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Structure of the gray matter of the spinal cord in cross section.
Anatomists have been working to fully understand the brain and spinal cord since as far back as the fourth century B.C. From Hippocrates in ancient Greece to well-known 20th century anatomists such as Paul Broca, Carl Wernicke, and Korbinian Brodmann. Some of these names may already sound familiar because certain regions of the brain are named after these guys. For example, Broca's area, Wernicke's area, and Brodmann areas. Another well-renowned neuroscientist is the Swedish scientist Bror Rexed who, in the 1950s, created a new functional division of the spinal cord gray matter called Rexed laminae, which brings us to the subject of this tutorial – the gray matter of the spinal cord.
You might be wondering what exactly we’ll be learning about today, so let me just give you a quick overview.
We'll start by having a brief recap of gross anatomy of the spinal cord and what it looks like in cross-section. We'll then move on to our main event – the gray matter. We'll see how it's divided into three areas, namely, the anterior horn, the posterior horn, and the lateral or intermediate horn. We'll then take a closer look at the structures found within each of the horns, namely, the nuclei and how the gray matter of the spinal cord can be further subdivided into areas called the laminae. We’ll then look at how the structures within the gray matter contribute to its appearance at different levels of the spinal cord and we'll finish up with some clinical notes.
Before we start learning about inner structures of the spinal cord, it's useful to quickly remind ourselves of how it looks from the outside because it is often related to what's on the inside. We'll start with a really brief recap of the gross anatomy of the spinal cord, but if you feel like you're struggling with any of the information or if you simply want some more detailed revision, don't forget to check out our website for more excellent videos to help you out with that.
The spinal cord is a tubular structure which extends from the foramen magnum of the skull to approximately L1 to L2 spinal level of the vertebral column. It begins as an elongation of the medulla oblongata of the brainstem and terminates as conus medullaris. Below it, we find the cauda equina, and there are two enlargements along the length of the cord – one in the cervical region and one in the lumbosacral region. And we’ll actually learn why they're there later on in this tutorial.
At each vertebral level, a bilateral pair of nerve roots leaves the spinal cord via rootlets to form the thirty-one pairs of spinal nerves. Let's take a closer look at the cross-section of the spinal cord because that's the area we'll be focusing on today. We have the anterior aspect here, and over here, we have the posterior aspect. You'll also often encounter the terms ventral and dorsal, which mean anterior and posterior, respectively, especially in relation to the spinal cord structures.
So, you've obviously already identified the gray matter due to its distinct butterfly-like shape, and as you can see, it's surrounded by the white matter. We're not going to be going into any more detail on the white matter in this tutorial, but you can find out all about it in our white matter tracts tutorial.
Before we look at gray matter in detail, we should recap some of the anatomical landmarks that we find in a cross-section of the spinal cord. The spinal cord is very clearly divided into right and left halves anteriorly by the anterior median fissure and posteriorly by the posterior median septum with the posterior median sulcus referring to the indent at the septum. At the site of the posterior root attachment is the posterolateral sulcus, and in the cervical region between the posterior median sulcus and the posterolateral sulcus, you'll also find a longitudinal furrow called the intermediate posterior sulcus.
The anterolateral sulcus is found at the site of anterior roots joining the spinal cord, and immediately posterior to the anterior white commissure, you'll find the gray commissure connecting the two halves of gray matter. Bang in the middle of it is the central canal and some myelinated fibers run across the gray commissure just posterior to the central canal and form the posterior white commissure.
Okay, so we're now ready to learn all about the structures found in the gray matter and what role they play in the body.
True to its name, the gray matter of the spinal cord is distinguishable from the surrounding white matter by its grayish appearance. This grayish color is a result of the abundance of neuronal cell bodies in this area and relative lack of myelinated axons, which give the white matter its color. In addition to motor cell bodies, the gray matter also contains cell bodies of interneurons as well as some unmyelinated axons and glial support cells.
