Internal Spinal Cord (Gray Matter, White Matter, Funiculus) - Anatomy

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 Internal  surface of the Spinal Cord. Basically go through everything you need to know regarding the anatomy  of the tracts and nuclei within 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 internal surface of the Spinal Cord and talk about the distribution of white and grey  matter within it. Then we’ll look detailed into the anatomy of the grey matter and the anatomy of  the white matter. Then at the end of this video, I’ve made a quiz which you’ll hopefully  be able to pass based on this video.

In our previous video, we looked at the Topography  and the external structures of the Spinal Cord, as well as the anatomy of the spinal nerve  and the two types of reflex arches we have through the spinal Cord. So if this is the  first time you’re studying the spinal Cord, I recommend you to watch that first because this  video will make much more sense with you knowing the external surface of the Spinal Cord. Alright.

We can start by taking a small segment of the spinal Cord, and look at  its internal surface. It’ll look like this. The internal surface of the Spinal Cord consists  of grey matter and white matter.

The reason why they’re called grey and white matter is because  if you look at a neuron. A neuron is colorless in general. And we know that because if you cut the  spinal Cord physically and look at it underneath the microscope without any significant staining,  you’ll see that the neurons are grey.

So the grey matter consists of nerve tissue rich in nerve  cell bodies and dendrites. White matter consists of nerve tissue rich in myelinated axons and  glial cells, as you see here. And the reason why myelinated axons are white is because they’re  rich in lipid, and lipids are white.

And keep this in mind throughout this video because you’ll  notice as we go through the structures in the grey matter, we’ll be talking about nuclei or nucleus  because cell bodies are in the grey matter. When we’re talking about the white matter, we  have structures called tracts, which are axons. So let’s start with the grey matter first. 

The grey matter consists of three horns. We have an anterior horn and a posterior horn, or  conu anterius and conu posterius. And where is the last horn?

The last horn is located in the  spinal segments associated with the sympathetic and the parasympathetic fibers, between the  C8 and L2, and S2 and S4 spinal cord segments. So if you cut the spinal Cord within those  areas, you’ll see that we have an anterior horn, a posterior horn, and a lateral horn  that give off either sympathetic fibers or parasympathetic fibers. So this segment  is from an area outside of the sympathetic or parasympathetic segments.

-- Between the anterior and posterior horn, there’s  the Intermediate column. Then in the middle, there’s the Central Zone, with the central canal  in the middle. The central canal is filled with cerebrospinal fluid, which is the same fluid as in  the subarachnoid space.

Around the central canal, there are cells called ependymal cells, forming a  central gelatinous substance of the Spinal Cord, which is the same lining as the rest  of the ventricles in the spinal Cord. So now I wanna focus on each of these  zones, basically go through the most important nuclei you’ll find, and  we’ll start with the Anterior horn. So the anterior horn is pretty easy as  it primarily consists of motor nuclei.

Their axons leave the spinal Cord as  the motor root of the spinal nerve. So that’s mostly it for the anterior horn. The  posterior horn is associated with receiving sensory information.

You’ll notice that it  consists of several nuclei responsible for certain types of sensory information. So the  first one is the marginal nucleus consisting of interneurons triggered by any sensory neuron  coming into the spinal Cord. Then there’s the gelatinous substance, which also consists  of interneurons that modulate sensory input Then there’s the Nucleus Proprius, which consists  of neurons that modulate sensory input like pain, touch, and temperature.

They take the information  and send them upwards to the higher senses After that, we have the posterior thoracic  nucleus, which receives unconscious proprioceptive movement. Basically giving information  about the position and posture of the body. So that was the posterior horn.

Now let’s do the intermediate zone. And the first nucleus here is the intermediomedial nucleus since  it’s located medially within the grey matter. This nucleus has more or less the same  function as the posterior thoracic nucleus, which is unconscious proprioception.

And some  sources consider the posterior thoracic nucleus as a part of the intermediate zone,  not a part of the posterior horn, so keep that in mind. But there are two  more nuclei within the intermediate zone, and they’re located within the lateral  horn. Remember we mentioned this earlier?

So in the lateral horn, we’ll find two different  nuclei depending on where we are within the spinal Cord. If we’re between spinal segments C8 through  L2, then we have the IntermedioLateral nucleus, which consists of sympathetic fibers for the  fight or flight response. They will send their fibers together with the motor fibers down  through the anterior root of the spinal nerve.

