External Spinal Cord (Surface, Segments, Spinal Nerve, Enlargements, Reflex Arch) - Anatomy
Summary
TLDRIn this educational video, Meditay explores the Central Nervous System's spinal cord, focusing on its external anatomy. The video covers the spinal cord's topography, including its length and termination at the L1/L2 vertebra region. It delves into the meninges, the external surface features, and the spinal cord's segments in relation to the vertebral column. The script also discusses the spinal cord's enlargements, the anatomy of spinal nerves, and the types of reflex arches. The video promises to continue with the internal structures in a subsequent part.
Takeaways
- đ§ The Central Nervous System (CNS) is composed of the encephalon (brain) and the spinal cord.
- đ The spinal cord extends from the Foramen Magnum to the L1/L2 vertebra region, averaging about 40-45cm in length.
- 𧏠The spinal cord is covered by three meningeal layers: Dura Mater, Arachnoid Mater, and Pia Mater, which protect the CNS.
- đ The spinal cord features distinct topographical landmarks including the Anterior Median Fissure and Posterior Median Sulcus.
- đ The spinal cord is segmented, with 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segments, totaling 31 segments.
- đ The spinal nerves emerge from the spinal cord through intervertebral foramina, aligning with the vertebral column in a specific pattern.
- đȘ The cervical and lumbosacral enlargements of the spinal cord are crucial for the formation of the brachial and sacral/lumbar plexuses, which innervate the limbs.
- đ§Ș Spinal nerves are divided into sensory (posterior root) and motor (anterior root) roots, with the posterior root containing a spinal ganglion.
- đ The spinal cord segments C8 to L2 are responsible for sympathetic responses, while S2 to S4 segments control parasympathetic responses.
- đ Spinal nerves branch into ventral, dorsal, white ramus communicans, and meningeal branches, serving various functions in the body.
- đââïž Reflexes in the body are either monosynaptic (quick and unconscious) or multisynaptic (slower and conscious), with examples like the patellar tendon and withdrawal reflexes.
Q & A
What are the two main parts of the Central Nervous System?
-The Central Nervous System consists of two parts: the encephalon (which includes the brain) and the spinal cord.
Where does the spinal cord begin and end anatomically?
-The spinal cord begins at the Foramen Magnum and ends at the L1/L2 vertebra region, forming the Medullary Cone or Conus Medullaris.
What is the average length of the spinal cord?
-The length of the spinal cord varies but is generally about 40-45cm long.
What are the three layers of the meninges that cover the spinal cord?
-The three layers of the meninges that cover the spinal cord are the Dura Mater, the Arachnoid mater, and the Pia mater.
What is the significance of the Anterior Median Fissure and the Posterior Median Sulcus on the spinal cord?
-The Anterior Median Fissure is a deep groove on the anterior side of the spinal cord, while the Posterior Median Sulcus is a shallow groove on the posterior midline of the spinal cord.
How does the spinal cord's segmentation relate to the vertebral column?
-The spinal cord is divided into 31 segments, which correspond to spinal nerves emerging from either side of the cord, whereas the vertebral column consists of 33 vertebrae.
What are the two enlargements of the spinal cord and their significance?
-The cervical enlargement (from C3 to T2) and the lumbosacral enlargement (from T12 to the medullary cone) are significant because they form the brachial and sacral/lumbar plexuses, which supply nerves to the upper and lower limbs respectively.
How does the spinal nerve divide upon entering the spinal cord?
-The spinal nerve divides into the posterior root (sensory) and the anterior root (motor) upon entering the spinal cord through the posterolateral and anterolateral sulci respectively.
What are the four branches of a spinal nerve after it exits the spinal cord?
-A spinal nerve branches into a ventral branch, a dorsal branch, a white ramus communicans, and a meningeal branch after exiting the spinal cord.
What are the two types of reflex arches in the body?
-The two types of reflex arches are monosynaptic reflexes, which are quick and unconscious, and multisynaptic reflexes, which are slower and conscious.
How does the spinal cord's position within the vertebral column change as one moves down from the cervical to the lumbar region?
-As one moves down from the cervical to the lumbar region, the spinal nerves bend more, and the spinal cord segments become increasingly higher relative to the corresponding vertebrae, with cervical segments aligning with the vertebrae above and lumbar segments aligning with vertebrae below.
