Sensation from the Body: Neuroanatomy Video Lab - Brain Dissections

Today we're going to talk about the two important sensory pathways coming from the body.

There are two pathways.

The first one is the anterolateral system also better known as the spinothalamic tract.

So that's this green one.

Then there is the blue one, which is the dorsal column medial lemniscus system.

So these are two pathways that are going from, let's say, the right side of my body to the left side of my brain.

Now what is carried in these two tracts is different.

In common is sensation from the skin for example.

Crude touch as we kind of call that.

So crude touch is actually carried in both of these tracts.

So these are my tracts.

Now what is carried in the spinothalamic tract?

It is the tract that we call for pain and temperature.

All right?

So you can test pain.

And this will be this tract.

You can test warm and cold.

So this is the pain and temperature pathway whereas the other pathway, the dorsal column medial lemniscus pathway is testing for a very fine touch, two point discrimination.

Or, for example, think of it this way, you know exactly where that mosquito lands.

That's very fine touch.

Or the two ends of a paperclip that can be perceived as one.

And then we have the pathway also carrying vibration.

So it carries vibration.

I would test that, for example, with a tuning fork on a joint.

And the other major test we do for this pathway's integrity is joint position.

Can I tell with my eye is closed if my big toe is being moved up or down.

So we have these two pathways, and let's assume that they are coming in on the right side of my body through the right spinal cord.

So remember, right body is going to go to the left side of my brain.

And so right body, left brain for perception.

So we have two tracts, two pathways.

Now the difficult thing about this is not to remember that right body goes to left-brain, all right?

What is difficult to remember is where these pathways cross from the right side of the body to the left side of the nervous system.

And that we're going to go over as we turn our attention now to the gross specimens, and let us begin with the spinal cord.

So here we have a brain.

And fortunately this spinal cord is still attached.

And so we can look at the entire length of the spinal cord.

Remember, the cauda equina is carrying sensory information over the dorsal roots from the leg region.

And here's our thoracic region.

Then up here we have our cervical region.

And what's really nice if you look closely at the dorsal surface here, you can actually see the dorsal columns.

This groove here and this groove here demarcate the dorsal columns, this is the cervical region of the cord, so here is the right dorsal column, and here is the left dorsal column.

And here are the dorsal roots.

They're mostly torn off, but here are some remnants of dorsal roots.

So you can see this is where they are entering.

So let's look at the cell body location of those roots.

This dura, we're looking at the outside of the dura of another spinal cord.

And these, each of these are dorsal root ganglia.

They are paired.

They're nicely paired.

You can see them here.

Here's a pair, right and left along here.

And you can see the dorsal roots coming in on one side, and the ventral roots are mostly torn, but you can see them coming off on the other side.

So this is our sensory dorsal column for two point discrimination vibration and joint position.

The pathway for the anterolateral or spinothalamic system is not visible on the cord.

So, in common, both of these tracts have their cell bodies in the dorsal root ganglia.

And the interesting thing about this pathway is where these axons synapse.

Where these dorsal root ganglia or primary axons end or synapse.

So where is the second neuron?

The second neuron in this system for the spinothalamic tract is in the cord very near, within a few segments, of where the information comes in, whereas for the dorsal column system, the second neuron is on the same side it comes in on up in the caudal medulla.

Up in this region here.

So that's something we have to keep in mind.

The pain and temperature pathway crosses very soon after entering, and the dorsal column medial lemniscus ascends the medulla before crossing.

And we'll come back to that in a moment, but let's follow these pathways now in cross-sections through the brain stem.

So now I've selected some nice specimens for you to look at.

Here is the cervical part of the spinal cord.

Here's the caudal medulla.

Here's the more rostral part of the medulla.

Here's our classic pons.

This is more rostral pons as we move towards the midbrain.

Here's our midbrain, and then finally our thalamus in a coronal section.

And finally we're going to end up in the post-central gyrus somatosensory cortex.

So now let's start with the spinal cord.

The dorsal roots are clearly visible here in this cervical area.

These were our dorsal columns.

This cord has been cut for pathological examination, but we'll pretend that it's whole.

And notice how many fibers there are in the dorsal roots.

This is probably a cervical eight level where we have lots of information coming in from our hands.

Fingers are very, very sensitive.

