Hey everybody, welcome to chapter 13. So we've introduced ourselves to basic nervous tissues. Now we're going to take a look at a couple of things here. One is the spinal cord. So it's our first look at, in lecture here, at the central nervous system, the brain and spinal cord or the CNS.
We're going to look at spinal nerves a little bit, although you'll do most of your learning there in lab. And then in the spinal cord itself, we're going to take a look at the ascending and descending spinal cord tracts, the ascending tracts of the sensory tracts, the descending tracts of the motor tracts. So there's a number of those, and there's some learning that goes on there. And then we'll also take a look at spinal reflexes, those reflex arcs that occur in the spinal cord. which is a part of the central nervous system processing that takes place outside of the brain.
So here we're going to start with some imagery that you should know from lab, really. So we're doubling up a little bit here with lab. You know, here's our basic look at the spinal cord. We know starting at the foramen magnum of the skull, passing down, we have the cervical enlargement here.
in the lower cervical and upper thoracic vertebrae, and that accommodates the increased neural traffic through the brachial plexus in and out of the arms. We pass inferior to that, we go through the thoracic region, and we get to the lumbar enlargement, which is where our lumbar plexus nerves pass out into the leg, and we have increased neural traffic in and out of the legs. so that we need an enlarged portion of the spinal cord for that.
Very abruptly inferior to the lumbar enlargement we have the conus medullaris where the spinal cord comes to an end so the the medullary cone there. The physical spinal cord there stops growing before you reach the age of 10 but your body keeps growing. And so as you continue to grow your skeleton, the spinal cord stays right there.
It ends up being right around L1 to L2. And then the spinal nerves that come out inferior to that have to stretch way down, way down. And this bundling of the spinal nerves, which are L3 through S5 in the coccygeal nerve, all have to travel through the vertebral canal together.
And we call this the cauda equina because it looks like a horse's tail. And that's what in Latin, cauda equina means horse's tail. Now, before I went any further talking about like whole spinal cord structures, I wanted to come to this diagram first or set of diagrams really.
To talk about the meningeal layers, it's important, and the positioning. of the spinal cord in the vertebral foramen, which we can see on the top of this. First of all, let's come down here and see some stuff that, you know, we talked about in lab, you know, so you have the meningeal layer, right, the three meninges. These are connective tissue coverings of the central nervous system. They have various functions.
The innermost layer is the pia mater here. Mater means mother in Latin, so these are the mother layers across the central nervous tissue. So the pia mater is like a shrink wrap right on the surface of the spinal cord and brain.
The arachnoid mater here is quite different. You can see here it's superficial to the pia, so it's between the pia mater and the dura. And here's the dura mater out here, the tough mother, or the dural sheath here.
So this arachnoid mater here is responsible for containing and circulating the cerebrospinal fluid. So how do we do this? If you look up here, what we notice is that between these meningeal layers, there are spaces or potential spaces. So, for example, between the pia mater, I'll zoom this up even really much closer than that.
So if we look in here, here's the pia mater on the surface, and here's the arachnoid mater. So between the pia mater and arachnoid mater, we have this subarachnoid space, and you can see that labeled way over here, subarachnoid space. That subarachnoid space is where the cerebrospinal fluid is circulating. Now, We will talk extensively about cerebrospinal fluid, its production and circulation in the next chapter, okay?
But I wanted to introduce the concept here so we could talk about a few aspects here of spinal cord anatomy. If we look here, there is the dura mater in orangish-red, whatever that color is, right? There's actually a space between the dura mater and the orangish-red.
the arachnoid monitor it's called the subdural space it's a potential space only there's no actual gap there but it's it's anatomically defined outside the dura though you see here they show adipose tissue in the epidural space epi means above or outside so there's no subdural space in all practical sense, but there is an epidural space and that lies between the dura mater and the bone of the vertebrae of the vertebral canal. So these are, you know, this image here shows us some interesting things. First of all these spaces here and the primary space we pay attention to is this subarachnoid space, all this shaded blue here, and that's where our cerebrospinal fluid is. the epidural space out here with its fat.
Now this fat has zero physiological significance to the spinal cord here, but it has a lot to do with cushioning of the spinal cord in the vertebral canal. Listen, your central nervous tissues don't do well when you bruise them. So we have many, many things in place to make sure that you don't do that.
One of those is that we have in the spinal cord in the vertebral canal we surround the spinal cord with adipose tissue so it's cushioned on all sides. We also, if I back off here, when we talk about protection of the spinal cord the dura mater here is a very tough connective tissue layer. What you notice is that the arachnoid mater and dura mater follow the nerve roots away from the spinal cord and out into the nerves.