Outlines
đ§ Anatomy of the Central Nervous System
The video segment introduces the Central Nervous System (CNS), focusing on the spinal cord's external anatomy. It explains that the CNS is composed of the encephalon and the spinal cord. The video will cover the spinal cord's topography, external surface features, segments, relationship with the vertebral column, and enlargements. It also mentions the meninges covering the CNS: Dura Mater, Arachnoid Mater, and Pia Mater. The spinal cord starts at the Foramen Magnum and ends at the L1/L2 vertebra region, with a length of about 40-45cm. It ends in the Medullary Cone and continues as the Filum Terminale, which is part of the Cauda Equina, a bundle of nerves supplying the lower body.
đ Topography and Segments of the Spinal Cord
This part of the script discusses the spinal cord's topography, explaining its start and end points, and how it relates to the vertebral column. It highlights the spinal cord's segments, which differ in number from the vertebrae, totaling 31 segments compared to 33 vertebrae. The video describes how spinal nerves emerge from the spinal cord and their arrangement within the vertebral canal. It also covers the cervical and lumbosacral enlargements of the spinal cord, which are significant for the brachial and sacral/lumbar plexuses that innervate the limbs and pelvis.
đ Anatomy of Spinal Nerves and Reflexes
The script delves into the anatomy of spinal nerves, explaining their division into sensory (posterior) and motor (anterior) roots, and the presence of spinal ganglia on the posterior roots. It describes how sensory information from the body enters the spinal cord and can either synapse with interneurons or connect directly to motor neurons to facilitate movement. The video also touches on the sympathetic and parasympathetic nervous systems, their segments of origin in the spinal cord, and their respective roles in stress and rest/digest responses. Lastly, it introduces the concept of reflex arcs, distinguishing between monosynaptic and multisynaptic reflexes.
đ Reflex Arcs and Internal Structures
This final paragraph summarizes the types of reflex arcs in the body: monosynaptic, which are quick and unconscious, exemplified by the patellar tendon reflex, and multisynaptic, which are slower and conscious, like the withdrawal reflex. The video concludes by indicating that the next segment will cover the internal tracts and nuclei of the spinal cord, promising further exploration of these complex structures.
Mindmap
Keywords
đĄCentral Nervous System
đĄSpinal Cord
đĄMeninges
đĄForamen Magnum
đĄMedullary Cone
đĄCauda Equina
đĄSpinal Nerves
đĄSegments
đĄEnlargments
đĄPlexuses
đĄReflex Arches
Highlights
The Central Nervous System consists of two parts: the encephalon and the spinal cord.
The spinal cord's topography starts at the Foramen Magnum and ends at the L1/L2 vertebra region.
The spinal cord is approximately 40-45cm long.
The spinal cord ends with the Medullary Cone and continues as the Filum Terminale.
The Filum Terminale is part of the Cauda Equina, a bundle of nerves supplying the lower body.
The spinal cord is covered by three meningeal layers: Dura Mater, Arachnoid Mater, and Pia Mater.
The spinal cord has an Anterior Median Fissure and a Posterior Median Sulcus.
The spinal cord is divided into 31 segments, corresponding to spinal nerves.
Spinal nerves emerge from the spinal cord through intervertebral foramina.
The spinal cord's enlargements, cervical and lumbosacral, are significant for nerve plexuses supplying limbs.
The cervical enlargement forms the brachial plexus, and the lumbosacral enlargement forms sacral and lumbar plexuses.
Spinal nerves are divided into posterior (sensory) and anterior (motor) roots.
Spinal ganglia contain nerve cell bodies and are part of the sensory system.
Sensory information from the body enters the spinal cord through pseudounipolar neurons.
The spinal cord's sympathetic and parasympathetic fibers are responsible for stress and rest responses.
A spinal nerve branches into a ventral branch, a dorsal branch, a white ramus communicans, and a meningeal branch.
There are two types of reflex arches: monosynaptic and multisynaptic.
Monosynaptic reflexes are quick and unconscious, like the patellar tendon reflex.
Multisynaptic reflexes are slower and conscious, involving more neurons, like the withdrawal reflex.
