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Spinal membranes and nerve roots

Section of the spinal cord showing the spinal membranes and nerve roots.

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Hello everyone! This is Megan from Kenhub, and welcome to another anatomy tutorial. In today's tutorial, we're going to be looking at the spinal membranes that cover the spinal cord as well as the roots that emerge from the spinal cord to form the spinal nerves. Before we begin describing the spinal membrane, nerve roots, and all the other structures that we are going to look at in this tutorial, I just want to give a brief general description of the spinal cord.

The spinal cord and the brain together make up the central nervous system. The spinal cord extends from the foramen magnum all the way down to the first or second lumbar vertebra and it's enclosed in the vertebral column. It is surrounded by the meninges which we will be discussing later on in this tutorial and cerebrospinal fluid. It consists of gray and white matter and it can be subdivided into a cervical, thoracic and a lumbosacral portion with the spinal nerves arising from each region of the spinal cord.

If we look closely at the transverse section of the spinal cord, we can see that it's composed of white matter which is located at the periphery and is highlighted in green in the left image, and gray matter in the center which is seen here in the image on the right. There is a tiny central canal located in the very center of the gray matter which is barely visible in this magnification. This central canal is filled with cerebrospinal fluid.

The white matter surrounds the gray matter and contains myelinated and unmyelinated nerve fibers. These fibers conduct information either upward making them ascending or sensory fibers or downward making them descending or motor fibers. Therefore, the white matter is responsible for the conduction of the information inside the nervous system.

On the other hand, the gray matter contains the cell bodies of the neurons giving it a light gray appearance, and it is for this reason that gray matter is gray. The gray matter is responsible for receiving of the sensory information from the periphery and for sending motor stimuli to the periphery. In the same transverse section of the spinal cord, we can see that the gray matter is shaped like the letter H, or like a butterfly.

The anterior wings of the butterfly that you now see highlighted are known as the ventral horns and they contain cell bodies of the motor neurons that innervate the skeletal muscles. The posterior wings are known as the dorsal horns and they contain the cell bodies of the sensory neurons – in other words – of the neurons that receive the somatosensory information from the periphery. Finally, between the two wings, we can see this portion of gray matter that is called the intermediate gray matter.

In the thoracolumbar region, the intermediate gray matter projects two small horns – the lateral horns of the gray matter – that are clearly seen in this image. These horns contain autonomic neurons that innervate the visceral and pelvic organs.

Now that we've looked at the general structure of the spinal cord, let's move on to the principal topic of this tutorial – the membranes of the spinal cord.

The spinal cord is surrounded by meninges in the same way that the brain is surrounded by meninges. The spinal dura mater is the outermost of the three meninges that surround the spinal cord. The name dura mater means "strong or tough mother" due to the fact that it's the strongest of the three membranes covering the spinal cord. It is separated from the periosteum-like lining of the vertebral canal by a space that contains adipose tissue and the internal vertebral venous plexus known as the epidural space. There also exists a potential space between the inner surface of the dura mater and arachnoid mater which can widen into a real space under pathological conditions such as in the case of a subdural bleed when the space fills with blood. This is known as the subdural space.

Superiorly, the dura mater is continuous with the inner meningeal layer of the cranial dura mater at the foramen magnum of the skull. Caudally, the dura mater forms a dural sac that envelopes the cauda equina and extends downwards together with the filum terminale to the periosteum of the coccyx.

Another point I'd like to mention is that if you look at this image on the left, you can see that as the spinal nerves and the roots pass laterally, they are surrounded by tubular sleeves of dura mater which merge and become part of the outer covering of the epineurium of the nerves.

The spinal arachnoid mater is the second layer of the meninges surrounding the spinal cord and it's continuous with the cerebral arachnoid mater above. As we saw on the previous slide, there exists a capillary cleft between the inner surface of the dura mater and the outer surface of the arachnoid mater known as the subdural space. Between the arachnoid mater and the pia mater is the cerebrospinal fluid-filled space known as the subarachnoid space. In the same way as the previously discussed dura mater, the arachnoid mater accompanies the spinal roots through the intervertebral foramina.

Even though the spinal cord ends at about the level of L1 or L2, the arachnoid mater and the subarachnoid space continue down to the level of S2. This allows for physicians to access the cerebrospinal fluid below the level of L1 or L2 by inserting a needle between the vertebral lamina in a procedure known as a lumbar puncture or a spinal tap. By this means, the pressure of the cerebrospinal fluid can be measured, the fluid can be analyzed, and a spinal anesthetic can be introduced. In the next image, we can see a transverse view of the arachnoid mater of the spinal cord.