The gray matter has three main divisions of projections and these are called horns. They are the anterior horn, the posterior horn, and the intermediate or lateral horn that pops up in the thoracic and sacral regions, and it might be easier to imagine them as three little communities in a gray matter village. What you find in all these communities are occasional axons, dendrites and support cells, but what makes up the majority of gray matter are neuronal cell bodies. These cell bodies form little clusters of cells based on their functions and these are called nuclei. There are nuclei present in each of the three horns or communities, so you could probably imagine them as little residents of that community, and the residents of each community are responsible for a different job.
The anterior part of the spinal cord is responsible for motor function. So, if you want to, you could probably think of the ventral horn nuclei as workers who get told by the brain where to carry information. The posterior part is responsible for sensory function. So, in this case, you could perhaps imagine the nuclei in the dorsal horn as researchers, collecting information and sending it away to the brain to be processed.
The intermediate horn is a little bit of a strange one. It's like a group from a different tribe – the autonomic nervous system – that's been allowed to create a community in the gray matter village. It's mostly concerned with the sympathetic function – meaning our fight-or-flight involuntary response and with the parasympathetic function of the autonomic nervous system, which means the resting involuntary functions are rest and digest times.
Just as a reminder, the somatic nervous system controls the voluntary movements and includes both afferent or sensory and efferent or motor functions, which receive their innervation from the anterior and posterior horns of the gray matter in the spinal cord.
Alright then, so let's first travel to the anterior horn of the gray matter to meet the workers, or motor nuclei, which take information the brain sends them and directs it to the appropriate parts of the body. The nuclei of the anterior horn are arranged into three groups, namely, the medial group, the lateral group, and the central group.
So, the first group that we'll begin with is the medial group. The medial group is the one closest to the center, so it's a pretty easy one to remember. In addition, the medial group can also be further divided into ventromedial – so, anterior – and dorsomedial – or posterior – parts. So, if you remember, we said that the anterior horn houses the workers motor nuclei and that this specific area holds the cell bodies of nerves which innervate the muscles of the neck and trunk. All of these nuclei stretch the whole length of the spinal cord.
The lateral group follows the exact same principle. So, the nuclei in this group can also be subdivided into a few different groups. The most anterior group is called the ventrolateral group, while the one posterior to it is called the dorsolateral group, and the last one is the retrodorsolateral group which literally means behind the dorsolateral group and that makes it the most posterior of the three groups.
Unlike the medial group, the nuclei of the lateral group are only present in the cervical and lumbar regions of the spinal cord, and if you remember, I mentioned cervical and lumbar enlargements of the spinal cord a little bit earlier on in the tutorial. Well, the reason why they're present is because those regions of the spinal cord supply massive structures - the limbs. So, this is your big hint that the nuclei in the lateral group supply exactly these structures – the upper and the lower limb.
And just one more thing, there's something else that's quite neat about these nuclei. If you have a hard time remembering their functions as we all do, you can easily work it out because the most medial part of the anterior horn supplies the most medial or axial structures, which are the trunk and the neck, and conversely, the most lateral or distal part supplies the most distal part of the body, which are the limbs.
Of course, anatomy wouldn't be anatomy if it didn't complicate things just a little and this is where the central group of the anterior horn swoops in.
So, the central group comprises three nuclei, and the first one is called the phrenic nucleus. It only extends from the C3 to C5 spinal segments and innervates the diaphragm. So, you may already know the little poem to remember the roots of the phrenic nerve – feel free to say it along with me – C3, 4, and 5 keeps the diaphragm alive.
The next one is the spinal nucleus of the accessory nerve. Again, this shouldn't be too difficult to remember. The spinal accessory nerve is a cranial nerve, so it's in the head. So, do you know what the spinal section closest to the head is? I think you probably get where I'm going with this. So, this nucleus is only present between C1 and C5 vertebral levels and it innervates the trapezius and sternocleidomastoid muscles.