Transcripts
Whatâs up. Meditay here. Letâs talk about the anatomy of the Central Nervous System. Â
In this segment, we will be talking about the external anatomy of the spinal cord. Â
basically, go through everything you need to know in regards to what the spinal cord is Â
and what youâll find grossly on the spinal cord. Alright, so the Central Nervous System consists of Â
two parts. The encephalon, and the spinal cord. So in this is video, weâre first going Â
to go through the Topography of the Spinal cord, basically where it is, Â
where it starts and ends. Then weâll focus on the external surface of the spinal cord, Â
basically going through all the grooves and fissures you see there. Weâre also going to go Â
through the segments of the spinal cord and look at its relationship with the vertebral column. Â
Then weâll go through the enlargements we see on the spinal cord. After that weâll look at Â
the anatomy of a spinal nerve, and understand its 4 branches, and then quickly understand the Â
types of reflex arches we can have through the spinal cord. The internal structures and all the Â
nuclei and tracts will be covered in the next video so that this video doesnât get too long.Â
Alright, so here we see a posterior view of the vertebral column, if we remove one vertebra and Â
zoom in, youâll see the spinal cord right here, going through the vertebral canal.Â
So letâs go ahead and take the spinal cord out. Now the spinal cord is covered by a meningeal Â
layer called Dura Mater. And if we remove the dura mater, youâll find the Arachnoid mater, Â
and if w remove the arachnoid mater, weâll see a very thin and delicate connective tissue covering Â
called the Pia mater, and if we remove that, weâll finally get to the actual spinal cord. Â
These three are what we call Meninges and they cover the whole central nervous system. Weâll go Â
through these in a separate video. But now, letâs do the topography of the spinal cord.Â
So the spinal cord starts off at the Foramen Magnum, all the way to the L1/L2 vertebra region. Â
The length of of spinal cord varies a lot, but in general itâs about 40-45cm long. If we now Â
remove the bones, youâll see that the spinal cord ends by a structure called the Medullary Cone, Â
or Conus Medullaris, from here, a very thin thread goes out called Flium Terminale, which literally Â
translates as the terminal thread because itâs consider as the continuation of the spinal cord.Â
And if we see here, the Filum Terminale continues downwards together with a lot of different nerves Â
supplying the lower part of your body with nerves. And this area with all the nerves, is called Cauda Â
Equina. So that was it for the topography. Letâs now take a small part of the spinal cord Â
and look at the external surfaces of it. So here is the external surface of the spinal cord. Â
This is the internal surface of the spinal cord which we will talk about in the next video, Â
and these nerves we see here on the sides are the Spinal Nerves.Â
Alright, now for some orientation. The anterior part is where youâll find this deep fissure, Â
and the posterior part is more flat, with small bumps. This fissure we see on the anterior side, Â
is called the Anterior Median Fissure. Then posteriorly on the midline, Â
youâll find a Posterior median sulcus. On the sides of the spinal cord, youâll find Â
the Right and Left posterolateral sulci, and the right and left, anterolateral sulci. From where Â
the anterior and posterior root of the spinal nerve are going to go through as you see here, Â
weâll get back to this later when we talk about the spinal nerve.Â
But now, letâs talk about something called segments. Alright, so the vertebral column Â
consist of 7 cervical vertebra, 12 thoracic vertebra, 5 lumbar, 5 sacral and this varies, Â
but usually you have 4 coccygeal bones fused together. Adding a total of 33 vertebraes.Â
Now the spinal cord is different in that, itâs divided into 8 cervical segments, not 7, Â
but thereâs still 12 thoracic segments, 5 lumbar, 5 sacral, but then only 1 coccygeal segment, Â
which adds up to 31 spinal cord segments. Now why is the spinal cord divided into segments? Â
Because at each segment, thereâll be one spinal nerve emerging from either side, like you see Â
here. Meaning that picture we saw earlier, with one spinal nerve going out, is one segment. So we Â
have 31 of those you see here. And if we add the nerves from my 3d anatomy program, youâll see that Â
it looks like this. Now if we take the vertebral column and the spinal cord, and fuse them Â
together, youâll see this. Youâll see the spinal nerves going out from the vertebral column through Â
the Intervertebral openings, or the intervertebral foramina. But if the spinal cord has 31 segments, Â
and the vertebral column has 33 vertebra. And as we know now from the topography, with the spinal Â
cord ending at L1/L2 region. How are the spinal nerves arranged within the vertebral canal? And Â
to answer that, we need to look at the spinal cord and the vertebra from this perspective.Â
Letâs now fade this picture a little bit, and go through them part by part.Â
