Sensation from the Face: Neuroanatomy Video Lab - Brain Dissections

It's time to consider the sensation coming from the face.

So focusing on facial sensation I brought my friend along here, and these green branches of the trigeminal nerve are going to give you an idea of the extent and the magnitude and the importance of the trigeminal nerve.

So the trigeminal nerve has three branches.

From the vertex here, we have branches all the way down to the middle of the orbit, and we call that the ophthalmic division.

Then below that there is the maxillary division, which includes innervation of the teeth and the gums. And below that we have the mandibular division, also includes the lower jaw, teeth, and gums. Now, if I turn this around, what you can see is there are green nerves on the posterior aspect of the skull, but these are not trigeminal.

These are high cervical innervation of the back.

So it's only from the vertex, and it only goes down and excludes the angle of the jaw.

The ear -- this is the external auditory meatus -- the ear is very complicated because embryologically it formed from different regions.

And so it's innervated partly by the trigeminal, but it also has some innervation from the 7th and 9th nerves.

So the ear is not a good thing to test when you're testing sensation from the face.

So now let's look at some of the internal structures that are also innervated by the trigeminal nerve.

Here's a hemisection of the head.

And I just want to remind you that the anterior two-thirds of the surface of the tongue, and also the mucosa of your cheeks, are innervated by the trigeminal.

So if you bite your cheek or burn your tongue, that information is conveyed by the trigeminal nerve.

Similarly, the nasal mucosa is also supplied by the trigeminal nerve, as are the linings of the sinuses and most of the dura mater that covers the surface of the brain.

So now we're going to quickly start with the pathway from its origin in the trigeminal ganglion.

So these nerves have come in from the surface, and now sitting here in the middle cranial fossa, this cavity here, is the trigeminal ganglion.

It's green and big; it's the biggest ganglion.

These are the cell bodies, the primary cell bodies, coming from all those three divisions, and they are going to enter into the center of the pons, the middle of the pons.

The pons, recall, is lying right here over the basal artery.

So in the body, carrying sensation up from the spinal cord, we had two major pathways: We had one called the spinothalamic and we had the other called the dorsal column-medial lemniscus.

We have those same two types of tracts coming from the trigeminal nerve, and one is going to involve a synapse in a descending, or spinal nucleus, and that's going to be one route.

And the other one is going to involve another nucleus, the principal, or chief, trigeminal nucleus.

And so we have these two pathways.

And the first one carries, just like from the body, pain and temperature: hot and cold or warm and cold.

And the other one carries vibration, two-point discrimination -- like knowing if my face is being touched in two places or one when two points are placed on my skin -- and also joint position, joint -- well, this must be the jaw; that's about the only joint we have on the face.

Now, light touch, just like from the body, is carried in both of these pathways.

So light touch will tell you that something's working in the trigeminal, but it doesn't distinguish between these two pathways.

So now let's look at an animation of how this information comes in and is carried up to the thalamus and onto the cortex.

I'm going to show you this animation, which covers the entire nervous system from the tip of the spinal cord to the top of the cortex.

And here we have the midbrain, the pons, the medulla, and the spinal cord.

Now, pain and temperature, which we're going to go through first, comes in from the face and it does something very strange.

This first neuron, the trigeminal ganglion neuron, comes in and it descends.

It descends all the way down, some people say, C4, C5 to the cervical level.

Then it synapses and the second neuron crosses and forms the trigeminothalamic tract and goes up to the thalamus and then onto the cortex.

Let's animate that.

You see this is the first order neuron.

You've been painfully stimulated on your face.

Information comes in and it descends from the middle of the pons to the top of the cord, crosses to the left side and then goes to the left cortex via the thalamus.

Now let's look at fine touch, vibration, joint position coming from the face.

It goes to a different nucleus.

It comes straight in and synapses in the chief, or principal, nucleus right there in the middle of the pons.