The last nucleus in this region is the lumbosacral nucleus, and the name pretty much speaks for itself. It stretches between L2 and S3 and its function is as yet unknown.
Are you with me so far? I hope so because we're about to visit the researchers in the posterior horn, which is the elongated part of the gray matter containing sensory nuclei. And we're going to start this journey from the apex and work our way towards the base of this column to meet the marginal nucleus, the substantia gelatinosa, the nucleus proprius, and the posterior thoracic nucleus.
Starting at the apex of the posterior horn, the first nucleus we encounter is the marginal nucleus. You can see it in this enlarged area of a transverse section of the spinal cord, and the marginal nucleus is also often referred to as either the posteromarginal nucleus or the marginal zone. It spans the whole length of the spinal cord and its main functions are relaying information from pain and thermal stimuli, and some fibers of the spinothalamic tract of the opposite side originate here too.
So, as we travel away from the apex, we meet the marginal nucleus’ next-door-neighbor – the substantial gelatinosa. It's also referred to as gelatinous substance. The substantia gelatinosa has a similar job to the marginal nucleus relaying peripheral pain and thermal stimuli and also stretches over the entire length of the spinal cord. If you follow its pathway into the brain, you'll see that it is continuous with the nucleus of the spinal tract of the trigeminal nerve and it also modifies transmissions of sensory input.
Deep to the substantia gelatinosa lies our big boy – the nucleus proprius. It contains second-order sensory neuron cell bodies which send out their central processes to form the lateral spinothalamic tract, and just like its neighbor, the nucleus proprius extends the full length of the spinal cord.
Deeper still, lives the posterior thoracic nucleus which is also known as the nucleus dorsalis, Clarke's nucleus, and the column of Clarke. Yeah, I know. Anatomist really couldn't make up their minds on this one. It constitutes the base of the posterior horn and it's a little bit different because it only stretches between C8 and L3. It receives proprioceptive feedback and it gives rise to the dorsal spinocerebellar tract.
Okay, and this region brings us to our foreign land – the lateral horn – which is quite different in its function to its neighbors. It's also known as the intermediate zone because it is squished between the two other horns. The lateral horn contains two discernable nuclei – the intermediolateral nucleus and the intermediomedial nucleus – which we're going to take a look at next.
The intermediolateral nucleus is the first one that we're going to be looking at and its name is rather explanatory of its location as it is the intermediate section and the more lateral of the two. It only stretches between T1 and L2 and it gives rise to preganglionic sympathetic fibers. It leaves the spinal cord through the anterior nerve root and reaches the ganglia in the sympathetic chain through the white rami communicantes forming the thoracolumbar outflow.
A bit more medially lies the intermediomedial nucleus. So, this one is a little bit less defined and some textbooks tend to leave it out completely or just refer to it as a group of cell bodies. It extends from S2 through to S4 and it gives rise to preganglionic parasympathetic fibers. These fibers then leave the spinal cord via the anterior roots of the spinal nerves and separates to form the pelvic splanchnic nerves.
So, I know what you're thinking. Doesn't the spinal cord finish at L1 to L2? Well, yes, but the spinal areas on the cord are not necessarily the same as the vertebral levels that they supply. If you pause the video to study in detail the image of the sagittal section through the spinal cord and the vertebral column we've been using, you'll see that the green and blue regions, in particular, which corresponds to the lumbar and sacral sections, respectively, actually sit superior to the vertebrae of the same level although their spinal nerves still leave the spinal canal at the level of the Associated vertebra.
Okay, so we're finished with learning all the nuclei. There's just one more thing I want to talk about which is another way to divide the gray matter into areas called the Rexed laminae.
So earlier on, we met Bror Rexed – the creator of the Rexed laminae – and he observed that the organization of cells and fibers within the gray matter exhibits a defined pattern which allowed him to identify ten layers known as the laminae. He observes that the cell bodies within the laminae numbered one to ten were grouped according to structure and function. While most things in anatomy are named anteroposteriorly, this guy just had to be a little bit different and named the laminae starting from the posterior horn. So, let's jump right into it.
