Hi, this is Dr. Kingston, and in this video,
I'll be talking about the regions
of the tongue and their innervation.
The objective for this video is to
describe those regions of the tongue and identify
the nerves and modalities, so motor or
sensory, that serve those regions.
We're going to start off by very simply
dividing the tongue up into four regional directions.
So that first division will be superior/inferior.
The superior surface of the tongue-
so we'll divide it up just like that.
The superior surface of the tongue,
we will call the dorsum of the tongue,
and the inferior surface, we'll call the ventral surface.
The second division will separate
the anterior two-thirds of
the tongue from the posterior third.
So this anterior is called the body,
and this posterior section
is called the root of the tongue.
We're going to start off talking
about the dorsum of the tongue
as there's a lot more going on
here in terms of surface anatomy.
There are two large sulci that are going to divide
the tongue into anterior and posterior, right and left.
The terminal sulcus is what divides
the anterior body from the posterior root on each side.
So there is technically one on
the left, one on the right.
The median sulcus then is going to
divide that posterior tongue into right and left halves.
This is actually sitting directly over
a fibrous septum in the midline that extends all
the way down into
the body of the tongue here and is going to
separate the right musculature from the left musculature.
Now where all of these sulci meet up,
there is a small depression called the foramen caecum.
This makes for a nice little intersection point,
but it's actually an embryological remnant
left behind by the formation of the thyroid gland.
We're going to use that terminal sulcus
to divide the tongue up
into the lingual root and the lingual body.
The root of the tongue sits roughly in the oropharynx.
And because of this,
it's going to follow the pattern of
sensory innervation for the oropharynx.
It doesn't have any sensory papillae.
It doesn't have any taste buds or touch papillae,
but it does have a very rugose appearance
that comes from the lingual tonsil,
which is a mass of lymphatic nodules
that cover the dorsal surface.
So this is part of a larger lymphatic ring in
the pharynx that we'll talk about
more in future sessions.
So for right now,
it's enough to just understand that it's there.
The body of the tongue, or
the anterior two-thirds, sits roughly in the oral cavity.
And again, it's going to share
that pattern of innervation.
The body of the tongue is very muscular.
It's home to all of those intrinsic muscles
that we looked at in a previous video.
And it has attachment points for
all of the extrinsic muscles as well.
This is additionally where all
of the lingual papillae are found.
These are often referred to as "taste buds,"
but they're also going to include some touch receptors.
We think of the tongue as being
primarily in the oral cavity,
but it's also important to take note of
its relationships with the pharynx and larynx.
So to orient you here a little bit,
here is the soft palate and the uvula.
Here is that very rugose lingual tonsil
on the root of the tongue.
The tongue is anchored to the epiglottis, so that
first most superior laryngeal cartilage.
And there are three mucosal folds that do this.
So we have on the lateral sides,
the lateral epiglottic folds there and there.
And then there is a medial epiglottic fold
attaching up to the midline of the tongue as well.
In-between these folds are the epiglottic
vallecula, or spit traps,
as I like to call them, which are
going to collect saliva that
flows into them until you're ready to swallow it.
So that's this area right in there.
This keeps the saliva from heading down into
your larynx and trachea unexpectedly, which would be bad.
Now, the ventral tongue is not divided
up into any additional regions.
It's tethered to the floor of
the oral cavity by the lingual frenulum.
And on either side of that you can find
those sublingual caruncles and
the opening of the submandibular ducts
Running laterally from those, we have
the sublingual folds which are
covering up the sublingual salivary glands
and house the openings of
their many little ducts onto the oral floor.
If we move up off the oral floor,
we can find these little fimbriated folds
of mucosa that are going to
be tying the floor mucosa up to the lateral tongue.
Medial to those we have this dark stripe.
These are the deep
lingual veins that are visible through the mucosa here.
And the reason they look so dark is
that they're very superficial from this angle.
So they're sitting just beneath that mucus membrane.
You can take advantage of this placement
when you're administering medication.
Medicines that are given sublingually will absorb into
the bloodstream very quickly thanks to
those veins proximity to the surface.
Alright, now that we've got
some regions of the tongue delineated,
let's use those to talk about innervation.
Motor innervation is going to be
the easiest to begin with because
both the root and the body of
the tongue are innervated by the same nerve.
All of the muscles, minus palatoglossus,
which is a pallet muscle,
that move and change the shape of the tongue,
are going to be innervated by the hypoglossal nerve.
The hypoglossal nerve exits
the cranium through the hypoglossal foramen
near the foramen magnum.
And it travels inferiorly
and anteriorly through the neck.
So we can see that coming down here as it crosses lateral
to the external carotid branches and
enters into the sub-mandibular triangle.
It is going to pass deep to
mylohyoid to enter the oral cavity so we can see it
coming up here and it is going to send its branches
up to those tongue muscles from an inferior position.
Alright, so far, so easy.
So let's go ahead and complicate it a little bit.
Sensory innervation from the tongue is carried
by separate nerves from motor innervation.
Not only that, but we have
two types of sensory innervation here.
We have somatic touch
sensory and special taste sensory.
