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Dr. P here talking about the 12th cranial nerve,

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the hypoglossal nerve,

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which innervates all of the extrinsic and intrinsic

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muscles of the tongue except for one.

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Which one? The palatoglossus muscle,

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which is innervated by the vagus nerve.

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The hypoglossal nerve is the sole nerve

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with a purely motor function.

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I love this nerve because it innervates the tongue.

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And my father told me the tongue is the most dangerous

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organ in the body, too much eating and too much talking.

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Now, the nerve arises from the hypoglossal nucleus in the

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brain stem, which we'll talk about separately,

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but it comes out as a number of small rootlets.

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Now, we can't see these rootlets,

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but I'm going to draw where they would

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be and how you would locate them.

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So you find this olivary bump right here and you identify

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the olivary bump in the coronal projection.

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And right behind that olivary bump,

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you're going to have the rootlets coming directly out

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in a somewhat caudad fashion.

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They're going to converge very quickly within less

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well under a millimeter into a single nerve.

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I'm making it a little bigger so you can see it,

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where they'll enter the hypoglossal canal,

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and then down through the neck,

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eventually passing up over the tongue muscles,

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supplying the tongue.

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There are two hypoglossal nerves in the body,

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one on the left and one on the right.

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Let's take a look at the hypoglossal nerve in the axial

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projection, where you can see it a little better.

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Best seen this time on the T1.

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Now because it's going caudal,

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I have to scroll caudal to continue following this.

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You can see it's right behind this protuberance right here.

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Now, let's try and follow it.

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We can see a little bit of it right there,

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and then we kind of lose it for a short period of time.

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And then it comes back right in the hypoglossal canal.

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There it is, this linear, darker structure.

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I might even have to blow it up a little bit.

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Let's do that. Just so you can see it a little better.

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Right there in the hypoglossal canal.

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And if we want to cross-reference it,

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there's the hypoglossal canal in the coronal projection.

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So if you were looking for a hypoglossal,

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say, schwannoma,

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and you saw tongue atrophy,

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these would be two places to go.

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Here and here,

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where you are prone to get these.