So, we're going to be starting by looking at the lamina I to VI and describing their contents. So, starting with lamina I, lamina I contains the marginal nucleus and, of course, we already know this guy. We saw him located at the very tip of the posterior horn of gray matter. Lamina II, in turn, holds the substantia gelatinosa at the head of the dorsal horn and, interestingly enough, the nucleus proprius sits in lamina III and IV which you can see highlighted. Together, laminae I to IV form the head of the posterior horn of the gray matter and are responsible for relaying mostly sensory information from the skin.
The neck of the posterior horn is formed by lamina V and it is formed by a variety of sensory afferent cell bodies and can become the relay center for pain. The base of the posterior horn is formed by lamina VI, which deals with the sensory elements of the reflex arc of fast pain – for example, moving your hand away when you accidentally touch a hot hob which you probably do quite a lot if you're a total klutz like me. It also gets pretty tricky here as Clarke's nucleus falls both under laminae VI and VII.
You can see all the laminae annotated on the screen now and this will help you appreciate where they lie in relation to one another. It'll also make it easier to see that the border between the anterior and posterior horns is roughly the same as between laminae VI and VII.
Okay, so next, let's look at laminae VII through X starting with lamina VII. So, this lamina also contains the elements of the intermediate horn, the intermediomedial, and intermediolateral nuclei. It has connections to the midbrain and cerebellum through white matter tracts which are discussed in a separate tutorial, and it also houses some Renshaw cells, which form a part of the negative feedback loop.
Lamina VIII is a bit of a tricky one because it varies throughout the length of the spinal cord, so let's first look at its thoracic level. So, here, it takes up most of the ventral horn, but is reduced to the medial parts at the cervical and lumbar enlargements. This lamina mostly contains interneurons connected to fibers from different sources which makes it into a sort of relay point where information is sorted and passed on to other appropriate neurons.
Lamina IX contains various clusters of cell bodies of alpha and smaller gamma neurons which send information through their fibers to muscle spindles, and finally, lamina X is the no man's land – the area around the central canal. The function of the neurons in lamina X still remains unknown; however, it has been established that this area plays a role in somatosensory integration, autonomic regulation, visceral nociception, and the modulation of motor neuron output.
You can see the laminae VII to X labeled on your screen now, which will help you appreciate where they are in relation to one another.
The last thing I want to show you is how all of these structures in the gray matter affects the shape and size of the cross-section of the spinal cord at different levels. The gray matter will vary in size depending on the structures it supplies, so if you think back to the nuclei we've learned today, you'll not be surprised that the gray matter is largest in the cervical and lumbar enlargements where the lateral group of the anterior horn supplies the limbs.
So now that we've talked through the layers of laminae, let's have a quick chat now about some clinical notes that are relevant to this tutorial.
So, you've probably heard about poliomyelitis which is more commonly known as just polio. Polio is a viral infection which was highly feared in the past, but no longer due to the discovery of a vaccine in the 1950s. So, this vaccine led to the almost complete eradication of the disease around the world and there hasn't really been a recorded case of polio in the Americas since 1994. So, if this is the case, then why do we need to worry about it? Well, it still hasn't been eradicated completely, and in some cases, it can attack and destroy motor neurons in the anterior horn of the gray matter which is called spinal polio. and from what we've learned today, we know that it houses nuclei responsible for motor function.
The disease spreads by infected fecal matter entering the mouth often through contaminated water and sometimes through the infected person's saliva. Muscle weakness is an early symptom of the disease, but most often the disease is asymptomatic and can lay dormant in the carrier just to resurface years later. There's no cure for polio, only medications that can relieve the symptoms, and that's why the vaccine is so important.
