Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where I’m going to be focusing on the spinal cord in situ.
Now, what we’re going to be doing here on this tutorial is discussing the different parts and structures which surround the spinal cord by looking at this image here of the spinal cord in situ.
Now, you might be asking now, “What does ‘in situ’ mean?” This is a Latin phrase that translates literally into “on sight” or “in position.” So we’re going to be looking at the spinal cord with all the structures that are supposed to be surrounding it or in the same area.
And to do so, we’re using this image that you’re looking at right now, which is a section of the spinal cord and the structures that surround it at the level of a thoracic vertebra.
We’re looking at it superiorly, so this is a section at the thoracic vertebrae, and we’re looking at it superiorly, where we can, then, highlight the different structures, starting off with this one that you see now on the screen, which is, then, the spinal cord.
The actual spinal cord, which is a long, thin, tubular bundle of nervous tissue and support cells that extends from this structure that you also see here from a medial view of the brain, that I just highlighted – this is the medulla oblongata. And the spinal cord will be extending from the medulla oblongata.
The brain and the spinal cord, together, make up the central nervous system, also known as CNS for abbreviation.
Now, looking at this image of the entire spinal cord, as you can see here, the spinal cord will begin at this bone here. This is a cut or section of the occipital bone on the skull, and then it will extend all the way down to the space between the first and second lumbar vertebrae.
And keep in mind that it does not extend through the entire length of the vertebral column.
In terms of length, the spinal cord is going to be 45 centimeters in men on average and 43 in women. Also, the spinal cord has a varying width ranging from 13 millimeters thick in the cervical and lumbar regions and about 6.4 millimeters thick on the thoracic area.
The spinal cord also has a function that I would like to cover here on this tutorial—functions primarily in the transmission of neural signals between the brain and the rest of your body but also contains neural circuits that can independently control numerous reflexes and central pattern generators.
The spinal cord has three major functions that I would like to highlight here. The first one is that works as a conduit for motor information, which travels down the spinal cord, and also as a conduit for sensory information, which will be travelling in the reverse direction.
And finally, it will work as a center for coordinating certain reflexes.
Now, let’s go back to this image here, where I'm now highlighting a structure that is in the proximity of the spinal cord. It doesn’t have any direct relationship to the spinal cord, but for us to understand what is happening here in situ, we need to highlight this one, which is the thoracic aorta, also known as descending aorta, which is part of the main blood vessel in your body, the aorta (the largest artery in your body).
Now, the thoracic or descending aorta starts at the aortic arch and runs down through the chest and abdomen. And like I mentioned, this artery is not directly relevant for the spinal cord, but it can be found in its vicinity.
We’re not going to go into a lot of details, but I just wanted to locate this structure on this image.
Like same thing, I would like to do here with these structures that you just see a bit of them—you just see a small cut of the two lungs—so the lung is an essential respiratory organ. And the two lungs are located near the backbone on either side of the heart as you can clearly see here. That’s why we’re highlighting them on this image.
Just for a little bit of information on this tutorial, there is a function associated to the lungs. You probably know this one. The main function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere.
So same thing here with the lungs. We’re not going to go into a lot more detail like we did with the aorta. Just for a bit of location.
Same thing happens to these structures that we’re now highlighting. This is known as the pleura. And each lung is covered by an exceedingly delicate serous membrane that we call then pleura, which is arranged in the form of a closed invaginated sac, and we can also see here, highlighted or in close proximity to the spinal cord in this particular area of the body.
Let’s highlight now a part of the vertebra. This is known as the body of the vertebra. And the body is the largest part of this bone and is more or less cylindrical in shape, as you can see on the image.
And on this particular illustration or image, we’re looking at a thoracic vertebra.
The upper and lower surfaces of the body of the vertebra will be flattened and rough and give attachment to the intervertebral discs.
Its anterior surface have a few, small apertures for the passage of nutrient vessels, and its posterior surface, you will find single, large, irregular aperture or occasionally more than one for the exit of the basivertebral veins from the body of the vertebra.
Believe it or not, there is also a function or a role associated to the body of the vertebra. And as part of the vertebral column, its role is to enclose and protect the spinal cord, as you can clearly see here by its shape around the spinal cord.
We’re moving on to this structure that you now see here, highlighted in green. This is a space known as the epidural space. And this is an anatomical space that is the outermost part of the spinal canal. It is the space within the canal formed by the surrounding vertebrae lying outside the dura mater.