If we’re looking at a segment between S2 and S4,  we will see the Sacral Parasympathetic nuclei, which also send their fibers through  the anterior root of the spinal nerve, responsible for the rest and  digest state of the person. So the intermediate column consists of the  Intermediomedial nucleus, Intermediolateral nucleus, Sacral parasympathetic nucleus, and the  posterior thoracic nucleus if your sources say so. Alright, so now we’ve covered the majority of  nuclei within all three zones of the grey matter.

Along the outer part of the grey matter, you’ll  find a very thin layer of white matter called fasciculi proprii, which are fibers that  connect adjacent parts of the grey matter together or adjacent segments because you can  also divide the grey matter into segments. Another thing you’ll find is grey matter  that is pressed into the white matter called spinal reticular formation, or Formatio  retucularis spinalis, which continues upwards into the brainstem. It consists of  neurons that make a communicative network for activating certain sensory information for basic  living.

So it regulates respiration, heartbeat, blood pressure, and all of those things. Here’s maybe a better representation of the spinal reticular formation, and again  it goes upwards into the brainstem. So that was all for the grey matter of the  Spinal Cord, now let’s do the white matter.

The white matter of the Spinal Cord, remember  its nerve tissue rich in myelinated axons, so we’re not talking about nucleus anymore.  We’re talking about tracts or bundles of fibers. And the white matter is divided into three  portions because you’ll find connective tissue separating these three portions.

We have  the Posterior Funiculus, the Lateral funiculus and the Anterior funiculus. So again, these  funiculi are bundles of fibers that are surrounded by connective tissue. Now before  we go detailed into each of these funiculi.

I wanna spend a little bit of time  explaining the general arrangement of fibers within the white matter. And by understanding  that, the tracts will get much more logical. Alright, so nerve fibers are  arranged in bundles or tracts, right?

So here are many neurons. There are  connective tissue around all of those neurons, and that was what we call a tract. And there are  three .

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directions these tracts can go. They can either go upwards, as ascending tracts, or afferent tracts, remember a stands for  arrive, so these tracts arrive to the brain, but they can also descend, go down as descending  tracts, or efferent tracts e stands for exit. So the asceding tracts receive sensory  information from anywhere in the body, and send them up to your higher senses to make  sense out of them.

And then once you’ve done that, you’d wanna react to the sensory information,  or you just wanna move a muscle in general, so you activate the descending tracts. So you accidentally put your hand above something hot, sensory information is sent  up, and then you react by descending tracts activating muscles to remove your hand. And  again, all of that happens through funiculi, which means long ropes, found within the posterior,  lateral, and anterior parts of the Spinal Cord.

The ascending and descending tracts  can further be divided into two parts. The ascending tracts are generally divided into  unconscious and conscious sensory information. Now, what is the difference?

Well, everything  that goes to the cortex of your cerebrum is considered conscious. Unconscious sensory  input goes to the cerebellum. The cerebellum is an organ responsible for balance and posture, so  the unconscious ascending tract relays unconscious proprioceptive sensation.

You’ll see this word a  lot as you study the tracts of the central nervous system. So it’s important that you have a general  knowledge about it. Unconscious proprioception is responsible for posture, meaning sensory  fibers from muscles are all the time sent to the cerebellum so that it can activating  the necessary muscles to keep your posture, as well as activating muscles that support your  joints and the natural position of your limbs, so information about joint stability is  considered unconscious proprioception as well.

But there is one more thing. Imagine you’re  outside, walking and minding your own business. All of a sudden, YOU SEE A CAR DRIVING  TOWARDS YOU AND BREAKS RIGHT IN FRONT OF YOU.

Your initial response is tensing your  muscles, a process called feedforward control. Your muscles tenses due to something in the  external environment. And in the majority of times, you’re not in control of it, and we  consider this as unconscious proprioception as well.

They’re unconscious because they go to  the cerebellum, which is this organ right here. In our spinal Cord, we have tracts called  the anterior spinocerebellar tract and the lateral spinocerebellar tract. They end with the  word cerebellar, so they go to the cerebellum.

Conscious sensory information relays conscious  proprioception, like kinesia, which is conscious muscle movement. The conscious joint position  is considered a part of this system, as well as a sense of force. Meaning as your standing,  you consciously know that there’s a force acting against you, which is the floor against  your feet.

The conscious sensory information is also responsible for sensing certain things,  like touch, and pain and pressure and temperature. Examples of conscious ascending tracts are  the anterior and lateral spinothalamic tracts, the cuneate, and the gracilis fascicle, which send  their information to the cortex of the cerebrum. So that is the ascending tracts, but we also  divide the descending tracts into two parts.