At the beginning, the spinous process of the cranial cervical vertebra, Â
cranial meaning up towards the head, correspond to the same level as the spinal cord segments, Â
and the spinal nerve leave above the first cervical vertebra as you see here. Â
Then as we continue slowly downwards. Youâll see that the spinal nerve starts to bend. And now, Â
the spinous process of the caudal cervical vertebra, caudal meaning towards the tail, Â
or away from the head, it correspond to one above the corresponding cervical spinal cord segment. Â
And by this I mean Vertebra C7 is at the same height as Spinal segment C8. So you could Â
take Segment +1 at this point. Because the spinal cord is getting compressed.Â
Then as we continue down, the spinal nerves bend even more and at this point, the spinous Â
process of the cranial thoracic vertebra, correspond to the thoracic spinous segment +2. Â
So you add two numbers to the vertebra youâre looking at, so vertebra T3, is at the level of Â
Spinal segment T5. And as you slowly continue down, the differences start to be greater. Â
The Caudal thoracic vertebrae start to correspond to the Spinal segment +3⊠and then as you continue Â
further down.. the T10-T12 vertebra becomes at the level of L1-L4 spinal cord segment, Â
and then at the Vertebra T12, L1 weâre starting to reach the end of the spinal cord, but not yet, so Â
we call this area the Epiconus, because remember the medullary cone is at the end, epi means above, Â
so above the end of the spinal cord. Thatâs what epiconus mean. This correspond to L5-S2 spinal Â
cord segment, and then lastly at the L1/L2 region, you have the rest of the spinal cord, Â
from S3 to S5 plus the one coccygeal segment. This scheme is just to help you visualize Â
how the spinal cord is arranged within the vertebral column. Now.Â
As you look at the spinal cord anteriorly, youâll notice two distinct enlargements. Â
One called the cervical enlargement, or intumenencia cervicalis. Which goes from the Â
segment C3 to T2, and a lumbosacral enlargement, or intumenencia lumbosacralis, going from T12 Â
down to the medullary cone. Now why are these significant? Because at these regions, you have Â
a bundle of nerves called plexuses, supplying the upper limb and the lower limb with nerves. Â
And these nerves have to be large and in a large quantity, in order to innercate all the muscles of Â
the lower limbs and the upper limbs with nerves. So the cervical enlargement forms the brachial Â
plexus for the nerves that goes to both arms, and the lumbosacral enlargement is for the sacral and Â
lumbar plexuses, innervating structures in the pelvis and the legs. So these are very Â
important. Now since weâre talking so much about the spinal nerves, letâs really understand the Â
anatomy of the spinal nerves. Because once you understand that, the actual internal and external Â
surfaces of the spinal cord becomes more logical. So if we take a segment of the spinal cord again, Â
youâll see the internal surface here. We will talk about this in detail in the next video, Â
but the internal surface consist of grey matter, and white matter. And how does this coorelates Â
with the spinal nerve? Because all the small neurons that go within the spinal nerves, Â
will synapse with nuclei in the grey matter. And Iâll show you how. So first you need to understand Â
where the spinal nerves enter the spinal cord. The spinal nerve enters the spinal cord through Â
the Right and left Posteriolateral sulci, and the right and left anterolateral sulci. Now lets Â
animate it a little bit and add some structures to make it look a little more realistic. Â
Here we can see the meninges. So the red thatâs closest tot eh spinal cord, Â
is the pia mater, the blue lining is the Arachnoid mater. And between the pia mater and the arachnoid Â
mater is the subarachnoid space, filled with cerebrospinal fluid that provide nutrients to Â
the spinal cord tissue. And then the outermost dense structure is the dura mater. So these are Â
the meninges. And remember, the deep fissure is anterior, and the more flat surface is posterior.Â
Now here you see the spinal nerve. The spina nerve is divided into two roots Â
before it enters the spinal cord. Itâs divided into the posterior root Â
or the sensory root, and itâs divided into the anterior root, Â
or the motor root. And Iâll mention this again, because I really want you to not forget this. Â
Posterior root enter through the right and left posterolateral suci, and anterior root Â
enter through the anterolateral sulci. But youâll notice that on the posterior root, thereâs a bulb, Â
a small enlargement called a Spinal ganglion, or sometimes referred to as dorsal root ganglion. Â
Youâre gonna have many ganglions in the body, and the reason why ganglions are bubbly, is because Â
they contain many nerve cell bodies as you see here. So dendritic fibers go from the periphery Â
towards the spinal ganglion, and then the axons of these neurons go into the spinal cord.Â
Now remember from previous video when we went through the different types of neurons. Â
