External Spinal Cord (Surface, Segments, Spinal Nerve, Enlargements, Reflex Arch) - Anatomy

Summary

In 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

• 🧠 The Central Nervous System (CNS) is composed of the encephalon (brain) and the spinal cord.
• 📏 The spinal cord extends from the Foramen Magnum to the L1/L2 vertebra region, averaging about 40-45cm in length.
• 🧬 The spinal cord is covered by three meningeal layers: Dura Mater, Arachnoid Mater, and Pia Mater, which protect the CNS.
• 🔍 The spinal cord features distinct topographical landmarks including the Anterior Median Fissure and Posterior Median Sulcus.
• 🔗 The spinal cord is segmented, with 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segments, totaling 31 segments.
• 🌐 The spinal nerves emerge from the spinal cord through intervertebral foramina, aligning with the vertebral column in a specific pattern.
• 💪 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.
• 🧪 Spinal nerves are divided into sensory (posterior root) and motor (anterior root) roots, with the posterior root containing a spinal ganglion.
• 🔋 The spinal cord segments C8 to L2 are responsible for sympathetic responses, while S2 to S4 segments control parasympathetic responses.
• 🔄 Spinal nerves branch into ventral, dorsal, white ramus communicans, and meningeal branches, serving various functions in the body.
• 🏃‍♂️ 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

00:00

🧠 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.

05:03

🌐 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.

10:06

💉 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.

15:07

🔄 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.

Keywords

💡Central Nervous System

The Central Nervous System (CNS) is the part of the nervous system consisting of the brain and spinal cord. It plays a crucial role in receiving, processing, and responding to information from the body and the environment. In the video, the CNS is introduced as having two main components, the encephalon (brain) and the spinal cord, setting the stage for the detailed discussion of the spinal cord's anatomy that follows.

💡Spinal Cord

The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata as part of the brainstem to the lumbar vertebrae of the vertebral column. It is responsible for transmitting signals between the brain and the rest of the body. The video script focuses on the external anatomy of the spinal cord, including its topography, segments, and relationship with the vertebral column.

💡Meninges

Meninges are the三层保护膜 that envelop the brain and spinal cord, consisting of the dura mater, arachnoid mater, and pia mater. They provide protection and support to the delicate neural tissues. In the script, the presenter describes the removal of these layers to reveal the spinal cord itself, emphasizing their role in the CNS's anatomy.

💡Foramen Magnum

The Foramen Magnum is the large opening at the base of the skull through which the spinal cord passes to connect with the brain. It is the starting point of the spinal cord, as mentioned in the script, indicating the anatomical continuity between the brain and spinal cord.

💡Medullary Cone

The Medullary Cone, also known as the Conus Medullaris, is the tapered lower end of the spinal cord. The script describes how the spinal cord ends at this structure, which is significant as it marks the transition to the Filum Terminale and the beginning of the Cauda Equina.

💡Cauda Equina

Cauda Equina refers to the bundle of nerve roots (spinal nerves) that emerge from the lower end of the spinal cord and continue down through the vertebral column. The term is Latin for 'horse's tail,' reflecting its appearance. In the video, it is mentioned as the area where these nerves supply the lower part of the body.

💡Spinal Nerves

Spinal Nerves are the paired nerves that originate from the spinal cord and are responsible for transmitting sensory and motor information to and from the body. The script discusses how these nerves emerge from the spinal cord at each segment and their critical role in the nervous system.

💡Segments

Segments in the context of the spinal cord refer to the division of the cord into regions corresponding to each pair of spinal nerves. The video script explains how the spinal cord is divided into 31 segments, which is fewer than the 33 vertebrae of the vertebral column, highlighting the mismatch and the way spinal nerves exit the spinal column.

💡Enlargments

Enlargments on the spinal cord refer to areas where the cord widens to accommodate the increased number of nerve fibers. The script mentions two significant enlargements: the cervical enlargement (C3 to T2) and the lumbosacral enlargement (T12 to the medullary cone), which are related to the brachial and lumbosacral plexuses, respectively.

💡Plexuses

Plexuses are networks of interconnected nerves that branch out from the spinal nerves. They are significant for distributing nerve signals to specific regions of the body. The script explains how the cervical and lumbosacral enlargements form the brachial and lumbosacral plexuses, which innervate the upper and lower limbs.

💡Reflex Arches

Reflex Arches are neural pathways that mediate rapid, involuntary responses to stimuli. The script differentiates between monosynaptic reflexes, which are quick and unconscious, and multisynaptic reflexes, which are slower and conscious. Understanding reflex arches is crucial for grasping how the spinal cord processes and reacts to sensory input.