Two-point discrimination is very good on your finger when you compare it with your back.

So now this is the dorsal root.

These are the primary axons.

I'm going to turn this over here so that posterior or dorsal is up.

So now I've turned it so that you're looking at a cross-section of the cord.

And dorsal is up.

Ventral is down.

And you can perhaps see an outline of the more whitish gray matter, which are the ventral horn and our dorsal horn.

And this slightly darker, beige area in here are our dorsal columns.

So our axons are coming in.

Now what we're going to do is look at the anterolateral system.

Those axons come in and synapse in the dorsal horn at approximately where they come in, and they cross right here in the anterior white commissure right below the central canal, and they traverse the second order neurons because this is the second neuron here.

The axons cross.

That's the important thing.

They cross.

And they form a tract here.

And this green tract, we call the spinothalamic tract.

Now let's talk about the dorsal column system.

The dorsal column system, those axons come in from the dorsal roots, but they do not cross.

So this first neuron still continues to ascend.

And it's going to ascend in the dorsal column in this area as it goes up to the medulla.

And there's an organization.

It's a somatotopic organization in that the parts of the body that are from the lumbar and sacral region are more medial.

And the parts of the body, those axons that have come from the hands and the neck, are more lateral.

So we have a somatotopic organization.

So sensory information from our blue tract is traveling on the right side still whereas information from the spinothalamic, green tract, has crossed.

And this decussation, this difference where they cross is important.

Now let's go to the level of the caudal medulla.

This, to orient you, is the caudal medulla.

There's no fourth ventricle yet, but you still have the pyramids on the bottom, the motor pathway.

And the dorsal columns, which were on our right side, are going to be synapsing in this caudal medulla and some nuclei here called the dorsal column nuclei, and those nuclei are going to send their axons across to form the medial lemniscus.

So the blue medial lemniscus is going to be right in this area, right above these pyramids where as the spinothalamic pathway, which crossed, is already traveling on the left side.

Remember? It crossed from the right to the left side.

So we have now both types of information, both pathways are on the opposite side.

They've both crossed now.

The blue pathway crossed here in the caudal medulla.

And the green pathway crossed down in the cord.

But they're still rather far apart.

Our green pathway is over here laterally.

That's why it's called the anterior and lateral system, whereas our dorsal columns are right here in the midline.

Now, if we follow them to the next level, here is a typical medulla with the fourth ventricle.

One of the cranial nerves, perhaps number nine, and here are the pyramids, the motor pathway.

And our medial lemniscus, our blue medial lemniscus is right here, between the two olives, are the medial lemnisci.

And here are the pyramids.

So they're like standing on the pyramids.

Think of it that way.

And down over here is our green pathway.

This is the somatic pain and temperature pathway.

And they're still quite far apart.

And they are supplied by different arteries.

So you can have one group of fibers affected in a diseased situation like a stroke and the other group not.

And now let's continue marching up to the pons.

We move now to the pons.

The pons, get your bearings, this is our nice, big fourth ventricle now.

Here is the groove on the base of the pons where the basilar artery was.

And here are the large, middle cerebellar peduncles.

And all of this area from here down to here is the base of the pons.

And the area, the tegmental area, that we're going to look at, is this region here.

All right?

So our pathways are now on the left side.

And this kind of brownish line across here, that represents our blue medial lemniscus.

And now the anterolateral system or spinothalamics are right out here to the side, so they're really quite close together.

I think maybe you can see here.

So from the middle of the pons, or rostral pons, as we can see in this next section here, the more rostral pons, we have all of these fibers together.

So on the left side here are the spinothalamic and the medial lemniscus fibers together.

The spinothalamic or green pathways a little more lateral, and the medial lemniscus is a little more medial.

And they're traveling now together.

And so they're often affected together.

As we move up to the midbrain, and you'll notice typical features, the aqueduct, the colliculi, the cerebral peduncles, and the substantia nigra.

Our fibers now are going to ascend toward the thalamus.

And so we're still here on the left side, so they're right in here.

And now we're going to move over to the thalamus.

And in this coronal section with the large lateral ventricles, and here's the inferior horn of the lateral ventricle.

Here's the third ventricle.

So if you have third ventricle, what you have out to the side is thalamus.

And down here with the mammillary bodies, the hypothalamus.

So this is hypothalamus.