What kind of nerve do you think this is? These are pseudounipolar neurons, going into he spinal cord, Â
so they take sensory information from anywhere in the body, and then enter the spinal cord so that Â
you can sense whatâs happening. So if you blow on your arm, that cold sensation is gonna enter Â
the spinal cord through this neuron. And once it enters the grey matter of the spinal cord, Â
it can either synapse with an interneuron and go further up towards your higher senses so Â
that you can make sense of whatâs happening, or it can connect directly to a motor neuron, Â
which go out from the spinal cord, to move a muscle to react in any way. And thereâs gonna Â
be a lot of connections to the motor neuron. Any voluntary movement you wanna do, either comes form Â
the interneurons ro the sensory neurons directly. So a spinal nerve consist of Sensory fibers, Â
Motor fibers, and either sympathetic or parasympathetic nerve fibers. So in the Â
spinal cord, youâll find the sensory fibers back here, the motor fibers are in the front here, Â
and the sympathetic and parasympathetic fibers, come from the lateral part fo the spinal cord. And Â
they come from specific areas within the spinal cord. The segments C8 to L2, are responsible for Â
sympathetic nerve response, while the segments S2 to S4 give parasympathetic nerve response. Â
And hereâs a quick way to remember this. S, stands from stress, to remind you Â
that sympathetic neurons are responsible for fight or flight response, or stress response, Â
meaning it increases your heart beat, makes you breath faster, more alert, all of those reacions Â
are gonna come from fibers that leave your spinal cord between the C8 and L2 segments. Â
The P in Parasympathetic stands for Peace, which is rest and digest. SO youâre chilling, Â
youâre sleeping, your intestines are doing its work to absorb the food, Â
and all of those things, come from the neurons that emerge between S2 and S4 spinal segments.Â
Now back to the spinal nerve. So once the spinal nerve leave the spinal cord. Â
It branches out into 4 parts. It becomes a ventral branch, which supply skin and muscles Â
of your limbs and the anterior and lateral part of the trunk. And as they do that, Â
they form plexuses. Weâll go through this when we go through the peripheral nervous system. Â
But plexuses are a huge network of neurons that supply regions of your body. So we have a cervical Â
plexus supplying areas associated with your neck and shoulders, the brachial plexus for your arms, Â
the lumbar plexus for your legs and pelvis and the sacral plexus for your pelvis and legs aswell. Â
So that is the ventral branch, forming these plexuses. The spinal nerve is also going to divide Â
into a dorsal branch for the skin and muscles of the back and neck. Then thereâs a white ramus Â
communicans, which relay sympathetic nerves. And those nerves are involuntary, so they reach out Â
to organs youâre not in control of, like your smooth muscles, glands and your visceral organs. Â
The last branch is a branch that goes back inside the vertebral canal to supply the meninges, Â
called the meningeal branch. So it goes back through the intervertebral foramen, to supply Â
the meninges. So that was the general anatomy of the spinal nerve. The last thing I wanna talk Â
about in this video, is a reflex arch. Within our body, we have two types of reflex arches. Â
Itâs either going to be a quick unconscious reflex through a monosynaptic reflex. And a more slower Â
conscious reflex, called multisynaptic reflex. Now the monosynaptic reflex are simple reflexes Â
that go through one synapse, remember synapse is when one neuron connect to another neuron. Â
So there are only two neurons involved here. And a famous example si the patellar tendon reflex. Â
Imagine youâre at your doctors office, and your doctor wants to assess your peripheral nerve Â
reflex response. So he takes up a hammer and taps your patellar tendon quickly, causing your leg to Â
kick out. What happens is that the impact of the hammer triggers a stretch receptor neuron Â
within your muscle, that quickly fires an action potential towards your spinal cord. Which then Â
quickly triggers a motor neuron to activate that muscle. We canât suppress this reflex, Â
because itâs physiological, it doesnât connect to an interneuron, which goes up to your brain.Â
A multi-synaptic reflex is different. And the withdrawal reflex is an example of that.Â
So letâs start here, by a candle triggering temperature Â
and pain receptors on your hand. That pain is sent through sensory neurons to your spinal cord, Â
which triggers interneurons that go up to your brain and trigger an ouch response, as well as Â
triggering a motor neuron to remove your hand as quickly as possible. It requires more neurons, Â
and itâs a conscious movement. So that was all I had for the external structure of the spinal cord Â
and the anatomy of the neuron and its reflexes. Letâs pause here so this video doesnât get too Â
long, and letâs do the internal tracts and nuclei of the spinal cord in the next video.
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