Sensation from the Face: Neuroanatomy Video Lab – Brain Dissections

>>It’s time to consider the
sensation coming from the face. So focusing on facial sensation
I brought my friend along here, and these green branches of
the trigeminal nerve are going to give you an idea of the
extent and the magnitude and the importance of
the trigeminal nerve. So the trigeminal nerve
has three branches. From the vertex here, we
have branches all the way down to the middle of the orbit, and we call that the
ophthalmic division. Then below that there is
the maxillary division, which includes innervation
of the teeth and the gums. And below that we have
the mandibular division, also includes the lower
jaw, teeth, and gums. Now, if I turn this around, what you can see is
there are green nerves on the posterior
aspect of the skull, but these are not trigeminal. These are high cervical
innervation of the back. So it’s only from the
vertex, and it only goes down and excludes the
angle of the jaw. The ear — this is the
external auditory meatus — the ear is very complicated because embryologically it
formed from different regions. And so it’s innervated
partly by the trigeminal, but it also has some innervation
from the 7th and 9th nerves. So the ear is not a
good thing to test when you’re testing
sensation from the face. So now let’s look at some
of the internal structures that are also innervated
by the trigeminal nerve. Here’s a hemisection
of the head. And I just want to remind you
that the anterior two-thirds of the surface of the
tongue, and also the mucosa of your cheeks, are
innervated by the trigeminal. So if you bite your cheek
or burn your tongue, that information is conveyed
by the trigeminal nerve. Similarly, the nasal
mucosa is also supplied by the trigeminal nerve, as
are the linings of the sinuses and most of the dura mater that
covers the surface of the brain. So now we’re going to
quickly start with the pathway from its origin in the
trigeminal ganglion. So these nerves have
come in from the surface, and now sitting here in
the middle cranial fossa, this cavity here, is
the trigeminal ganglion. It’s green and big; it’s
the biggest ganglion. These are the cell bodies,
the primary cell bodies, coming from all those three
divisions, and they are going to enter into the center of the
pons, the middle of the pons. The pons, recall, is lying right
here over the basal artery. So in the body, carrying
sensation up from the spinal cord,
we had two major pathways: We had one called
the spinothalamic and we had the other called the
dorsal column-medial lemniscus. We have those same two
types of tracts coming from the trigeminal
nerve, and one is going to involve a synapse in a
descending, or spinal nucleus, and that’s going
to be one route. And the other one is going
to involve another nucleus, the principal, or chief,
trigeminal nucleus. And so we have these
two pathways. And the first one carries,
just like from the body, pain and temperature: hot
and cold or warm and cold. And the other one
carries vibration, two-point discrimination — like knowing if my face is being
touched in two places or one when two points are placed on my
skin — and also joint position, joint — well, this
must be the jaw; that’s about the only
joint we have on the face. Now, light touch, just
like from the body, is carried in both
of these pathways. So light touch will tell
you that something’s working in the trigeminal, but
it doesn’t distinguish between these two pathways. So now let’s look
at an animation of how this information
comes in and is carried up to the thalamus
and onto the cortex. I’m going to show
you this animation, which covers the entire
nervous system from the tip of the spinal cord to
the top of the cortex. And here we have the
midbrain, the pons, the medulla, and
the spinal cord. Now, pain and temperature, which
we’re going to go through first, comes in from the face and it
does something very strange. This first neuron, the
trigeminal ganglion neuron, comes in and it descends. It descends all the way
down, some people say, C4, C5 to the cervical level. Then it synapses and the
second neuron crosses and forms the trigeminothalamic
tract and goes up to the thalamus and
then onto the cortex. Let’s animate that. You see this is the
first order neuron. You’ve been painfully
stimulated on your face. Information comes in and
it descends from the middle of the pons to the top of the
cord, crosses to the left side and then goes to the left
cortex via the thalamus. Now let’s look at
fine touch, vibration, joint position coming
from the face. It goes to a different nucleus. It comes straight in and
synapses in the chief, or principal, nucleus right
there in the middle of the pons. Then the second neuron crosses
— it’s always the second neuron that crosses — and synapses on
the third neuron in the thalamus in the same nucleus
but different cells, the ventral posterior, and