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

What’s up. Meditay here. Let’s talk about  the anatomy of the Central Nervous System.  

In this segment, we will be talking about  the external anatomy of the spinal cord.  

basically, go through everything you need to  know in regards to what the spinal cord is  

and what you’ll find grossly on the spinal cord. Alright, so the Central Nervous System consists of  

two parts. The encephalon, and the spinal cord. So in this is video, we’re first going  

to go through the Topography of the  Spinal cord, basically where it is,  

where it starts and ends. Then we’ll focus  on the external surface of the spinal cord,  

basically going through all the grooves and  fissures you see there. We’re also going to go  

through the segments of the spinal cord and look  at its relationship with the vertebral column.  

Then we’ll go through the enlargements we see  on the spinal cord. After that we’ll look at  

the anatomy of a spinal nerve, and understand  its 4 branches, and then quickly understand the  

types of reflex arches we can have through the  spinal cord. The internal structures and all the  

nuclei and tracts will be covered in the next  video so that this video doesn’t get too long. 

Alright, so here we see a posterior view of the  vertebral column, if we remove one vertebra and  

zoom in, you’ll see the spinal cord right  here, going through the vertebral canal. 

So let’s go ahead and take the spinal cord out.  Now the spinal cord is covered by a meningeal  

layer called Dura Mater. And if we remove the  dura mater, you’ll find the Arachnoid mater,  

and if w remove the arachnoid mater, we’ll see a  very thin and delicate connective tissue covering  

called the Pia mater, and if we remove that,  we’ll finally get to the actual spinal cord.  

These three are what we call Meninges and they  cover the whole central nervous system. We’ll go  

through these in a separate video. But now,  let’s do the topography of the spinal cord. 

So the spinal cord starts off at the Foramen  Magnum, all the way to the L1/L2 vertebra region.  

The length of of spinal cord varies a lot, but  in general it’s about 40-45cm long. If we now  

remove the bones, you’ll see that the spinal cord  ends by a structure called the Medullary Cone,  

or Conus Medullaris, from here, a very thin thread  goes out called Flium Terminale, which literally  

translates as the terminal thread because it’s  consider as the continuation of the spinal cord. 

And if we see here, the Filum Terminale continues  downwards together with a lot of different nerves  

supplying the lower part of your body with nerves.  And this area with all the nerves, is called Cauda  

Equina. So that was it for the topography.  Let’s now take a small part of the spinal cord  

and look at the external surfaces of it. So  here is the external surface of the spinal cord.  

This is the internal surface of the spinal cord  which we will talk about in the next video,  

and these nerves we see here on  the sides are the Spinal Nerves. 

Alright, now for some orientation. The anterior  part is where you’ll find this deep fissure,  

and the posterior part is more flat, with small  bumps. This fissure we see on the anterior side,  

is called the Anterior Median Fissure.  Then posteriorly on the midline,  

you’ll find a Posterior median sulcus. On the sides of the spinal cord, you’ll find  

the Right and Left posterolateral sulci, and the  right and left, anterolateral sulci. From where  

the anterior and posterior root of the spinal  nerve are going to go through as you see here,  

we’ll get back to this later when  we talk about the spinal nerve. 

But now, let’s talk about something called  segments. Alright, so the vertebral column  

consist of 7 cervical vertebra, 12 thoracic  vertebra, 5 lumbar, 5 sacral and this varies,  

but usually you have 4 coccygeal bones fused  together. Adding a total of 33 vertebraes. 

Now the spinal cord is different in that,  it’s divided into 8 cervical segments, not 7,  

but there’s still 12 thoracic segments, 5 lumbar,  5 sacral, but then only 1 coccygeal segment,  

which adds up to 31 spinal cord segments. Now  why is the spinal cord divided into segments?  

Because at each segment, there’ll be one spinal  nerve emerging from either side, like you see  

here. Meaning that picture we saw earlier, with  one spinal nerve going out, is one segment. So we  

have 31 of those you see here. And if we add the  nerves from my 3d anatomy program, you’ll see that  

it looks like this. Now if we take the vertebral  column and the spinal cord, and fuse them  

together, you’ll see this. You’ll see the spinal  nerves going out from the vertebral column through  

the Intervertebral openings, or the intervertebral  foramina. But if the spinal cord has 31 segments,  

and the vertebral column has 33 vertebra. And as  we know now from the topography, with the spinal  

cord ending at L1/L2 region. How are the spinal  nerves arranged within the vertebral canal? And  

to answer that, we need to look at the spinal  cord and the vertebra from this perspective. 