In fact, the dura mater here becomes the outer connective tissue covering the epineurium of the nerves. So remember this diagram here we just came from? If we consider... Each of these spinal nerves that come off at every level, followed out by the dura and arachnoid, that anchors like a bunch of buttons all the way up and down, like a zipper, locking the spinal cord into place because it's completely enveloped by that dura mater. that then gets locked into place at every spinal nerve.
It's kind of cool actually. So that's another aspect of spinal cord stabilization. Another one here is these denticulate ligaments. I'll use a different color here. These denticulate ligaments here.
And what we these denticulate ligaments occur at every spinal level also. And I'll show you a better picture of these on a cadaver section on the next slide. They also stabilize the spinal cord laterally left to right. Okay. And then at the termination of the spinal cord, we'll come back up to this upper right diagram here in a minute.
The determination of the spinal cord at the conus medullaris, the arachnoid and dura mater. layers keep going and align the vertebral canal all the way down to the base of the sacrum. But the pia mater doesn't.
The pia mater ends at the conus medullaris with one small exception. So we'll come back in here and remember what we're looking at here. So here's that cauda equina and the medullary cone, the conus medullaris.
Let's come back up to that conus medullaris. So here's the end, the termination of the spinal cord. Look at what you have here. The dura mater and arachnoid mater keep going. So that subarachnoid space is that blue space where the cervical spinal fluid is circulating.
It keeps going. So the spinal nerves of the cauda equina are completely bathed inside this chamber. And that's actually called the... lumbar cistern.
But the pia mater, the pia mater off the tip of the medullary cone keeps going as the phylum terminal. And when it reaches the very end here of where the dura and arachnoid mater stop. It continues on where it gathers with it also the dura and the arachnoid, and they travel together all the way down to the coccyx, and that becomes the coccygeal ligament down here. So the pia mater, really that connective tissue, is what makes that phylum terminal inside the cauda equina region.
And as it passes out of that cistern area, it gathers with it the dura and arachnoid, and they travel together as one band of connective tissue, and that provides stabilization vertically for the spinal cord. So that phylum terminal is pulling down. and stabilizing spinal cord vertically where the spinal nerves and the denticulate ligaments are stabilizing it laterally and it's being cushioned by adipose tissue all up and down so the spinal cord is well taken care of inside its vertebral canal there Here's a look at some cadaver sections of human spinal cords. And I show you this to kind of give you a little bit better idea of the root layout and the meningeal layers. So let me zoom in over here for a second.
What are we looking at here? Here's the actual spinal cord, right? And there's your anterior median fissure. So we're looking at this from the anterior or ventral side. And what do we have here is a ventral root, right?
Because we're looking at it from the ventral side. And so the ventral roots, the anterior roots are facing us. And you can see on this one right here, you can see the dorsal root going away from you in the screen.
That's kind of cool, but I want to look at the meningeal layers. If this is the actual bare surface of the spinal cord, this is the pia mater, so that lighter gray tissue. And it's right on the surface. whoever did this dissection has resected it off a little bit and then here this gap right here is your uh sub arachnoid space because this tissue right here is the arachnoid mater and they have it labeled right there so here's your arachnoid mater here's your sub arachnoid space and this tissue layer out here is the dura mater they've pulled that way aside so it's over here too And you can see the nerve roots passing through the dura.
And the dura is going to follow those out into the spinal nerve. So you can see how the nerve roots have kind of punctured that and gone out through. So, yeah, I just wanted to show you this. And then they've got the denticulate ligaments here. So you'll see there's one here.
there's one here there's one there there's one there there's one there another one there look at that So at every, between actually every spinal level, there's a denticulate ligament stabilizing that spinal cord from side to side. And it's enveloped in those three meningeal layers. And so here's a different look.
And actually, if you notice, here we have the posterior median sulcus. Now we have a posterior view. This is the posterior root. There's the anterior root underneath there. There's the ganglion on that posterior root.
So we're just seeing it from another way. So here's the dura mater here. They have the arachnoid mater. I don't know if they actually have the pia mater on this.
But, yeah, I mean, the spinal cord is pretty neat. So I just wanted to show you a few things about this stuff so that we can. have some intelligent discussions later and in lab. This should be familiar territory to everybody, a look at the spinal cord itself. We have this idea of gray matter and white matter, the gray matter being on the interior of the spinal cord, the white matter being on the outside, the gray matter being the horns, we call them.
and the white matter being the columns. And we're going to see later in this chapter we call them columns because information is going up to the brain and back down from the brain through this white matter. So this is vertical information, whereas gray matter is horizontal information.