We divide it into Involuntary  movements and voluntary movements. So one of the important parts of our cerebral  cortex is the primary motor cortex. If we look at it underneath the microscope, you’ll  find that it consists of pyramidal cells.

They’re pyramidal because they literally  look like pyramids. So when you decided you wanted to click on this video or search for the  internal surface of the Spinal Cord on youtube, you activated the pyramidal cells  to give the command to your muscles. All the tracts that come from the pyramidal  cells are called voluntary movements.

And they help you make fine conscious movements like  when you’re writing with a pen. These movements are so precise that they make your handwriting  look good. These tracts usually start with the name Cortico- Like the anterior corticospinal  tract and the lateral corticospinal tract.

When motor fibers doesn’t  come from the pyramidal cells, they’re called extrapyramidal tracts meaning these  tracts originate from other parts of your brain instead of the primary motor cortex So these movements are movements you don’t really think about, like keeping your  balance and posture when you’re walking, as well as rough movements or coarse movements. These  tracts doesn’t come from the primary motor cortex, so they have other names like rubrospinal  tract, tectospinal tract and olivospinal tracts. So I hope this made a little sense to you  all because once you’ve visualized this part.

The next part of this video will  be much easier to understand. So let’s now start by talking about the  tracts within each of these funiculi, and we’ll start with the posterior one. Ok, so in this diagram, I’ve made the blue colors represent sensory fibers and the  red ones representing motor fibers.

The posterior funiculus has two sensory tracts.  The first one is called Fasciculus Gracilis. This tract will conduct impulses from the  lower part of the trunk and the lower limbs, so it’s present in all the segments of the Spinal  Cord, and it takes the information to the cortex.

And since it brings info to the cortex, then  remember its function is conscious sensation. In this case, it’s responsible for the epicritic  sensibility of the lower parts of the body. Epicritic sensibility means Conscious  proprioception, which remember is Kinesia, joint position, and sense of force.

But epicritic sensibility also means receiving info from the mechanoreceptors. Now, what does  that mean? It senses two-point discrimination, so the minimal distance between two  touchpoints, until you actually sense that there are two objects touching you, that is  the two points discrimination.

Mechanoreceptors also sense vibration and touch. So all of those  things are what we call epicritic sensibility. The other ascending tract of the posterior  funiculus is the Fasciculus Cuneatus, which brings sensory information from the upper body and sends  it to the cortex.

So it does exactly the same as Fasciculus Gracilis, just for the upper body.  And since this tract si only for the upper body, you’ll find this tract only above the thoracic  spinal segment number 6. So that was all of the posterior funiculus.

Fasciculus Gracilis for lower  limb and Fasciclus Cuneatus for Upper limb. I use the letter G in Gracilis as Genitals to remember  that gracilis is for the lower part of the body Next, Let’s do the Lateral and the anterior  funiculus together because you’ll find tracts that do the same but are present on both  the lateral and the anterior funiculus. Just remember that they’re divided by  connective tissue.

These two funiculi are divided by connective tissue, and we’ll represent  the connective tissue using these two brown lines. The first tracts are ascending tracts  located on the lateral funiculus, called the SpinoCerebellar tracts. We have  two, there’s the anterior spinocerebellar tract and the posterior spinocerebellar  tract.

Before ascending up, the posterior spinocerebellar tract receives  sensory input from the posterior thoracic nucleus, and the anterior spinocerebellar tract receives  sensory input from the intermediomedial nucleus. They then ascend to the cerebellum because they  both end with the word cerebellum. And remember, if something ascends to the cerebellum and not  the cerebral cortex.

Then they provide unconscious proprioception, which later will provide  unconscious contraction of muscles for posture, joint stability, and the feedforward control  system. So that is the spinocerebellar tract. Next, we have the spinothalamic tracts, and there are two spinothalamic tracts in our  spinal Cord.

There’s the anterior spinothalamic tract in the anterior funiculus and a lateral  spinothalamic tract in the lateral funiculus. These two tracts are formed by axons coming from  the nucleus proprius, and then they ascend to the cortex. And since they go to the cortex, they  are responsible for the conscious sensation of pain, temperature, pressure, and touch.

So that  was all the sensory tracts of the Spinal Cord. Now let’s do the motor tracts  or the descending tracts. This includes the corticospinal tracts. 