The innermost layer of the membranes of the spinal cord is the pia mater. The pia mater is very thin and transparent which is the reason that the only visible part of this structure is its vascular plexus. The word pia comes from the Latin meaning "delicate mother". It firmly adheres to the surface of the spinal cord and is vascularized containing numerous small blood vessels that penetrate into the spinal cord. Here in this illustration, you can see the vascular plexus of the pia mater. Now that we've described the meninges of the spinal cord, let's have a look at the spinal nerves and the way these nerves are formed.

As I mentioned previously, the ventral horns of the gray matter contain motor neurons that innervate the skeletal muscles. The axons of these neurons exit the spinal cord as six to ten short, thin filaments which are known as the rootlets of the anterior root. After a very short distance – as you can see in the image on the left – these rootlets merge to form the anterior or ventral root of the spinal nerve. So, the anterior root of the spinal nerve contains motor fibers which carry signals away from the central nervous system. But what about sensory information? How does information from the skin, the muscles, and the joints come to the central nervous system?

This information is relayed to the spinal cord by sensory cells located in the structure you can see on the far left, which is known as the dorsal root ganglion. The axons of these cells form the posterior root of the spinal nerve and finally these axons enter the spinal cord again as short, thin filaments – the rootlets of the posterior root. The anterior or ventral root and the posterior or dorsal root come close laterally and merge to form the spinal nerve. So – and this is important to remember – the spinal nerve contains both motor fibers from the ventral root and sensory fibers from the dorsal root.

There are thirty-one pairs of spinal nerves and they can be grouped according to each section of the spinal cord. So, to break it down, there are eight pairs of cervical spinal nerves, twelve pairs of thoracic spinal nerves, five pairs of lumbar spinal nerves, five pairs of sacral spinal nerves, and one pair of coccygeal spinal nerves. All of these nerves emerge from the vertebral canal through the intervertebral foramina to reach and innervate one part or aspect of the body. Shortly after the spinal nerve exits through the intervertebral foramen, it splits to give two major rami – the anterior ramus and the posterior ramus. Please note that these two rami carry both motor and sensory information.

The anterior ramus of the spinal nerve – also known as the ventral ramus – is the thickest of the two. Adjacent anterior rami communicate to form large networks or plexuses particularly the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus. However, in the thoracic region, the anterior rami of the spinal nerves remain distinct from each other and become continuous with the intercostal nerves. The nerves of the plexuses and the intercostal nerves generally provide motor and sensory innervation to the upper and lower limbs and to the ventral abdominal wall and the thoracic wall.

The second major branch of the spinal nerve is the posterior ramus or the dorsal ramus. As I mentioned previously, this branch is a mixed branch as well and carries both motor and sensory fibers. The motor fibers innervate the autochthonous or deep muscles of the back whereas the sensory fibers innervate the skin of the back on both sides of the vertebral column. In the next image, we can see the patterns of innervation more clearly.

Now, let's see something that is more complicated but we'll explain it in a simple way in order to understand it. The anterior ramus of the spinal nerve gives off two other branches – the rami communicantes or the communicating rami. Their names suggest that they are responsible for the communication of the spinal nerves with the autonomic nervous system. If you remember in the beginning of the tutorial, I mentioned that in the thoracolumbar region of the spinal cord, we see the lateral horns of the gray matter which is where the autonomic neurons are located. The axons of these neurons travel through the anterior roots and then through the spinal nerves.

As you can see in this image on the right immediately after the division of the spinal nerve into an anterior and posterior ramus, the autonomic axons leave the spinal nerve in order to reach the sympathetic trunk and especially the sympathetic ganglion. This thin branch that leaves the spinal nerve to reach the autonomic ganglion is called white ramus communicans because it's composed mainly of myelinated fibers and it has a white color. In the next image, we can see a transverse view of the white ramus communicans.

Some of the axons that arrive at this ganglion makes synapse here and then return to the spinal nerve in order to innervate organs in the periphery. This branch of fibers that leave the sympathetic ganglion to reach the spinal nerve is the gray ramus communicans and it's called gray because it's the unmyelinated fibers that give a gray color to these branches. In both this image and the following image, we can see the gray ramus enters the spinal nerve medial to the white ramus, making this unusual loop in the spinal nerve. So, in this close-up, we can see the white ramus communicans laterally, the gray ramus communicans medially, and the sympathetic ganglion.

The final structure that we're going to look at here is a blood vessel of the spinal cord. Here, we see the anterior spinal artery which supplies the anterior aspect of the spinal cord. This artery arises from the vertical arteries as the left and right anterior spinal arteries. These two arteries then unite at the level of the foramen magnum and descend along the anterior aspect of the spinal cord as one single trunk all the way down to the filum terminale. Along its course, the anterior spinal artery gives off several small branches. In the next image, we can see the anterior spinal artery at the thoracic level.

Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.

Now, good luck everyone, and I will see you next time.

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