In the spinal cord, like virtually everywhere else in the body, we have bilateral symmetry, so we have left side, right side. We have to be able to determine anterior from posterior though. Once you can do that, you're in good shape.
The best and easiest way is to look at the roots out here. So look, here we have the dorsal root and the ventral root. So ventral, dorsal.
How do you know the dorsal? The dorsal has this ganglion. here. So the dorsal root ganglion. There's a bulge in the dorsal root, not in the ventral.
Keep in mind here that the dorsal root represents incoming sensory information. The ventral root is outgoing motor. So these are one way. Ventral out, dorsal in. Once they reach the spinal nerve, then they become two-way traffic within the same nerve.
And you can kind of see a little bit better of that up here. So once you know dorsal here, there's some other ways to do it. First of all, on the ventral or anterior side, there's a fissure here called the anterior median fissure. On the dorsal side, it's not a fissure. They're not separated.
It's what's called a sulcus. And a sulcus, there's a couple of sulci here. They only label the median one.
That's where two pieces of tissue come together. They are separate tissues. but they are physically bound together. So it just makes a crease here that separates from left and right.
So that's a sulcus, that's a fissure. Okay, so on the ventral side, you have a root with no ganglion, and you have the fissure. Dorsal side, you have a root with a ganglion and just a sulcus. Once you get used to looking at things, you can look at the gray matter here, and you can pretty much tell the dorsal or posterior gray horn will be sharper and pointier, and the ventral gray horn will be wider and rounder. Sometimes there's a lateral gray horn.
We will wait until we do the autonomic nervous system chapter to talk about that, because that's an important aspect there. With the columns here, you know, if you're from the root, From the ventral root to the fissure, we call that the anterior column or ventral column. Between the roots, it's the lateral column.
And between the dorsal root and the sulcus is the posterior or dorsal column. If we come over here, this is stuff for lab, but you can see the same thing on a tissue slide. Here's a look at...
at the gross anatomy of a nerve. This will look very familiar to anybody who knows about the gross anatomy of whole muscle. You have a connective tissue layer on the outside that's very tough.
This is the epineurium, and that's going to be contiguous with the dura mater. So when the dura mater follows the roots out, it becomes the epineurium. And then you have fascicle bundles inside carrying axons, myelinated or unmyelinated, but they're carrying axons inside the fascicles.
And so the fascicles tend to bundle together axons that are going to and from the same region. It's just like any kind of wiring setup you might have. And then inside the fascicle, which is surrounded by a perineurium, this purple layer, you have the axons that are then cushioned in an areolar tissue, and that's called the endoneurium. So this can be very similar to muscle layout.
This set of diagrams is an interesting look at kind of the intersection here of the two spinal roots, the dorsal and ventral root, and the spinal nerve. I'm going to key in here on the top portion first. All right, so remember with the ventral root here, that's outgoing motor traffic.
So those are signals from the central nervous system going out to the skeletal muscles. and such and the dorsal root here is bringing sensory information in right and that comes into the dorsal gray horn okay so the spinal nerve itself is two-way traffic right outgoing and incoming but the the two-way traffic splits at the root so they become one way outgoing and eventual and going in incoming on the dorsal the reason that we have a dorsal root ganglion here is because the sensory neurons for your sense of touch, which is what these are, is that somatic sensory. You know, you don't have vision coming in on these.
You don't have hearing. You don't have smell. You don't have taste, right?
What's coming in on the spinal nerves is your sense of touch and what we call somatic sensory neurons. So I want to remind us here of the structure of these neurons. And these are unipolar neurons. So we went over that in Chapter 12. All right, so sensory, the touch sensory neurons.
are unipolar. And so when we take a look at that, these cell bodies have to be somewhere. Remember, the cell bodies on a unipolar neuron are off of the axon. They're not in line with the axon, like a bipolar neuron.
The cell bodies are off to the side. So all of the sensory neurons, come up here again. If you have a sensory neuron coming in on this spinal nerve, so it comes in this way and enters, you know, that like that, its cell body is here in the dorsal root ganglion.
So that's that bulge is for all. of the sensory neurons coming in at this particular spinal root. They're all in here.
That's what all these little jelly beans are in here. You can see the little stalks coming off of the axon and going into that cell body. So there's one right here, see? So here comes this particular sensory neuron. And there is its soma right there.