We have two corticospinal tracts. There’s the Anterior corticospinal tract  in the anterior funiculus and the lateral corticospinal tracts in the lateral funiculus These tracts start with the name ‘’Cortico’’, meaning they come from the pyramidal  cells of the primary motor cortex. So they are pyramidal tracts.

Now the Lateral and Anterior corticospinal tracts descend differently, but they both synapse  with the motor nuclei to stimulate the motor fibers in the anterior root of the spinal nerve. Alright. So the Lateral Corticospinal tract starts off at the cortex and then descend,  but they decussate at the medulla oblongata, meaning they cross to the other side as you  see here at the medulla oblongata, and form the decussation of pyramids, and then they descend  further as the lateral corticospinal tract in the spinal Cord and synapse with the motor nuclei.

The Anterior corticospinal tract is a little bit different in that they descend without  crossing. Instead, they cross at each segment they’re going to leave from. So in this  example, if they’re gonna leave at this segment, they cross to the other side and synapse with  the motor nuclei.

And since they come from the cortex, they’re responsible for conscious  movement. So that’s the corticospinal tracts Next, we have the Rubrospinal tract. Rubro means  red, and the reason why they’re called rubrospinal tract Is because we have red nuclei located  inside the midbrain of the brainstem.

So these fibers are extrapyramidal fibers because they  originate from the red nucleus of the midbrain and go down as rubrospinal tract. And remember,  extrapyramidal tracts are responsible for fine coordination of movements and support voluntary  movements. They make our voluntary movements more precise.

SO that is the rubrospinal tract. The next tract is the tectospinal tract, located in the anterior funiculus. It transmits  motor impulses for the eyes and neck muscles meaning they coordinate the eyes  and the neck muscles when you look at something.

I’ve animated this very badly, but  imagine you’re looking at a hamburger, you look at it, and you keep looking at it as it passes you,  and your neck muscles follow your eyes. That’s what this tectospinal tract is responsible for. It’s called tectospinal tract because it comes from the tectum of the midbrain, it’s located  on the posterior surface of he midbrain, which is on your brainstem, and then descend  down and synapse with the motor nuclei.

It’s extrapyramidal, so it unconsciously  moved your neck muscles with your eyes. Next is the vestibulospinal tract. Inside of your  ear, the inner ear, you have a system called the vestibular system.

The vestibular system has  crystals within it sensing the position of your head, whether your head is tilted upside down or  to the side, all of that is sensed and through nerves, it’s sent to the brainstem, and then  down to your spinal Cord to keep your balance and posture. So the vestibulospinal tract is  responsible for keeping your balance and posture. So if you’re running and trip on a rock and you’re  about to fall, your vestibular system, along with other systems, will quickly react to that and  quickly activate the necessary muscles in order for you to keep your balance.

And this happens  involuntarily because this tract doesn’t come from your cortex. It comes from your brainstem. So that is the Vestibulospinal tract.

Next, we have the olivospinal tract. The olivospinal  tract is also responsible for balance and posture. Inside of your medulla oblongata, there  are olivary nuclei that send out fibers to the spinal Cord as the olivospinal tract,  which also synapse with the motor nuclei.

Keep that in mind all motor tracts will synapse  with the motor nuclei of the grey mater. Then we have the Reticulospinal Tract.  We have a lateral Reticulospinal tract and a medial reticulospinal tract which are  also a part fo the balance and posture system.

They come from the reticular  formation inside the brainstem and also from the reticular formation of  the Spinal Cord, remember we talked about that earlier? Just to remind you again. The  Reticular system are responsible for Sleep, alertness, cardiovascular control, breathing  and all of those vital things.

But they’re also responsible for motor control like your balance  and posture, through the reticulospinal tract. So that is all of these. Then lastly there’s  the medial longitudinal fascicle, located in the anterior fasciculus only in the cervical segments,  which coordinates involuntary movements of the head neck and eyes through synapses between  the cranial nerves.

And then they’re going to synapse with the motor nuclei of the grey matter  as well. So that was all I had for the grey and the white matter of the spinal cord. So I made  this table for the nuclei in the grey matter And this is where this video  gets scary, I am going to make all of the names disappear.

Now can you, from the  beginning, tell me what is the name of number 1, what is the name of number 2, and so on. And here is one for the white matter as well. You can pause the video if you want and have a  look, and then here is an empty table for you to fill.

So that was everything I had for the  anatomy of the Spinal Cord. If you found this video helpful, please put a like, comment,  share, whatever you find convenient to you. The next video is going to be  about the Medulla Oblongata.