Video: Spinal nerves

Picture yourself a keen Anatomy student, living in a big, bustling city within whose center has a main highway with many roads that connect its neighborhoods together. At the end of that highway is your school. To reach the highway which leads to the school, you commute along the local roads which provide you with a vital and smooth pathway to go and learn all about the wonderful workings of the human body.

Now imagine the same concept within your body. This time, your body is the bustling city, the school is your brain, the central highway is the spinal cord, the cars are packets of vital information, and finally, instead of the roads, we have spinal nerves, ensuring the information is able to be smoothly transported between the periphery and the spinal cord.

Welcome to the world of spinal nerves.

In this tutorial, we'll begin by briefly reviewing some anatomy of the vertebral column in order to understand how spinal nerves are formed and how they exit the vertebral column. We'll also be looking at how spinal nerves are numbered and how many spinal nerves arise from the different regions of the spinal cord. So to best understand how spinal nerves are formed and classified, let's quickly review some basic anatomy of the vertebral column.

You probably already know that there are five distinct regions which can be identified by the different characteristics of the vertebrae. Starting most superiorly, we find seven cervical vertebrae in the neck. Right below it are the 12 thoracic vertebrae which span the thorax. Lower still, in the lower back, we find the lumbar spine with its five lumbar vertebrae. It is followed by five sacral vertebrae which are normally fused into one bone called the sacrum. Right at the inferior tip of the vertebral column is a little structure called the coccyx, also known as the tailbone. It consists of approximately four coccygeal vertebrae fused to a variable extent.

Together, these 33 vertebrae form the vertebral column, which spans from the atlas, or C1, superiorly right to the tip of the coccyx inferiorly. We also have a couple of important openings to note. In the middle of each vertebra is a vertebral foramen. When all the vertebrae is stacked together, these vertebral foramen form the vertebral canal throughout the length of the vertebral column. From this lateral view, we can also see another opening formed between two adjacent vertebrae called the intervertebral foramen.

Now let's add the spinal cord to the vertebral column. We can see that it occupies the vertebral canal as far as the L1 to L2 vertebrae. Now, the bilateral pairs of nerves that you see exiting the vertebral column are the spinal nerves, which arise from the spinal cord. These spinal nerves exit the vertebral column on either side through the rounded openings that we've just seen which are the intervertebral foramina.

Spinal nerves are the largest component of the peripheral nervous system with cranial nerves being the other component. Spinal nerves go on to transmit afferent and efferent information between much of the periphery and central nervous system.

So now that we've seen how spinal nerves make their way out of the vertebral column, let's take a look at how they're formed.

In this section of the spinal cord, we can see a series of six to eight anterior and posterior rootlets emerging from either side at each level which then unites to form the anterior root and posterior root, respectively. The anterior root contains efferent or motor nerve fibers which carry stimuli away from the central nervous system towards peripheral target structures, while the posterior roots contain afferent or sensory nerve fibers that return sensory information from the periphery to the central nervous system.

After a short distance, the anterior and the posterior roots unite to form the spinal nerve. As spinal nerves are a combination of anterior and posterior roots, they are termed mixed nerves as they contain both afferent and efferent fibers and thus carry information in both directions between the periphery and the central nervous system, specifically, the spinal cord.

We can get a good view of the whole story in this transverse section where we can see the rootlets emerging from the spinal cord, forming the roots which unite into the spinal nerve that we can also see exiting from the intervertebral foramen here. You might have also noted that there's a swelling along the posterior root. This is known as the spinal ganglion, or dorsal root ganglion, and it contains the cell bodies of sensory neurons. The anterior root does not possess such a swelling as the cell bodies of the motor neurons are located in the anterior horn of the gray matter of the spinal cord instead.

Now once the spinal nerve exits the intervertebral foramen, we can see that it divides into two branches, or rami. We have the smaller posterior rami which provide innervation to the deep back muscles as well as the skin of the back while the anterior rami provide motor innervation -- somatic or visceral -- to most of the rest of the body related to that segmental level.