Let’s now fade this picture a little  bit, and go through them part by part. 

At the beginning, the spinous process  of the cranial cervical vertebra,  

cranial meaning up towards the head, correspond  to the same level as the spinal cord segments,  

and the spinal nerve leave above the  first cervical vertebra as you see here.  

Then as we continue slowly downwards. You’ll see  that the spinal nerve starts to bend. And now,  

the spinous process of the caudal cervical  vertebra, caudal meaning towards the tail,  

or away from the head, it correspond to one above  the corresponding cervical spinal cord segment.  

And by this I mean Vertebra C7 is at the same  height as Spinal segment C8. So you could  

take Segment +1 at this point. Because  the spinal cord is getting compressed. 

Then as we continue down, the spinal nerves  bend even more and at this point, the spinous  

process of the cranial thoracic vertebra,  correspond to the thoracic spinous segment +2.  

So you add two numbers to the vertebra you’re  looking at, so vertebra T3, is at the level of  

Spinal segment T5. And as you slowly continue  down, the differences start to be greater.  

The Caudal thoracic vertebrae start to correspond  to the Spinal segment +3… and then as you continue  

further down.. the T10-T12 vertebra becomes  at the level of L1-L4 spinal cord segment,  

and then at the Vertebra T12, L1 we’re starting to  reach the end of the spinal cord, but not yet, so  

we call this area the Epiconus, because remember  the medullary cone is at the end, epi means above,  

so above the end of the spinal cord. That’s what  epiconus mean. This correspond to L5-S2 spinal  

cord segment, and then lastly at the L1/L2  region, you have the rest of the spinal cord,  

from S3 to S5 plus the one coccygeal segment. This scheme is just to help you visualize  

how the spinal cord is arranged  within the vertebral column. Now. 

As you look at the spinal cord anteriorly,  you’ll notice two distinct enlargements.  

One called the cervical enlargement, or  intumenencia cervicalis. Which goes from the  

segment C3 to T2, and a lumbosacral enlargement,  or intumenencia lumbosacralis, going from T12  

down to the medullary cone. Now why are these  significant? Because at these regions, you have  

a bundle of nerves called plexuses, supplying  the upper limb and the lower limb with nerves.  

And these nerves have to be large and in a large  quantity, in order to innercate all the muscles of  

the lower limbs and the upper limbs with nerves.  So the cervical enlargement forms the brachial  

plexus for the nerves that goes to both arms, and  the lumbosacral enlargement is for the sacral and  

lumbar plexuses, innervating structures in  the pelvis and the legs. So these are very  

important. Now since we’re talking so much about  the spinal nerves, let’s really understand the  

anatomy of the spinal nerves. Because once you  understand that, the actual internal and external  

surfaces of the spinal cord becomes more logical. So if we take a segment of the spinal cord again,  

you’ll see the internal surface here. We will  talk about this in detail in the next video,  

but the internal surface consist of grey matter,  and white matter. And how does this coorelates  

with the spinal nerve? Because all the small  neurons that go within the spinal nerves,  

will synapse with nuclei in the grey matter. And  I’ll show you how. So first you need to understand  

where the spinal nerves enter the spinal cord.  The spinal nerve enters the spinal cord through  

the Right and left Posteriolateral sulci, and  the right and left anterolateral sulci. Now lets  

animate it a little bit and add some structures  to make it look a little more realistic.  

Here we can see the meninges. So the  red that’s closest tot eh spinal cord,  

is the pia mater, the blue lining is the Arachnoid  mater. And between the pia mater and the arachnoid  

mater is the subarachnoid space, filled with  cerebrospinal fluid that provide nutrients to  

the spinal cord tissue. And then the outermost  dense structure is the dura mater. So these are  

the meninges. And remember, the deep fissure is  anterior, and the more flat surface is posterior. 

Now here you see the spinal nerve. The  spina nerve is divided into two roots  

before it enters the spinal cord. It’s divided into the posterior root  

or the sensory root, and it’s  divided into the anterior root,  

or the motor root. And I’ll mention this again,  because I really want you to not forget this.  

Posterior root enter through the right and  left posterolateral suci, and anterior root  

enter through the anterolateral sulci. But you’ll  notice that on the posterior root, there’s a bulb,  

a small enlargement called a Spinal ganglion, or  sometimes referred to as dorsal root ganglion.  

You’re gonna have many ganglions in the body, and  the reason why ganglions are bubbly, is because  

they contain many nerve cell bodies as you see  here. So dendritic fibers go from the periphery  

towards the spinal ganglion, and then the  axons of these neurons go into the spinal cord. 