So that's what the dorsal root ganglion is. It's a collection of all of the sensory neuron cell bodies for that particular side of that particular spinal level. And then out here in the spinal nerve, you get the motor out and the sensory in altogether because it's just axons. Hey, that's just axons. The axons for the motor neurons, their cell bodies are in that.
Ventral gray horn. We talked about that in lab. Now this diagram here is meant to help us build on some of the things that we saw in lab.
Some of the vocabulary that I asked you to learn. For example, I mean, so we've been doing, you know, ventral root and dorsal root and dorsal root ganglion. Notice here they have anterior and posterior as a vocabulary.
We've talked about this in lab. The posterior and dorsal are the same thing. Anterior and ventral are the same thing.
You kind of got to get used to having those used interchangeably. But another thing we had you learn was, for example, the sympathetic chain. And we had you learn that as those roots, dorsal and ventral root, come out. from the spinal cord, they merge into a spinal nerve, and we had you learn several branches off of that.
For example, we had you learn the posterior ramus of the spinal nerve, and we had you learn the anterior ramus of the spinal nerve. And then we had you learn the communicating rami, the gray and white communicating rami that go from the spinal nerve over to the sympathetic chain. And we said every time that a gray and white ramus communicate from the spinal nerve to the sympathetic chain, they connect at a ganglion.
So there's a bulge there. Just like the dorsal root ganglion, the sympathetic chain ganglion is a collection of cell bodies of nerves. And again, we'll talk about that in the autonomic chapter. So we had you learn those things, but you might not have had the best context. Again, we'll talk sympathetic chain and autonomic nervous system in another chapter.
But for example, if you talk about the dorsal and ventral ramus. Now, ramus in Latin just means branch. The anterior or ventral branch versus the dorsal or posterior branch. Look over here at this larger picture.
This is actually neat because it shows you the spinal cord in the vertebral canal, right? And then here's the roots coming out. They've got one set exposed where you can see inside.
Another one, you can just see the outer epineurium covering. But what do you got? As it emerges, you have the communicating rami coming over the sympathetic chain.
They're not showing much of the chain here because they're just showing one level. But that chain is coming at you through the screen and going away from you into the screen. And then what do we got?
We have, and I'll trace this, we have a anterior or ventral ramus. And it's called that because it's the branch that comes around the front of the body. I'll back off of this a little bit here. See, it comes around the front of the body.
So that's the anterior or ventral ramus. The posterior or dorsal ramus goes out. I can do it better. There we go.
Goes out the dorsal aspect and goes around the back. So you'll notice on all the models, the anterior ramus is much larger than the posterior or dorsal ramus because there's a lot more tissue out here and a lot more nerve. So if you go... posterior to the spinal cord, ain't much back there.
Don't need a very big nerve set there. What you need mostly is anterior ramus. So that image there, I hope, gives you a little more context, visual recognition for the types of things that you were looking at on those lab models. All right.
enough of that stuff because you know what everything we've talked about in this lead-up half hour here is stuff that you've covered in the lab already for the most part a little bit of value added here and there now we're talking about some new things here so here's where we're going to take a look at the the vertical aspect of the spinal cord and its physiology and the white matter columns So what do we have here? On the left we have the ascending tracks outlined in red. So these are sensory. So if they're ascending, they're coming in from the body through the dorsal root ganglia and into the dorsal gray horns, and they're bringing sensory information in, and that sensory information is going to then be shared upwards to the brain, and it's going to travel upward on one of these ascending tracks.
And so they've outlined the white matter column regions in red here that are carrying this ascending sensory information. they've labeled them so we'll take a look at the posterior column pathway the spinal cerebellar tracts and the anterolateral system so the spinothalamic track is one we will mostly consider and then over on the right we have the descending tracks so these are motor so when the brain makes a decision about movements then these motor commands go down the spinal cord and out to the muscles following one of these pathways. And they're highlighted in green here. Please don't get confused. I've had people confused by this, and they think that the ascending information is going up one side of the spinal cord and descending is coming down the other side.
No, no. If I pull out my green marker here, And if we consider, for example, the lateral corticospinal tract, for example. So the lateral corticospinal tract is in this region of the lateral column. That's why it's called lateral, this. The lateral corticospinal tract is also present right over here in the same position mirror image on the other side of the spinal cord.