This is thalamus.

This, from here to here to the ventricle and over to the internal capsule.

And then out to the side we have the basal ganglia and the temporal lobe.

So where do these fibers end?

These fibers coming from the midbrain come and terminate in the ventral and posterior nucleus of the thalamus in approximately this region here.

You don't have to identify it.

It's in the ventral posterior part of the thalamus and more in the lateral part.

And all the way through this entire system, the somatotopic organization has been maintained, so that if you were very discrete, and you could record with an electrode, you would see that the toes are more lateral, and the neck is more medial.

Now, this is where the third neuron is located.

Remember the first neurons were in the dorsal root ganglia.

The second neurons were either in the spinal cord, for the green spinothalamic pathway, or in the caudal medulla, for the dorsal column medial lemniscus (blue) pathway.

From rostral pons up, they're together.

And now they are together also, but the lines are still labeled.

You don't change from being a hot fiber to being a pain fiber.

We have labeled lines.

And these labeled neurons here that continue to carry that information now will proceed out into this white band, this internal capsule, this posterior part of the internal capsule and go up to the cerebral cortex to the postcentral gyrus, and we have a homunculus.

We have an organization such that the genital and leg area are medial.

And then we have a smaller trunk representation and arm, a very large hand, and face, and lip, and larynx area out here to the side.

Our homunculus.

So now let's assume that our fibers from the internal capsule have reached the cerebral cortex postcentral gyrus.

OK? So now we are in the left hemisphere.

Right body goes to the left hemisphere.

And here is our central sulcus.

No two are the same.

Here it's coming down from the crest here, from the vertex down our central sulcus all the way to the lateral fissure.

So this would be the face region and the hand region and the trunk and the upper leg and then over onto the medial surface for the lower leg, feet, and genitals.

And this area behind here is association cortex for somatic sensation, for information, then from this postcentral gyrus, is processed in further detail by these parietal regions that are important for integration of all these different sensory modalities.

This animation is going to show you the difference in the crossing location for these two pathways.

The anterolateral system or spinothalamic tract enters in the spinal cord, and the second neuron crosses to the opposite side almost immediately whereas the dorsal column-medial lemniscus system enters in the spinal cord

and ascends on the same side and does not cross until the caudal medulla.

In both cases, it is the second neuron that crosses in this three-neuron chain to the cerebral cortex for perception.

What you have to keep in mind is that pain and temperature pathways from the cord are separated from the vibration, joint position, and fine touch pathways.

Now, let's continue.

So the take home point of part of this discussion is that we have the ability to lose all sensation if we involve the postcentral gyrus or the internal capsule or the midbrain or the rostral pons.

And we have a possibility for sensory dissociation.

That is you lose pain and temperature, but not vibration, joint position, and fine touch or vice versa when you have a lesion below the middle of the pons.

Let's look at a slide or a section or an image that can make that distinction of how far apart those pathways are in those lower levels.

[ Silence ]

So now what I want to do is turn this over and look at the mid-sagittal surface of the brain.

You can see our corpus callosum, our septum, our thalamus, midbrain, pons, medulla, cerebellum, fourth ventricle, aqueduct, third ventricle, in between the two thalami.

So this is very nice.

And notice that our central sulcus is a little bit off center, and things aren't about as symmetrical as your textbooks, but this represents the area then on the mesial surface that is somatosensory and motor cortex with the sensory area being this part behind the sulcus

and the part in front of it being more motor.

This is sometimes called the paracentral lobule.

But this isn't a very typical example.

But this is still part of that somatosensory and motor cortex.

Now, we want to remember that the homunculus, the genitals, and the foot, and a little bit of maybe the lower leg are on this side.

And this vessel over here, can you see this anterior cerebral artery coming up here off of the internal carotid?

Here it comes very nicely.

And it's going to come, and it's going to supply this mesial surface of the brain all the way back here to the occipital parietal notch.

So, this surface is supplied by the anterior cerebral artery including a bit of this crest along here, whereas if I turn it again around, this surface, all of this surface here, is supplied by the middle cerebral artery.

So now you can see that if a person had an occlusion of the middle cerebral artery, somatic sensation from the hand would be much more affected than somatic information from the foot.

And so we have to think about the organization of sensory cortex and its blood supply together when we are looking at and examining a patient.

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