Now remember from previous video when we  went through the different types of neurons.  

What kind of nerve do you think this is? These are  pseudounipolar neurons, going into he spinal cord,  

so they take sensory information from anywhere in  the body, and then enter the spinal cord so that  

you can sense what’s happening. So if you blow  on your arm, that cold sensation is gonna enter  

the spinal cord through this neuron. And once  it enters the grey matter of the spinal cord,  

it can either synapse with an interneuron and  go further up towards your higher senses so  

that you can make sense of what’s happening,  or it can connect directly to a motor neuron,  

which go out from the spinal cord, to move a  muscle to react in any way. And there’s gonna  

be a lot of connections to the motor neuron. Any  voluntary movement you wanna do, either comes form  

the interneurons ro the sensory neurons directly. So a spinal nerve consist of Sensory fibers,  

Motor fibers, and either sympathetic or  parasympathetic nerve fibers. So in the  

spinal cord, you’ll find the sensory fibers back  here, the motor fibers are in the front here,  

and the sympathetic and parasympathetic fibers,  come from the lateral part fo the spinal cord. And  

they come from specific areas within the spinal  cord. The segments C8 to L2, are responsible for  

sympathetic nerve response, while the segments  S2 to S4 give parasympathetic nerve response.  

And here’s a quick way to remember this. S, stands from stress, to remind you  

that sympathetic neurons are responsible for  fight or flight response, or stress response,  

meaning it increases your heart beat, makes you  breath faster, more alert, all of those reacions  

are gonna come from fibers that leave your  spinal cord between the C8 and L2 segments.  

The P in Parasympathetic stands for Peace,  which is rest and digest. SO you’re chilling,  

you’re sleeping, your intestines are  doing its work to absorb the food,  

and all of those things, come from the neurons  that emerge between S2 and S4 spinal segments. 

Now back to the spinal nerve. So once  the spinal nerve leave the spinal cord.  

It branches out into 4 parts. It becomes a  ventral branch, which supply skin and muscles  

of your limbs and the anterior and lateral  part of the trunk. And as they do that,  

they form plexuses. We’ll go through this when  we go through the peripheral nervous system.  

But plexuses are a huge network of neurons that  supply regions of your body. So we have a cervical  

plexus supplying areas associated with your neck  and shoulders, the brachial plexus for your arms,  

the lumbar plexus for your legs and pelvis and  the sacral plexus for your pelvis and legs aswell.  

So that is the ventral branch, forming these  plexuses. The spinal nerve is also going to divide  

into a dorsal branch for the skin and muscles  of the back and neck. Then there’s a white ramus  

communicans, which relay sympathetic nerves. And  those nerves are involuntary, so they reach out  

to organs you’re not in control of, like your  smooth muscles, glands and your visceral organs.  

The last branch is a branch that goes back inside  the vertebral canal to supply the meninges,  

called the meningeal branch. So it goes back  through the intervertebral foramen, to supply  

the meninges. So that was the general anatomy  of the spinal nerve. The last thing I wanna talk  

about in this video, is a reflex arch. Within  our body, we have two types of reflex arches.  

It’s either going to be a quick unconscious reflex  through a monosynaptic reflex. And a more slower  

conscious reflex, called multisynaptic reflex. Now the monosynaptic reflex are simple reflexes  

that go through one synapse, remember synapse  is when one neuron connect to another neuron.  

So there are only two neurons involved here. And  a famous example si the patellar tendon reflex.  

Imagine you’re at your doctors office, and your  doctor wants to assess your peripheral nerve  

reflex response. So he takes up a hammer and taps  your patellar tendon quickly, causing your leg to  

kick out. What happens is that the impact of  the hammer triggers a stretch receptor neuron  

within your muscle, that quickly fires an action  potential towards your spinal cord. Which then  

quickly triggers a motor neuron to activate  that muscle. We can’t suppress this reflex,  

because it’s physiological, it doesn’t connect  to an interneuron, which goes up to your brain. 

A multi-synaptic reflex is different. And  the withdrawal reflex is an example of that. 

So let’s start here, by a  candle triggering temperature  

and pain receptors on your hand. That pain is  sent through sensory neurons to your spinal cord,  

which triggers interneurons that go up to your  brain and trigger an ouch response, as well as  

triggering a motor neuron to remove your hand as  quickly as possible. It requires more neurons,  

and it’s a conscious movement. So that was all I  had for the external structure of the spinal cord  

and the anatomy of the neuron and its reflexes.  Let’s pause here so this video doesn’t get too  

long, and let’s do the internal tracts and  nuclei of the spinal cord in the next video.