this time it’s the medial part of the thalamus, and then on through the internal
capsule up to the cortex. Let’s animate that. Information comes in, synapses,
crosses, goes to the thalamus, through the internal capsule to
the face region of the cortex. Recall, this is only information
from the face and it’s out on the lateral cortex,
the lateral postcentral gyrus. So now we are looking at the
ventral surface of the brain, and the pons extends
from here to here. And right in the middle
of the pons is the largest of all the cranial nerves,
the large trigeminal nerve. So it comes in on the side
of the pons on either side. We can see it better on
a brainstem specimen. We’re looking at
the brainstem here. The surface here
is the midbrain. And as I tilt it up, you can see
the pons that extends from here to here and the medulla
coming down here, connecting with the spinal cord. And right here is the large
trigeminal nerve burrowing through the pons here, and
here it is on the other side. And so what we want to do is to
follow this nerve as it comes in and makes its connections. Now we’re going to trace the
entrance of the trigeminal nerve that carries information
for pain and temperature. So here we have in the middle of
the pons the trigeminal nerve. So information coming in from
the right trigeminal nerve enters into an area
approximately here, and then those axons descend. They’re going back down
through the medulla — we’re still on the right side
— back down through the medulla down near the level
of the decussation, even down to the high
cervical spinal cord. And there in these areas you
will find the second neuron involved in crossing
to the other side. So the decussation of this
trigeminothalamic tract occurs all the way from the middle
of the pons all the way down to the high
cervical spinal cord. Now we’re going to consider
vibration, joint position, and fine discrete touch
and how that enters. Similarly, it comes in through
the right trigeminal nerve. And right here at the level where it enters there
is a nucleus: the chief, or principal, sensory
nucleus of the trigeminal. There, the second neurons
cross to the other side and are located close
to the medial lemniscus that we saw coming from the
spinal cord connections. So we now have facial
sensation located very close to the medial lemniscus that
is carrying body sensation for fine touch, joint
position, and vibration. Now, the pathway that
came up with pain and temperature is
also in this area. And so we have both of
these pathways converging in the middle of the pons. And from there on
they are called the trigeminothalamic pathway. And they run close to the
body sensation pathway and they’re going to head
up toward the midbrain. This section is a transition
between midbrain and pons, and our fibers are
traveling in this area here. Facial sensation for all
modalities is very close to body sensation, and
everything has crossed. So damage in this area is likely
to result in the loss of pain, temperature, vibration,
joint position, et cetera, from both the body and the face. One more level up
is the midbrain. When we get to the midbrain,
our fibers now are right here on the surface, on their way
going around the midbrain and onto the thalamus. So finally we’ve
arrived at the thalamus. So we have to be going
to the left hemisphere. So we are now in
the left thalamus and facial sensation goes more
medial than body sensation. So body sensation was in the
ventral posterolateral region and facial sensation is in the ventral posteromedial
area of the thalamus. All of this is the thalamus. So we generically
call these nuclei VPM for the face and
VPL for the body. Label lines still exist: If you’re a pain fiber,
you’re still a pain. If you’re a vibration, you’re
still carrying vibration and you enter into the
internal capsule, and you head up now for the face area. Remember, the face is
lateral on our homunculus. So the face and larynx and
pharynx and that area is all over in this most lateral
part of the postcentral gyrus. Let’s look at the cerebral
cortex in a whole brain now. We’re back to our
whole brain again. And here is our central
sulcus coming down here to the lateral fissure, the
postcentral gyrus region here. All of this area is mostly
regarded as involving the face. Don’t forget that we have
lips and tongue and all of these other areas, but we
also have facial sensation. So there’s a very important
take-home message about the pain and temperature pathway
from the face. If you have a low lesion of
the brain stem or a high lesion of the spinal cord and you
see loss of facial sensation for pain and temperature,
that doesn’t mean that the lesion is in the pons. It could be at any
one of these levels. And if all the sensation
is lost from the face, you know it’s either
the nerve or the middle of the pons farther
forward or rostral, anywhere from mid pons
to cerebral cortex.


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