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Now, before I show you some sixth nerve anatomy on MRI,

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let's just do a quick review of what we've

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covered so far in the cranial nerves.

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We talked about the olfactory nerve,

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cranial nerve number one.

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And then as it comes back, we don't show it here,

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it divides up into three or four different segments

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which are outlined in the olfactory section.

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Then we get into the optic nerve.

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The optic nerve starts at the level of the retina.

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You've got photoreceptors in the retina,

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just like you have receptors in the ethmoid

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sinus mucosa for the olfactory nerve.

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And then it comes back as the intracanalicular nerve,

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the optic nerve, the chiasm, the optic tract,

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and then goes on to the lateral geniculate nucleus.

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It has a synapse at the superior colliculus,

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the stria gennari, the calcarine cortex.

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It also has a pulvinar synapse.

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So that's a quick review of the second cranial nerve,

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which some consider an extension of the brain with

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arachnoid around it and a subarachnoid space,

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which you'll be able to see.

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Then we said the third nerve.

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What characterizes the third nerve is its

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exit in the interpeduncular cistern.

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And then we described its course into the ocular

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motor sulcus, then into the cavernous sinus,

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and then on into the orbit where it services

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the extraocular muscles, all of them,

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except for the superior oblique and the lateral rectus.

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Then we get into the fifth nerve, very complex.

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But the fifth nerve is mostly composed of a Portio major,

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which is sensory with three branches:

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an ophthalmic, a maxillary, and a mandibular.

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There's also a Portio minor, which is motor.

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And we'll describe in the fifth nerve section,

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if you go back to it,

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the different structures that are innervated by it.

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Then the characteristics of the trochlear nerve,

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the fourth nerve,

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the one before it are the following,

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it is the one nerve that comes

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out in the back, dorsally in the brain stem,

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and then decussates in the back.

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And then we're back up to the 6th nerve,

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which comes out at the pontomedullary sulcus ventrally.

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Remember the trochlear nerve?

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Dorsally and it starts out lateral.

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Its apparent exit is lateral to the pyramid of the pons.

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Very critical so you don't confuse it with pontine perforators.

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Now, let's take a look on MRI.