We can also see that just before the bifurcation into anterior and posterior rami, spinal nerves give rise to one or more recurrent meningeal branches which reenter the intervertebral foramen to supply the meninges and other structures of the vertebral canal. You can also see that communicating with the anterior ramus of the spinal nerves are the white and grey rami communicantes, which are responsible for relaying autonomic signals between the spinal nerves and the sympathetic trunk.

White rami communicantes arise from spinal nerves T1 to L2 and carry preganglionic nerve fibers from the spinal cord to the sympathetic chain ganglia. Grey rami communicantes, on the other hand, exist at all levels of the spinal cord and carry postganglionic nerve fibers from the sympathetic chain ganglia to their target locations.

So now that we have a good idea of how spinal nerves are formed, let's take a look at how many there are.

In total, we have 31 bilateral pairs of spinal nerves arising from the spinal cord. This includes 8 cervical spinal nerves, 12 thoracic, 5 lumbar, 5 sacral, and just 1 coccygeal spinal nerve.

Now you may have noticed these numbers are similar to the number of vertebrae in each region of the vertebral column with some small differences. One such difference is that there are eight pairs of cervical nerves but only seven cervical vertebrae, so how can this be?

Well, for the most part, the spinal nerves exit the vertebral canal through the intervertebral foramen below the corresponding vertebra. This is not the case for cervical nerves, however. Because the first spinal nerve exits between the skull and the first cervical vertebra, otherwise known as the atlas, spinal nerves C1 to C7 therefore exit the vertebral canal through the intervertebral foramen above their corresponding vertebra. Spinal nerve C8 does not have an associated vertebra and therefore exits the vertebral canal through the intervertebral foramen below the C7 vertebra.

Now as a result, each of the 12 pairs of thoracic spinal nerves will exit below the intervertebral foramina below their corresponding thoracic vertebra. The same is then true with the five pairs of lumbar spinal nerves which exit through the intervertebral foramina below their corresponding lumbar vertebra.

Now the sacral region is a little bit different. The five pairs of sacral spinal nerves technically still leave below their respective vertebrae, but it's important to remember that all five sacral vertebrae are normally fused together to form the sacrum. So instead of the intervertebral foramina being the final exit space, there is sacral foramina that parts of the spinal nerves travel through.

The sacral foramina are on both the posterior and anterior aspects of the sacrum. The anterior rami of S1 to S4 nerves exit through the anterior sacral foramina and the smaller posterior rami through the posterior counterpart. The S5 spinal nerves exit through the sacral hiatus, which is the inferior opening of the sacrum.

Lastly, we have the coccygeal region. This is another unusual arrangement as only one pair of nerves leaves the vertebral column over the anterior aspect of the coccyx, in spite of it having three to five segments.

So, looking at the spinal nerves, you might have noticed that something quite odd goes on in the vertebral canal. Although we have corresponding regions in the vertebral column and the spinal cord, they don't quite line up. The spinal cord terminates as the conus medullaris at around the L1 or L2 vertebral level, but the roots of many spinal nerves continue below that level in the vertebral canal and still leave through the intervertebral foramina below their respective vertebral levels.

Collectively, the spinal nerve roots that continue inferiorly beyond the conus medullaris are called the cauda equina which has the appearance of, and actually translates to, a horse's tail in Latin.

The other thing you may have noticed that didn't look quite uniform were the two swellings in the spinal cord. These are known as the cervical and lumbosacral enlargements. Now the sizes of different regions of the spinal cord are reflective of the number of structures that each region innervates. So more complex structures such as the limbs, which carry out functions like movement and interacting with what's around us, require greater neural output to innervate them. This means we have more wiring coming from the parts of the spinal cord responsible for those structures, and packing more wiring into a region will make it appear larger.

So with that in mind, you may be able to guess that the cervical enlargement contributes to the innervation of the upper limb and it involves spinal segments C5 to T1. So now you can imagine that the lumbosacral enlargement, which involves spinal segments L2 to S3, would be the spinal cord region that innervates the lower limb.

And that concludes our tutorial on the spinal nerves. Make sure to take the road to check out our other study units and content on the other topics of the nervous system.