Depending on the spinal level that is affected, polio can cause muscle weakness and paralysis which is usually asymmetric. For example, in this image on the right, we see a man with atrophy and paralysis of the right leg and foot which is a result of the polio. However, if the affected neurons are merely damaged, they'll recover and muscle function will return in a few days. If the motor neurons and the nuclei die, that creates irreversible paralysis and muscle atrophy as neurons can't regenerate.
Right, so it's time to recap everything that we've learnt today.
We started by looking at some anatomical features of the cross-section of the spinal cord and we first pointed out gray matter and white matter in our cross section, and we saw that gray matter mostly consists of cell bodies while white matter is mostly made up of myelinated axons. We then looked at some useful anatomical landmarks and these included the anterior median fissure and the posterior median septum which separate the two halves of the spinal cord, posterior median, posterolateral, intermediate posterior, and anterolateral sulci.
We then looked at the anterior white, gray, and posterior white commissures which allow the crossing over of fibers between the two halves of the spinal cord and central canal right in the center of it all. We then moved on to learn all the exciting things about gray matter. We started with its division into the anterior horn which houses motor or efferent neurons, the posterior horn which contains sensory or afferent neurons, and the lateral horn only found in thoracic and sacral regions which is a part of the autonomic nervous system as opposed to the other two which are part of the somatic or voluntary nervous system.
We then went on to learn about the little clusters of cell bodies in the gray matter – the nuclei. We started with the medial group in the anterior horn which is further subdivided into ventromedial and dorsomedial parts and supplies motor innervation to the neck and trunk muscles. It spans the whole length of the spinal cord and the lateral group had three divisions which were ventrolateral, dorsolateral, and retrodorsolateral nuclei.
You might remember that the naming of nuclei in the central group was a little bit different so we had the phrenic nucleus between C3 and C5 supplying the diaphragm, the spinal accessory nucleus found between C1 and C5 supplying trapezius and sternocleidomastoid, and the lumbosacral nucleus stretching between L2 and S3 with its mysterious function currently unknown.
We then worked our way through the posterior horn from the apex to the base starting with the marginal nucleus right on the tip. It spans the whole length of the cord and relays information for pain and thermal stimuli. Just deep to the marginal nucleus, we found substantia gelatinosa with the same function and span, and deeper still, we saw nucleus proprius giving origin to the anterior spinothalamic tract and spanning the whole length of the spinal cord. Forming the base of the posterior horn, we had Clarke's nucleus or nucleus dorsalis, and it is only found between C8 and L3 and it receives proprioceptive feedback as well as giving rise to posterior spinocerebellar tract.
We then moved on to the lateral horn or the intermediate zone where we met the intermediolateral nucleus stretching between T1 and L2 giving rise to preganglionic sympathetic fibers of the autonomic system. Between S2 and S4, we saw a group of cell bodies sometimes referred to as the intermediomedial nucleus giving rise to preganglionic parasympathetic fibers.
And that concluded our introduction to the different nuclei of the gray matter.
We then learned about a different division method of the gray matter called the Rexed laminae, and if you recall, there are ten laminae in each half of the spinal cord numbered posteroanteriorly. Lamina I contains the marginal nucleus, lamina II the substantia gelatinosa, and lamina III together hold the nucleus proprius. Lamina I to IV form the head of the posterior horn while lamina V forms the neck. Lamina VI forms the base and is the sensory pathway in the reflex arc of fast pain.
Together with lamina VII, lamina VI holds Clarke's nucleus, and also in lamina VII, you'll find elements of the intermediate horn. Lamina VIII varies through the length of the spinal cord being the largest in the thoracic region and mostly holds interneurons while laminae IX holds alpha and gamma motoneurons, and lamina X surrounds the central canal.
And we finished up, of course, by looking at how poliomyelitis can affect the motor neurons in the gray matter and cause irreversible paralysis and muscle atrophy.
And that brings us to the end of this tutorial. Thanks for watching, see you next time, happy studying!