
Now the anterior branch of the spinal nerve is longer and thicker than the posterior branches.
18:05
They provide motor innervation of the muscles of the anterior abdomen, thorax, and neck,
18:11
and they also provide sensory innervation of the skin on the anterior aspect of the
18:15
abdomen and thorax.
18:17
What’s unique with the ventral branches of spinal nerves, though, is that they form plexuses.
18:23
Between C1 to C4, we got the Cervical plexus.
18:27
Between C4 to T1, they form the Brachial plexus.
18:32
We got the lumbar plexus between T12 and L4 and the sacral plexus between L4 to S4.
18:39
Notice that there’s a gap between T1 and T12.
18:43
Those anterior branches don’t form plexuses.
18:46
They leave as Thoracic nerves, which innervate the intercostal spaces primarily.
18:52
So, that was the overview of the peripheral nervous system I wanted you to have a general
18:57
idea of.
18:58
In the next few videos, we’ll go through each of these plexuses shown there.
19:03
We’ll animate them, highlight the most important things, and make an easy mindmap that’ll help
19:09
you remember them quite easily.
19:10
With that being said, see you next time.