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Dr. P back with the dreaded trigeminal nerve.

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It's the biggest of the cranial nerves

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and somatosensory fibers of the trigeminal nerve

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enter the pons in the sensory root

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known as the portio major.

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It's the main root, even though it's sensory,

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and it's distributed to the principal sensory nucleus

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and spinal trigeminal nucleus.

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So, let's take a look at those.

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This gets a little bit scary.

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I'm going to do some very rudimentary

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but I think helpful drawing,

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I'm going to have you look at sort of an AP projection.

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I'm going to make a thalamus one side and a thalamus on the other side.

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It'll have a nucleus in there in a second.

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Then we're going to come down from the thalamus,

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midbrain and cerebral peduncles, and we'll get ourselves

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a little bit of a bump at the level of the pons.

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Then on this side, I'm going to draw in the forked

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or tri-forked entrance of the portio major,

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the sensory division of the trigeminal nerve.

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I'm also going to draw in a little bit smaller

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the motor portion of the trigeminal nerve.

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Then the bump continues on down a little bit

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and then starts to taper in

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as the medulla and as the spine.

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It's a little asymmetric,

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but that's okay.

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My drawing skills require a little bit of improvement.

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So now I'm going to change colors a little bit,

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just to show you how things are and where they are.

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I'm going to start up here in the region of the thalamus.

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And I'm going to highlight an important structure,

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the ventral posteromedial nucleus of thalamus,

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which is where the trigeminal system

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is going to end up.

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From here, fibers are going to go to the postcentral gyrus,

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which is the sensory gyrus,

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and they will arborize on the surface

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that's right above the temporal operculum.

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And I think that's a story for another day.

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As we go down, let's go down on this side.

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We've got a series of fibers that come down

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and they reach the principal sensory

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nucleus of the trigeminal nerve, right about here,

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and it's a little bit posterior in its position,

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even though this is an AP projection,

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if we were in cross-section, you could appreciate that.

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And so, these fibers are coming in

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from the portio major.

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Now, I'll make them blue.

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Give them a little bit of highlight.

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They come into the primary sensory nucleus of the trigeminal

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nerve in the posterolateral aspect of the pons.

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Now, right below this is the second important nuclear structure,

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which is the spinal trigeminal nucleus.

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I'm going to make that a little bit like a cone head.

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You remember the show from Saturday Night Live,

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there's the cone at the top.

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And then as we come down, this goes for quite a distance.

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And then eventually,

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it transitions into two other tracks.

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And I'm going to divide this one up into thirds.

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And the name of these thirds are the pars oralis,

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the second one is the pars interpolaris, I.

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And the third one is the pars caudalis, C.

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So that's a little bit confusing.

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All part of this spinal trigeminal

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nucleus that gets into the spinal canal,

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and then DV's up.

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Time for a new color.

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DV's up into...

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Let's pick something that the audience might like,

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something a little brighter.

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DV's up into two tracks right here,

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that are going to be seen in the posterolateral aspect of the spine.

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And these are known as the nucleus

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proprius and the substantia gelatinosa together.

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So, we're going to ignore these right now.

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I just want you to be aware of them.

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But for the portio major,

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coming in through the posterolateral pons,

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there is a synapse and connection to the principal nucleus,

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and there are multiple connections.

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Let's go back to blue.

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Multiple connections to the pars oralis,

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the pars interpolaris, and the pars caudalis.

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Now, you might say,

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well what about the poor little teeny weeny oft-forgotten

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motor portion of the trigeminal nerve?

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Well, it has something that's a little bit smaller

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and weenier and tinier to go to.

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But this is going to sit a little medial,

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and it's going to be quite a bit thinner than

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the principal nucleus,

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goes up a little further in the upper pons

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and reaches portions into the midbrain,

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and this is known as the trigeminal mesencephalic nucleus of five.

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Then you've got another structure called the motor nucleus

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and it sits right underneath directly adjacent to the

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principal sensory nucleus.

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So, we've got principal sensory here in blue,

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principal motor here in green,

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and then trigeminal mesencephalic nucleus of five,

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going up like that.

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And what goes there?

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Well, the motor portion,

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the portio minor of the trigeminal nerve is going to have

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synapse connections to this component,

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and to the primary

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motor nucleus, namely this component.

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So this over here is going to sit posterolateral in the pons.

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This is going to sit quite a bit higher and slightly

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more medial to the other structures.

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So again,

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motor medial, motor midbrain,

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sensory lower in the pons

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and that's the one you really want to remember.

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Dr. P out.