Okay, so everything you see here has a left and a right version here. Okay, so we're going to take a look at these, you know, one one set at a time. We're going to do sensory first, and then we're going to do motor, and I'm going to illustrate for you some basic vocabulary that we use in the spinal cord and central nervous system to follow these tracks. And we'll do a little bit of learning here. So let's take a look.
Okay, we're going to start here with the spinothalamic pathway. Keep in mind, these pathways are often named for their starting and ending point. Spinothalamic starts in the spine and goes to the thalamus part of the brain that we're going to learn about. So the spinothalamic pathway.
Now, when we do these things. First of all, we're starting with sensory. These are sensory pathways. These are ascending pathways.
So we'll keep this track. Sensory, ascending, okay? We want to know what information does it carry, and we want to know where does it decussate.
You don't know what that means yet. I'm going to explain it. All right, so let's talk about vocabulary here when looking at spinal tracts.
All right, I'm going to look up here. There's a key here. Look at this.
Notice here we have red, white, and black. So we're going to follow the arrows and notice what we have here is first order, second order, and third order neuron. So what we have here is when we take a look at sensory information, we can have up to three neurons in sequence synapsing with each other.
The first order is your sensory neuron, can be a second order, and can be a third order. So we're going to pay attention on these maps where the first order, second order, and third order is. And we're going to keep track of where it decussates. Now, what decussate means, where does it cross over from one side to the other? No And let me erase that.
There we go. Where does it cross over from one side to the other? And then the third thing here we're going to keep track of is how does it ascend? Again, I've got to fix my spelling here. Hold on.
Ascend the spinal cord. So let's take a look at these two pathways. And because there's two portions to the spinothalamic pathway, there is an anterior tract and a lateral tract. So you see the anterior over here, you see the lateral over here. The anterior...
Spinal thalamic tract here. This is this side. Anterior spinal thalamic tract. It says carry crude touch and pressure sensation. That's highlighted down here.
Crude touch and pressure sensations. So this diagram is showing it coming in from the right side. All right, look at what we got here.
Remember, red was the first order neuron. So you think first order is the first neuron. in order that's carrying information. So what do we have here? This little red neuron right here, that's the sensory neuron.
So on these ascending pathways, the first order neuron is always the sensory neuron. It's the one that's actually entering the spinal cord, okay? And then look what happens. There's a synapse here.
This is how they are going to communicate with you on these diagrams. Let me show you something. They are always going to have a situation where they're going to show some around soma, and a long axon, and an arrow. And then they're going to do a round soma, a long axon, and an arrow.
And then a round soma, a long axon, and an arrow. You get the idea? This is a neural circuit, a neural wiring diagram. This is how they're showing you synapses and communication between neurons.
So there's synapses. So what do we got going on here? The first-order neuron is going to synapse with a second-order neuron. And on these diagrams, the white is the second-order neuron.
And look what happens here. So we said one of the things we wanted to know is, I'll even put this back out. Where does it decussate? Where does it cross over from one side to the other? That's that term, decussate.
It looks to me like on the anterior spinothalamic tract, the second order neuron decussates immediately. So you decussate at the same level as you... enter the spinal cord and look it decussates through the anterior gray commissure right there and once it decussates then it enters a white column and goes north goes up towards the brain and that's the anterior spinal thalamic tract because it's in the anterior white columns there's your anterior median fissure this is the anterior white column This is the anterior spinothalamic tract.
So our first-order neuron enters the gray horn, the dorsal gray horn, immediately synapses with a second-order neuron, which immediately decussates, enters the anterior tract, and goes up through the spinal cord. So over here, it says, how does it ascend the spinal cord? In this case, The ascension through the spinal cord is on the opposite side of where it entered. So the term for this is contralateral. So I'm going to put this vocabulary down here.
I'll come over here for a second. If the information ascends. on the opposite side of the spinal cord. from where it entered this is called contralateral ascension i'm not exactly sure if that's how you spell ascension but it looks right so this is contralateral ascension if this over here Let me give myself a little more room. So if information ascends on the same side of a spinal cord from where it entered, we call this ipsilateral.
ascension, contralateral, and ipsilateral. These are your vocabulary terms for the information heading up and down the spinal cord. So in the case of the anterior spinothalamic tract, We have the first order neuron enters immediately synapses the second order, which immediately crosses over or decussates.
And then it ascends contralaterally. Now, keep in mind right here, medulla oblongata and midbrain. By the time you reach this slice here, you're in the brain. So this is the spinal cord. This is the spinal cord.
Here we're in the brain, brain, brain. So. Let's follow this second order neuron. It ascends contralaterally, enters the brain at the medulla. So now we're inside the form and magnum.
