Cranial Nerves: Neuroanatomy Video Lab – Brain Dissections


>>Well, this is supposed to
be a model of the brain stem with the cranial nerves. Some people say it looks like
a voluptuous dancing girl, other people say it looks
like a toy for their dog. However, the pharmaceutical
companies maintain that this is how neurologists
can learn their cranial nerves. There are 12 cranial nerves
and it takes about 12 years to learn all 12 of them
until it’s automatic. I don’t know why 12 nerves
can be so complicated, but what we’re going to
try to do is to take some of the mystery out of this model and help you associate
three things, the level of the brain stem, the
cranial nerves that supply it and its blood supply and
reflexes to be tested. So we have a medulla, we have
a pons, we have a midbrain, and all of this can be ignored because this is part
of the thalamus. So midbrain on the
ventral surface, by the cerebral peduncles, which
we’ll come to in another time, the pons on the ventral surface, where the basilar artery was
running, and the medulla, where the vertebral
artery was running. So here’s the first little
piece that you need to memorize. There are 12 cranial nerves. There are two cranial
nerves associated with this short little midbrain,
cranial nerves 3 and 4, there are four cranial nerves
associated with the pons, cranial nerves 5, 6, 7 and 8, and there are four
cranial nerves associated with the medulla,
9, 10, 11 and 12. Blood supply to the medulla
is the vertebral, blood supply to the pons is the basilar,
and the blood supply to the midbrain is variable
from the posterior cerebral to the superior cerebellar. If we turn it over, it gets
more complicated than we need so let’s now look
at the real brain. Cranial nerves one, the
olfactory nerve, and two, the optic nerve, are
not true cranial nerves. They are actually embryological
outgrowths of the brain, they are central nervous tissue,
they will not regenerate, and while they are called
cranial nerves one and two, they have their own
system of connections and are not usually
discussed when we talk about cranial nerves
of the brain stem. So let’s start with
the midbrain. What you’re looking at
here is the midbrain cut. Before what we could see were
these two bands of fibers on either side, the
cerebral peduncles. They can be seen here and here. These cerebral peduncles,
according to Dr. Renner, look like Mickey Mouse ears
when seen in this position. But the midbrain, with the
ventricular aqueduct here, and the colliculi here,
and the substantia nigra, and the cerebral peduncles, this is a classic
appearance of the midbrain. Sometimes we look at it
this way, anatomically, and sometimes we look at it
this way, radiologically. So the trick is eventually
to be able to turn things around in your mind,
but we’re not going to worry about that today. We’re just going to look at the cranial nerves
associated with the midbrain. And the first one is
the third cranial nerve. It’s missing on the other side. It emerges between two vessels,
the posterior cerebral artery at the top of the basilar, and
the superior cerebellar artery. It’s very important, the
third or oculomotor nerve, for visual reflexes, pupillary
constriction and the curvature of the lens of the eye for
sharp accommodated vision. The other cranial nerve
associated with this level, the midbrain, is this fine, small tenuous nerve
called the trochlear nerve. This is the fourth
cranial nerve. It’s the only cranial
nerve to come out on the dorsal
surface of the midbrain. Inside the brain it crosses,
but we don’t need to worry about that, and it supplies one
rinky-dink muscle called the superior oblique muscle. So this right trochlear
nerve goes to the right superior
oblique muscle. And clinically, it’s not
particularly important whereas the third nerve, the oculomotor
nerve, supplies all but two of the eye muscles
and the eyelid and is important
clinically because it’s easy to recognize deficits associated
with this cranial nerve. So cranial nerves three
and four, oculomotor and trochlear, midbrain. Next we go to the pons. We’ve taken the vessels off so you can more clearly see
the cranial nerves five, six, seven and eight. Let’s start with five. Five is the trigeminal nerve. It’s the largest of
all the cranial nerves, has the most axons in it
because our face, our lips, our nasal cavities, the inside
of our mouth, highly innervated by the trigeminal nerve. It comes in right in
the middle of the pons.>>So it’s sensory from the face
and other structures in the head and it is motor to the
muscles of mastication. You have four muscles for
chewing, the masseter, the temporalis and
the pterygoids. We don’t need to
remember that except that you can test those muscles
and if they’re not working, you suspect this trigeminal
nerve, nerve number five. Nerve number six has a lot in
common with the trochlear nerve, number four, in that it
goes to only one muscle, the lateral rectus muscle,
for moving your eye laterally. All right? And the seventh and
eighth nerves come out here in this region here. See, there’s quite a bit of space here called the
cerebellopontine angle, all right, the cerebellopontine
angle. There’s a lot of cerebral
spinal fluid in there, there’s a fair amount of space, and in there are two very
important cranial nerves. This is the fifth. Let me pull that aside. The more medial one
is the seventh nerve. It’s called the facial
nerve and it goes along with the eighth nerve, or the
auditory and vestibular nerve. The vestibulocochlear nerves,
the two of them travel together into the temporal bone, into
the internal auditory meatus. The seventh nerve goes to the
muscles of facial expression, your smiling muscles, your
wrinkling the forehead muscle, your kissing muscles,
your whistling muscles. And the eighth nerve goes
to the semicircular canals and to the cochlea for
balance and for hearing. They can be seen very nicely. So we have five,
six, seven and eight. Those are the cranial nerves
associated with the pons. Next we have the medulla. The medulla, you can see
stretching from here to here, and it has cranial
nerves nine and ten. You can see all these
little fascicles here. I kind of separate them
and pull them apart. These are cranial
nerves nine and ten, the glossopharyngeal
and the vagus. And I can’t tell you which
branches are going to which. The glossopharyngeal
is sensory for taste. It’s also motor to one muscle. And the vagus is also
sensory and motor to the autonomic nervous system. I might mention that the seventh
nerve also carries taste. So taste is carried in seven and
in nine and a little bit of ten. But clinically, taste is not
nearly as important as hearing and vestibular function and
facial expression and sensory from the face and muscles of
mastication or eye muscles or the control of eye muscles. So that’s nine and ten. What happens to eleven? Eleven is torn off of
every single brain I have because it has a unique
trajectory that comes up through the foramen magnum and it’s called the spinal
accessory because it has part of its origin and roots from
the upper cervical spinal cord and others from the medulla,
and then it takes a crazy course after coming up into the
cranial vault, it goes back out. And that can best be seen
in the following diagram, but this cranial nerve is
not particularly critical and is easily diagnosed
by a slumping shoulder because it supplies
the sternocleidomastoid and trapezius. The twelfth cranial nerve cannot
be seen either, but it emerges in a groove in the medulla
between the band of fibers here, which I’ll call the
pyramid, and this bulge here, which I’ll call the olive,
and in this groove along here, out come the multiple
little roots of the twelfth cranial nerve,
or the hypoglossal nerve. And that too can best
be seen on a diagram. [ Pause ]

42 Comments

Add a Comment

Your email address will not be published. Required fields are marked *