The epidural space contains a few structures, including lymphatics, spinal nerve roots, loose fatty tissue, small arteries, and a network of large, thin-walled blood vessels called epidural venous plexus.
Now, back to the highlight here of the spinal cord, to say that the spinal cord is protected by three layers of tissue called the spinal meninges, which surround the canal. They include the dura mater, the arachnoid, and the pia mater.
We’re going to start off with the very first one that you see now, highlighted in green. This is known as the dura mater of the spinal cord, which is a thick membrane that is the outermost of the three layers of the meninges that surround the brain and the spinal cord.
Now, the dura mater has two layers – a superficial layer, which is also called a periosteal layer, serves as a skull’s inner periosteum called the endocranium. There is also deep layer called the meningeal layer, the actual which is the actual dura mater.
The dura mater of the spinal cord is supplied by a few blood vessels including the anterior and the middle meningeal arteries and the accessory meningeal arteries.
In terms of drainage, the two layers of the dura mater run together throughout most of the skull, where they separate. The gap between them is called, then, the dural venous sinus.
These sinuses drain blood and the cerebral spinal fluid from the brain and empty into the internal jugular vein.
There is also innervation of the dura mater of the spinal cord, which is via the small meningeal branches of the trigeminal nerve and the upper cervical nerves.
The role or function associated to the dura mater is that the dura surrounds the spinal cord and is responsible for keeping in the cerebrospinal fluid.
The dura mater is a sac that envelopes the arachnoid mater, which we will talk about next. It also supports and surrounds the dural sinuses (also called the dural venous sinuses, cerebral sinuses, or cranial sinuses). It also helps carry blood from the brain towards the heart.
We’re moving on to the next layer that you now see, a very thin layer that you see here, highlighted a bit more into or closer to the spinal cord. This is, then, the arachnoid mater.
This is the middle protective layer. Its name comes from the fact that the tissue has a spider web-like appearance, hence the word “arachnoid.”
Next structure that we’re going to be highlighting now, on the screen, is known as the subarachnoid cavity or subarachnoid space, which is an anatomical space between the arachnoid membrane and the pia mater.
The subarachnoid space is occupied by spongy tissue consisting of trabeculae, which are delicate connective tissue filaments that extend from the arachnoid mater and blend into the pia mater.
The subarachnoid space is also occupied by intercommunicating channels in which the cerebrospinal fluid is contained.
And speaking of which, the cerebrospinal fluid circulates in the subarachnoid space. The cerebrospinal fluid is a transparent, colorless fluid, and it is produced at about 500 milliliters per day.
The medical procedure known as lumbar puncture or spinal tap—you probably heard about it—involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space, usually from the lumbar region of the spine.
The next layer that you’re now seeing, highlighted in green, is known as the pia mater of the spinal cord. And you see a very thin layer here surrounding the spinal cord. This is, then, the innermost, protective layer of the spinal cord. It is very delicate and is tightly associated with the surface of the spinal cord, as you can clearly see here from this image.
And I will zoom in here a bit more, so you can see now a bit of the highlight of the pia mater, to add here that the pia mater attaches to the dura mater through 21 pairs of denticulate ligaments that pass through the arachnoid mater and dura mater of the spinal cord. These denticular ligaments help to anchor the spinal cord and prevent side-to-side movement providing, then, stability of the structure.
There is a few roles or functions associated to the pia mater of the spinal cord. And in conjunction with the other meningeal membranes, the pia mater functions to cover and protect the central nervous system.
It also functions to protect the blood vessels and enclose the venous sinuses near the central nervous system and to contain cerebral spinal fluid.
The pia mater also functions to deal with the deformation of the spinal cord under compression. Due to high elastic modulus of the pia mater, it is able to provide a constraint on the surface of the spinal cord.
Ventral root afferents are unmyelinated sensory axons located within the pia mater, and these ventral root afferents relay sensory information from the pia mater and allow for the transmission of pain from the disc herniation and other spinal injuries.
We’re going to move on to another highlight that you see on the image. This is known as the anterior root of the spinal nerve.
Nerve fibers leave the spinal cord in order to reach the rest of the body, the anterior root, and come back again in order to provide the brain with sensory information through the posterior root.
The anterior or ventral root of each spinal nerve consists of axons of motor neurons whose cell bodies are found in the grey matter of the spinal cord, comes from the anterolateral sulcus of the spinal cord and contains efferent fibers that run towards the muscles in order to give them motor impulses.