Then the second neuron crosses -- it's always the second neuron that crosses -- and synapses on the third neuron in the thalamus in the same nucleus but different cells, the ventral posterior, and this time it's the medial part of the thalamus, and then on through the internal capsule up to the cortex.

Let's animate that.

Information comes in, synapses, crosses, goes to the thalamus, through the internal capsule to the face region of the cortex.

Recall, this is only information from the face and it's out on the lateral cortex, the lateral postcentral gyrus.

So now we are looking at the ventral surface of the brain, and the pons extends from here to here.

And right in the middle of the pons is the largest of all the cranial nerves, the large trigeminal nerve.

So it comes in on the side of the pons on either side.

We can see it better on a brainstem specimen.

We're looking at the brainstem here.

The surface here is the midbrain.

And as I tilt it up, you can see the pons that extends from here to here and the medulla coming down here, connecting with the spinal cord.

And right here is the large trigeminal nerve burrowing through the pons here, and here it is on the other side.

And so what we want to do is to follow this nerve as it comes in and makes its connections.

Now we're going to trace the entrance of the trigeminal nerve that carries information for pain and temperature.

So here we have in the middle of the pons the trigeminal nerve.

So information coming in from the right trigeminal nerve enters into an area approximately here, and then those axons descend.

They're going back down through the medulla -- we're still on the right side -- back down through the medulla down near the level of the decussation, even down to the high cervical spinal cord.

And there in these areas you will find the second neuron involved in crossing to the other side.

So the decussation of this trigeminothalamic tract occurs all the way from the middle of the pons all the way down to the high cervical spinal cord.

Now we're going to consider vibration, joint position, and fine discrete touch and how that enters.

Similarly, it comes in through the right trigeminal nerve.

And right here at the level where it enters there is a nucleus: the chief, or principal, sensory nucleus of the trigeminal.

There, the second neurons cross to the other side and are located close to the medial lemniscus that we saw coming from the spinal cord connections.

So we now have facial sensation located very close to the medial lemniscus that is carrying body sensation for fine touch, joint position, and vibration.

Now, the pathway that came up with pain and temperature is also in this area.

And so we have both of these pathways converging in the middle of the pons.

And from there on they are called the trigeminothalamic pathway.

And they run close to the body sensation pathway and they're going to head up toward the midbrain.

This section is a transition between midbrain and pons, and our fibers are traveling in this area here.

Facial sensation for all modalities is very close to body sensation, and everything has crossed.

So damage in this area is likely to result in the loss of pain, temperature, vibration, joint position, et cetera, from both the body and the face.

One more level up is the midbrain.

When we get to the midbrain, our fibers now are right here on the surface, on their way going around the midbrain and onto the thalamus.

So finally we've arrived at the thalamus.

So we have to be going to the left hemisphere.

So we are now in the left thalamus and facial sensation goes more medial than body sensation.

So body sensation was in the ventral posterolateral region and facial sensation is in the ventral posteromedial area of the thalamus.

All of this is the thalamus.

So we generically call these nuclei VPM for the face and VPL for the body.

Label lines still exist: If you're a pain fiber, you're still a pain.

If you're a vibration, you're still carrying vibration and you enter into the internal capsule, and you head up now for the face area.

Remember, the face is lateral on our homunculus.

So the face and larynx and pharynx and that area is all over in this most lateral part of the postcentral gyrus.

Let's look at the cerebral cortex in a whole brain now.

We're back to our whole brain again.

And here is our central sulcus coming down here to the lateral fissure, the postcentral gyrus region here.

All of this area is mostly regarded as involving the face.

Don't forget that we have lips and tongue and all of these other areas, but we also have facial sensation.

So there's a very important take-home message about the pain and temperature pathway from the face.

If you have a low lesion of the brain stem or a high lesion of the spinal cord and you see loss of facial sensation for pain and temperature, that doesn't mean that the lesion is in the pons.

It could be at any one of these levels.

And if all the sensation is lost from the face, you know it's either the nerve or the middle of the pons farther forward or rostral, anywhere from mid pons to cerebral cortex.