We're in the brainstem. Pass up through the brainstem and we hit this blue thing here. And let me zoom in on this blue thing here.
This blue thing is the thalamus. That's the blue thing. And so here we synapse with another neuron. This is our third-order neuron, the third in the pathway. So in the thalamus, we synapse with a third-order neuron, and that third-order neuron goes to the cerebral cortex, the gray matter on the surface of the brain that's going to interpret that information.
Please don't make any, try to make any sense. of this homunculus idea we're going to talk about this in another chapter try not to put too much effort into that yet so here's one of the two tracks in the spinothalamic pathways so the anterior carries crude touch and pressure sensations now there's also you know anterior spinothalamic information coming in from the other side. So it'd be mirror image on this left side here you would synapse in the dorsal gray horn and the second organ neuron would decussate and ascend contralaterally on the other side of the spinal cord. So the the two sides would be mirror images.
They're only showing one side on these diagrams to simplify them. Over here we have the lateral spinothalamic tract. Let's follow this. We have a first-order neuron, enters the spinal cord in the dorsal root, immediately synapses with a second-order neuron.
That second-order neuron immediately decussates and enters the lateral spinothalamic tract. Now because it's not in the anterior column, it's in the lateral white column. Ascends contralaterally to the brain, passes through the brainstem to the thalamus, synapses with a third-order neuron, and projects to the cerebral cortex.
So what's going on here? The lateral spinothalamic tract carries pain and thermal sensory information, temperature. Pain and temperature on the lateral, crude touch and pressure. on the interior.
these are called the spinothalamic pathways they project directly to the thalamus and then the thalamus relays that to this cerebral cortex so vocabulary vocabulary vocabulary oops sorry about that my uh screen's not cooperating with me okay here we go we want to know what information does it carry right there you got that Where does it decussate? We got that. How does it ascend? Contralaterally, we got that.
I want you to be able to follow these maps, and I want you to keep track of this stuff. Well, let's look at this one. This map is the posterior column pathway.
Posterior column pathway. It's called that because these tracks... here make up the entire posterior white column.
So all of the white matter of the posterior white column is this pathway, right? And it's pure sensory ascending information. Well, what do we got here?
We've got, what is it carrying? Fine touch, vibration, pressure, and proprioception. Okay. And again, this particular diagram, is showing from the right side of the body.
You will see a mirror image of the left side, okay? But we'll just follow it from the right side. So fine touch, vibration, pressure, and proprioception.
You might say, what the heck is proprioception? Okay, this is, these are joints and muscles have sensory receptors in them. And what this does is it gives your three-dimensional position of your body.
The idea is if you close your eyes and you reach your arm out, and then keep your eyes closed, pull your arm in, turn your hand over, turn it back. Squeeze your fingers, let your fingers out, do things with your eyes closed, and your brain knows exactly where your arm is in three dimensions. Because it actually has a dedicated set of sensory receptors in every knuckle, in your wrist, in your elbow, in your shoulder, and all the muscles that run all those.
There are sensory receptors there that are dedicated to telling the brain exactly what. angle every joint is and what position every muscle's in. It tells your brain what your body's position is in three dimensions.
That's proprioception. It's pretty cool stuff. So this is coming in on the posterior column pathway.
So that's what we're being carried. So let's follow the neurons. The first organ neuron comes into the dorsal root. and into the dorsal gray horn it does not synapse it immediately enters the posterior white column and a sends the spinal cord to the brain so what happens is is that sensory neuron enters the spinal cord it doesn't synapse it keeps going all the way up to the brain so I want you to think about this if you have a proprioceptor for example in your big toe. So we just said, hey, your brain knows your position of your big toe at all times because it has proprioceptors in those joints.
That sensory neuron that's entering your spinal cord from your big toe is going all the way up the spinal cord of the brain before it synapses. So that sensory neuron goes from your big toe to your brain. That's one soma.
and one axon, basically is almost as long as you are tall. That's a long neuron. So in the posterior column pathway, the sensory first-order neuron does not synapse immediately. It immediately ascends ipsilaterally on the same side. It enters the brain.
Here's the medulla oblongata. In the medulla, It synapses with a second-order neuron. Then the second-order neuron D-cussates enters a new tract inside the brain called the medial lemniscus, labeled there, and goes to the thalamus. We've seen that before. Synapses with a third-order neuron projects to the sensory cortex, the cerebral cortex.