If you have an anterior root, you should have, then, this one that we’re now highlighting: the posterior root of the spinal nerve. And the posterior or also called dorsal root of the spinal nerves comes from the posterolateral sulcus of the spinal cord and contains, now, afferent fibers that transport sensory information from the peripheral areas back to the spinal cord.
We’re also going to highlight this structure here that is known as the dorsal root ganglia. And a dorsal root ganglion consists of nerve cell bodies in the dorsal root of the spinal nerve.
And keep in mind that the dorsal root only contains afferent nerve fibers coming from the peripheral areas back towards the spinal cord. These neurons are what we call pseudounipolar, which means that they don’t synapse in the ganglion.
The dendrites collect sensory information in the peripheral areas and run towards the ganglion where the axon sends the information towards, then, the spinal cord.
Now, we’re going to go back to this image here, so we can just list a few things that we’re going to talk about about the spinal nerve, which is found right about here on the image. And the anterior and posterior roots unite in order to become, then, the spinal nerve, which we can see here, the ventral or the anterior and the dorsal or posterior groups. They come together form to form, then, what is known to be as the spinal nerve.
And shortly after a spinal nerve exits the intervertebral foramen, then will branch into the dorsal ramus or dorsal branch, or ventral ramus or ventral branch, and also the rami communicantes, which we will talk about now. So we just wanted to list a few structures of the spinal nerve that we’re going to now highlight.
Now, this one is known as the anterior or ventral ramus, which is responsible for sensory and motor supply of the skin and muscles of the ventral trunk, as well as skin and musculature of the extremities via the plexuses.
The next one that I just mentioned, this one is known as the posterior ramus of the spinal nerve. And the posterior or dorsal ramus of the spinal nerve is smaller than the anterior ramus, as we can clearly notice here on this image—notice the anterior one is a bit bigger—and is responsible for, then, sensory and motor supply of the skin and musculature, but this time, of your back.
We’re now going to highlight these structures here that look like two little horns, and for that reason, we’re going to call them the lateral horns of the spinal cord.
The lateral horn of the spinal cord can be found at the thoracic and lumbar parts of the spine. It contains the cell bodies of the vegetative neurons belonging to the sympathetic nervous system. The fibers run towards the sympathetic trunk via the white or communicans ramus or branch and then synapse or get sent back to the spinal nerve via, then, the grey communicans ramus or branch.
The lateral horns connections control the functions of the sympathetic nervous system which, for example, changes the activity of your heart, lungs, liver, and gastrointestinal system to, then, prepare your body for emergency situations.
Next highlight that we see here on this image, this is known as the ganglion of the sympathetic trunk (“ganglion” for singular, “ganglia” for plural).
So the ganglia of the sympathetic trunk are located next to the spine as you see on this image. And they will receive sympathetic fibers from the spinal cord for controlling the sympathetic activity of the body.
These fibers synapse in the sympathetic ganglia and run back towards the spinal nerve via the grey communicans ramus where they are led to the area of action.
We’re going to zoom on this image to now highlight this structure, which is known as the white ramus communicans. And the white ramus communicans is a connecting branch between the spinal nerve and the sympathetic trunk.
The sympathetic fibers from the thoracic and lumbar segments reach the sympathetic ganglia of the sympathetic trunk, where they can either synapse at the ganglion on the same level or travel up, down at the sympathetic trunk to arrive at the correct spinal level for their action then.
It is called white because the fibers are rich in myelin.
We also see this one now that we’re highlighting, which is known, then, as the grey ramus communicans. And the grey ramus communicans is the connecting branch from the sympathetic trunk back to the spinal nerve.
The grey rami consist of post ganglionic sympathetic fibers and lead them to their target destinations.
Their lack of myelination makes them, then, appear grey.
We’re going to go back to a few structures that come out of the posterior ramus of the spinal nerve. Notice here, now, these two structures that are highlighted in green.
On the left side, we know… we see here that is the medial muscular ramus, and on the right side, we’re looking at a lateral muscular ramus, based on their positions in your body. And the medial and lateral muscular rami are the two branches of the posterior ramus that go towards the dorsal trunk. They’re going to innervate the intrinsic back musculature.
And finally, we’re going to be highlighting these structures that you see now, which are known as the recurrent meningeal branches of the spinal nerve. And the meningeal branches of the spinal nerve (also known as recurrent meningeal nerves) are a number of small nerves that branch from the spinal nerve near the origin of the anterior and posterior rami but before the rami communicantes branch.
They re-enter the intervertebral foramen and innervates the facet joints, the annulus fibrosis of the intervertebral disc, and the ligaments and periosteum of the spinal canal carrying pain sensation.