Hello anatomy friends this is Dr. Alsup and we are going to discuss how to locate cranial nerves
in numerous views, including an inferior view of the brain at the cranial base through the dura
so through those foramina, and we are also going to look at a brain stem model. So much to do so
let's get started. And we're going to start with this inferior view of the brain. Now due to this
specific dissection, only certain cranial nerves are viewable and we're going to focus in on those.
And these typically are the ones that are easiest to locate on the inferior view of the brain.
All right here you can see the olfactory tracks which I'm going to outline
on either side. The bulbs have been cut but they would be kind of right in this region here.
And these are components of the olfactory nerve
or cranial nerve I. And they're located on the inferior portions of the frontal lobes
lateral to these anterior extensions of the anterior cerebral artery so kind of on either side
of that longitudinal fissure. Next we can see a bit of the optic nerve or cranial nerve II as well as
a good view of the optic chiasm right here towards the midline so kind of nestled within or medial
to much of the circle of willis. And this chiasm is going to be the means by which fibers from the
retina decousate or cross to the other side which is very important in terms of binocular vision.
You can see a bit I'm going to circle, you can see a bit of the oculomotor nerve or cranial nerve III
that is going to be nestled in between the posterior cerebral and the superior cerebellar
artery so a really close relationship between these two branches of the basilar artery. So some
type of increase in size in these arteries say due to an aneurysm that could compress the oculomotor
nerve. You can see the abducens nerve right here very closely associated to the pons of the brain.
And lastly you can see at least on this one side the accessory nerve right here kind of running
up from the spinal cord so obviously it's been cut closer to the spinal cord. As we know it will
ascend from the spinal cord up through the foramen magnum and it has this really close relationship
to the posterior inferior cerebellar artery or pica so more to come on that in the artery video.
All right let's work through a few slides showing most of the cranial nerves and how
they actually exit the skull. And it's important to note that in these particular dissections the
dura mater is still present so the foramina that we have discussed and looked at on
dry bone will certainly look very different so the dissections we are about to
look at are more similar to what we see in your dissections. So we're going to start anteriorly
and move posteriorly. And you can see a little bit of the optic nerves right here in this region. And
these are really close and just superior to the internal carotid arteries which have been cut
here and here is a portion of the pituitary gland right there. So all really close in this region.
Obviously these optic nerves are traversing the optic canal right here.
The oculomotor nerve which you can see here and you see that it was cut a little bit more distally
right here so these are cranial nerves III are going to be located on these lateral walls of
the sella turcica. They are heading up towards the superior orbital fissure which will be in
this region. And both the root of the trigeminal nerve which I am outlining right here as well as
the trochlear nerve which would be right around this region can be found in association with the
dura on this petrous part of the temporal bone so this whole region right here is the petrous part
of the temporal bone. And the trigeminal is very large and obvious or the trigeminal root whereas
the trochlear nerve is not so much which makes sense as it's the smallest cranial nerve.
The abducens nerve or cranial nerve VI is going to be very closely when we're looking at this
particular view very closely associated to the clivus or the basilar part of the occipital bone
so we're right here on the occipital bone. But eventually it'll make its way anteriorly to also
exit through that superior orbital fissure after traversing the lumen of the cavernous sinus.
So that cavernous sinus is going to be located all around this region right here.
The facial and vestibulocochlear nerves will enter the internal acoustic meatus which is what
I am outlining right here the internal acoustic meatus. And the majority of the facial nerve will
eventually exit the petrous part of the temporal bone
but that vestibulocochlear nerve will head towards the inner ear so enters and never
exits the petrous portion. And it's really impossible to tell which nerve is which
in this particular dissection and we would never ask you to differentiate between these two nerves
at least with this particular view. However you do need to know which two nerves traverse this region
so internal acoustic meatus is going to be the facial nerve and vestibular cochlear but just for
fun the vestibular cochlear tends to be the larger of the two whereas the facial is the smaller.
All right, last let's identify those last four cranial nerves. Recall that the glossopharyngeal
vagus and accessory nerves will exit the skull via the jugular foramen and the
jugular foramen is in this general region here you can actually see that the dura
has kind of created a little compartment within the jugular foramen which is not uncommon.
Similar to what we had with the internal acoustic meatus it's really difficult to tell which nerve
is which in this region. We would not ask you to differentiate between the glossopharyngeal and
the vagus because it's really impossible to tell. This would be IX this would be X just for kind of
clarification there. But you can tell which one is the accessory nerve and this is because that
accessory nerve will come up from or ascend through the foramen magnum which is right here.
It will ascend up through here and head towards the jugular foramen. So this nerve we're able to
tell is the accessory because of its relationship to both the foramen magnum and the jugular foramen.
Lastly the hypoglossal nerve will exit through the hypoglossal canal which is in this general region
right here very close to that foramen magnum. With the dura still present as we have here
you have a compartmentalization of the hypoglossal canal so it looks like the hypoglossal nerve
is kind of two different nerves these are just axons of the hypoglossal nerve that will coalesce
into one more more prominent nerve in other parts of the dissection. So this is going to be
your hypoglossal nerve here. Lastly let's look at an anterior view of the brain stem model
as this gives us good indications of where the cranial nerves originate or terminate
and most are going to be originating on the brain stem.
The optic nerves however do not. The optic nerves do not originate from cranial nuclei
of the brainstem. But you can see the optic nerve and the chiasm so this is going to be
cranial nerves II right here in this region with the chiasm in this general region right here.
Moving more towards the midbrain region you are going to have two cranial nerves
that will originate from here. You have the oculomotor nerves which I'm outlining
so this is the oculomotor nerves which are larger and more oriented towards the midline.
And then you have the trochlear nerves that are going to be the smallest cranial nerves
which we say quite often. And these are the only cranial nerves that exit the brain
dorsally or posteriorly and you can see them kind of sneaking in towards this more anterior portion.
Four cranial nerves originate from the pons or near the boundary of the pons and the medulla
right here. Six of the eight are really actually very closely associated to that boundary
between the two structures. Trigeminal is going to be the largest of your cranial nerve so you're
gonna have a large root associated with this or a large trunk on the sides lateral sides of the pons.
Abducens is going to be more medially located whereas the facial and the vestibulocochlear,
so this would be facial and this would be vestibular cochlear, are going to be
a little bit more lateral in comparison to the abducens and very close to one another.
Cranial nerves IX, X and XII will originate on the medulla oblongata and not too far from the pons
either. Glossopharyngeal is going to be more superiorly located, with vagus very close but
a little bit more distal. Oops not five getting my roman numerals wrong. X is going to be vegas.
Your hypoglossals are going to be more medially oriented and more inferiorly positioned. So these
will be your hypoglossal nerves. And lastly your accessory nerves are going to be originating from
the spinal cord so it makes sense that these are going to be more inferiorly located on this model.
All right so we have really explored a few very different ways to view cranial nerves.
Please take time to review on your own and as always feel free to reach out with any
outstanding questions. Thanks for your time and attention and have a great day.