So what are some things to know here? I want to pay attention to a few things here. First of all, we have some vocabulary here.
We have the nucleus gracilis and nucleus cuneatus here in the brain. We have the fasciculus gracilis and fasciculus cuneatus here in the spinal cord. So when we talk about fine touch, vibration pressure, and proprioception, That's actually a lot of information.
That's a really a lot of information. All right, so it has to be organized well. When we look at these posterior columns here, each one of them is actually split into two bundles. Each bundle's a fascicle. There's one fascicle that's the gracilli fascicle, fasciculus gracillus.
And then there's one fascicle that's the cuneate fascicle for the fasciculus cuneatus. They're just Latin names. Don't worry about the Latin names, right?
But there's two bundles within this column. And the information being carried on those bundles is coming from different sources. So I want to outline this for you.
so the the fasciculus gracilis here this is carrying information all right so it's it's all of this stuff this this fine touch vibration pressure proprioception but the fasciculus gracilis is carrying info the information from t7 below so you know thoracic vertebra number seven if this fine touch and then vibration pressure and proprioception is coming from the spinal nerve t7 or below all that information is going to be on the fasciculus gracilis if it's coming from t6 or above it's going to be carried on the fasciculus cuneatus. So we split it by upper body, lower body. And so basically T6 and above is going to be your thoracic cavity, arms and above.
And T7 and below is going to be the abdominal pelvic cavity and legs. So they're both carrying fine touch vibration pressure and proprioception, but it's... by position of where it's coming in at which one of these you're going up right one's the gracilai one's the cuneate then each one of these projects to a slightly different region in the medulla oblongata in the medulla amalgata you have two centers called nuclei the nucleus gracilis that's where the fasciculus gracilis is going to go to the nucleus cuniatus That's where the fasciculus cuneatus is going to go to.
Once you reach that nucleus, then you synapse with your second-order neuron, which immediately crosses over and then keeps going north, you know, up through the brainstem on a new track called the medial lemniscus. And look, lemniscus in Latin means ribbon. It's the medial ribbon because it has that kind of flat shape of a ribbon.
But, you know, don't read too much into it. They're just called by what they look like. But the information then goes up through the brainstem.
And again, there's that thalamus. So the ventral nuclei and the thalamus. And then the third order neuron.
projects to the cerebral cortex. So this is the posterior column pathway carrying a certain kind of information. Here the Information ascends the spinal cord ipsilaterally, decussates in the medulla oblongata, and then terminates in the cerebral cortex after synapsing in the thalamus. So there's another one of these pathways.
All right, so here's our last. sensory pathway that we'll look at, the spinocerebellar pathway. Now this diagram we'll see leaves a lot to be desired, but I'll use it because it has the same kind of visual conventions and I can illustrate a few things for you. But we're going to draw our own set of diagrams to illustrate these pathways. First of all, what's the spinocerebellar pathway?
So it's going from the spinal cord to the cerebellum. cerebellum. So let's look over here.
The cerebellum here. We haven't talked brains yet here, but the cerebellum, this is a motor control center. Your cerebellum, which is Latin for small cerebrum, is basically a subcontractor. for your brain who's dedicated to the sole purpose of refining motor movements, making your motor movements better, more coordinated, more highly controlled.
So if you learn to play the flute, you're really working your cerebellum to figure out your fingerings on the flute and all of the little things that go into that. So the cerebellum, it's a motor control center. Now for motor control, Let's take a look here.
What is the spinal cerebellar pathway carrying here? Get the whole thing in. What's the spinal cerebellar pathway carrying?
It's carrying proprioception. Proprioception. Now, we just talked about this on the previous slide. Your body knows your position of all your joints and all your muscles. Great.
So the posterior column pathway. Sends that information up to the spinal cord, through the spinal cord to the medulla oblongata to the, you know, nucleus cuneatus and the nucleus gracilis. And then it goes up through the medial amniscus to the thalamus and the thalamus to the cerebral cortex.
Okay, that posterior column pathway is how your cerebrum learns or knows about the position of your body. But if the cerebellum is going to be able to do its job, it needs to have its own proprioception information input. So the cerebellum has its own set of proprioceptor inputs.
So it doesn't have to go asking the cerebrum, hey what position is your big toe in? It knows directly because it has its own pathway for it. That's what the spinal cerebellar pathway is. So there's one difference from what we've seen before. What we've seen before is all this sensory information has gone to the thalamus and then gone up to the cerebral cortex.
Here, it's going to the cerebellum. Second, since we're not going to the cerebral cortex, we're not going to the thalamus. There's no third-order neuron here. there are only first order and second order in the spinal cerebellar pathway so two neurons instead of three the other thing is every other pathway we've looked at the other three really uh four you know two spinothalamic tracks and then two posterior column pathways all of them you start on one side where the information comes in And you eventually, somewhere, you decussate, and that information ends up on the other side.
So the central nervous system, for virtually all of the information that gets handled by the central nervous system, the right side of the brain is processing information from the left side of the body, because everything decussates. It goes over to the other side. So the right side of your brain is handling the left side of your body.
The left side of your brain is handling the right side of the body. That's how your central nervous system works, by and large, except in the spinal cerebellar pathway. In the spinal cerebellar pathway, the cerebellum is handling information from the same side of the body.
So that's where this diagram here leaves a lot to be desired. We're going to do this better because if you try to read this diagram, it's only going to confuse you. So we're going to do a custom diagram here. So let's go over here to the side. Kind of keep that a little bit over there.
Okay, so I'm going to draw the spinal cord. I'm going to draw nice and thick here. So here's my spinal cord. In case you were wondering, this is the cerebellum. All right, so there's the spinal cord and there's the cerebellum.
All right, we are going to draw our spinocerebellar pathway here. Let me get a red pen here to represent our first-order neuron coming in from the left side of the screen. Okay, so here we are.
We're inside the spinal cord, right? So we've got a first-order neuron here. So with the spinocerebellar pathway, we synapse now with a second-order neuron.
And if you look at that, the diagram, you'll notice that there are two different tracts in the spinocerebellar pathway. There's an anterior tract and a posterior tract. Now, If we are on the anterior, or I'm sorry, the posterior tract, we will synapse immediately on the same side, the dorsal grey horn, and the second order neuron is going to ascend ipsilaterally up to the cerebellum and is going to directly enter the cerebellum. This is the posterior spinocerebellar pathway. I'm going to zoom in here.
I'm going to label this thing. So here's the posterior spinocerebellar. posterior. So look, the cerebellum here is handling this information from the same side of the brain.
There's been no crossing over. There's no decussation. Everything is being handled ipsilaterally. Ascension is ipsilateral and processing is ipsilateral. Okay, well, what about that other one there, the other tract, the anterior tract.
Here comes another first-order neuron set, and we are going to synapse now. I'll use green for this one. So we synapse immediately in the dorsal gray horn.
And this time, the second-order neuron decussates immediately at the spinal level and is going to ascend now contralaterally. And so here, I'm going to zoom in again and label this thing like I did before. So this is the anterior.
Spino cerebellar tract. So one one decussates and ascends ipsila, contralaterally. One doesn't decussate and ascends ipsilaterally.
But here's the kicker. This is where the game gets exciting. This anterior spinal cerebellar, it decussates again in the brain and ends up on the same side that it entered.
So the cerebellum, it's weird. It's handling proprioceptive input from the same side where it entered. So the left cerebellar hemisphere is receiving proprioception from the left side.
One of the pathways stays on that side of the spinal cord. The other one goes across, goes up, and then comes back. It's the weirdest thing.
But, you know, I don't get to make the rules. I just teach them, right? It's going to be the same thing from the other side. So we can show, for example. uh let me see here we could come in from the other side i could show a posterior coming in here like that and then i could have my other one coming in here d you know synapse there decussates it goes up the other side of the spinal cord and then comes back.
And that's what the spinal cerebellar pathways look like. They're all carrying proprioception. It's just, this is the only example in the entire central nervous system where you have double decussation.
It's just, that's the only place you see it. It's kind of abstract. but there it is so don't please don't try to read this diagram okay it's just going to confuse you as far as understanding the tracts and the posterior versus anterior just use this custom diagram here and you're probably even better off just using one half of it i mean i added in the right side of the diagram just for illustration purposes but you might want to just stick to the first half that we did so there's the first half i guess of the chapter so what did we do we we kind of reviewed spinal cord anatomy and nerve anatomy and things like that which basically doubled up from lab and then we started in on these Spinal cord tracks and pathways.
So we talked about all the sensory ascending pathways, the major ones. So that's where I'm going to end it here on this particular lecture recording. So when we come back for part two of chapter 13, we'll do the descending pathways, the motor pathways. How does the brain send motor commands down the spinal cord? So that will...
occupy our time for a little bit and then we're going to take a look at spinal cord reflexes so mono and polysynaptic reflexes of the spinal cord so that's going to be the second part so i'll see you then