And the nerves that carry them are going to be different
for the anterior and posterior regions of the tongue.
We're going to start up here with this posterior third,
which is the root of the tongue,
plus this large of very large,
this row of very large taste buds
called vallate papillae.
So remember, the root
of the tongue sits in the oropharynx,
and so it shares innervation with that region.
Somatic sensory innervation to
the oropharynx is carried
by the glossopharyngeal nerve.
And lo and behold,
that's exactly what we're seeing here.
For this posterior region,
both taste and somatic sensory
are going to be carried by the glossopharyngeal nerve.
The glossopharyngeal nerve exits the cranium through
the jugular foramen along with
the vagus and accessory nerves
and the internal jugular vein.
It makes its way down into the pharynx by riding on
the posterior surface of the stylopharyngeus muscle.
We can see that happening right here.
Here is the glossopharyngeal nerve.
And this guy right behind it
is the stylopharyngeus muscle.
Stylopharyngeus muscle. From there,
it is going to enter into the pharynx by slipping
in-between the superior and
middle pharyngeal constrictor.
So we've got our superior constrictor up here.
We've got our middle constrictor right here.
So this is going to be slipping
in that gap in-between them.
From there it's going to distribute
its branches via the pharyngeal plexus.
And some of those fibers are going to make
their way to the posterior tongue.
While this area is mostly nice and simple,
there is just one little exception.
There is always one little exception in anatomy.
So recall that the vagus nerve innervates the larynx,
which begins at the epiglottis.
And some of its fibers are going to have
an overlapping territory on
the very posterior most aspect of the root of the tongue.
And these are going to be carrying
some somatic sensory innervation.
You can see this area here
indicated by the purple stripes.
The specific branch of vagus
involved here is the internal laryngeal nerve.
And that is a branch of the superior laryngeal.
So we can find the superior laryngeal
coming down right here.
That is going to pierce through the thyrohyoid membrane
and enter into the larynx to
become the internal laryngeal nerve.
So this will be sending off some fibers up
this way to innervate
that posterior most aspect of the tongue.
So that takes care of the posterior tongue.
The anterior tongue gets somatic sensory innervation from
the same source as the rest
of the inferior oral cavity proper,
which is the lingual nerve.
The lingual nerve is a branch of
the mandibular division of trigeminal that
originates in the infratemporal fossa,
deep to the mandibular ramus.
So in the photograph here,
you can see the mandibular ramus has been cut.
You can see your inferior
alveolar nerve heading that way,
and the lingual nerve heading
anteriorly and inferiorly into the oral cavity.
It's going to dive deep to the oral mucosa,
which sits underneath the tongue.
And we've seen a few times now that it
is attached to the submandibular ganglion,
which is where parasympathetics from
the facial nerve synapse to get to
the submandibular and sublingual glands.
So that ganglion is right here.
Okay.
Just colored a little bit so I
don't cross off the arrow there.
And you can see it's got some branches
heading this way to
the sublingual gland and
some others down here to the sub-mandibular glands.
The bulk of the lingual nerve, however,
is going to loop underneath the sub-mandibular duct.
So sub-mandibular duct, if you
follow the arrow, is right here.
So the bulk of the lingual nerve is
going to come up underneath of it, like so.
And it will distribute its branches to the tongue
thusly. The special sensory
for the anterior tongue is carried by the chorda tympani,
which is a branch of the facial nerve.
To get where it's going,
it's going to hitchhike with
the lingual nerve that we just saw.
So let's take a look at how it does that.
As we're gonna see in much greater detail
in future sessions,
the chorda tympani will branch off of
the facial nerve within the temporal bone,
and then it will emerge from
the cranium in the infratemporal fossa.
You can actually see it here in this image,
but it is very tiny,
so I will highlight it for you.
Alright, so here's chorda tympani.
It's coming down into
the infratemporal fossa and it's going to
run anteriorly and inferiorly.
As it comes up to the anterior edge of the mandible here,
it is going to join up with the lingual nerve.
And let me just choose
a different ink color here to highlight that.
So here's our lingual nerve in blue.
It's a branch of V3 going to run down,
enter into the oral cavity.
Here. You can see right around this area here that
the chorda tympani not
only runs as a neighbor to that lingual nerve,
but it actually joins right up against it.
Now, these are still considered to be separate nerves.
They're wrapped in their own fascicles,
but they do run pressed
along right up against each other.
Alright, so just to review here,
the somatic sensory from
the anterior tongue will follow that lingual nerve,
that blue nerve back up through V3 to get to the brain.
But taste sensation will follow the chorda tympani
back to the facial nerve to get back to the brain.
So two separate pathways once they leave the oral cavity.
Let's do a practice question to test what you've learned.
The correct answer here is C, dry mouth.
Now, this is a multi-step question
and a little bit complicated.
So let's walk through why this is the answer.
Alright, so we have a patient reporting decreased
sensation on the anterior right aspect of their tongue.
Alright. We think it's a nerve injury.
But to prove that to ourselves,
we'd like to see some additional symptoms.
So what symptom of these
is most likely to experience? Alright,
so we need to figure out what nerve is
responsible for taste sensation on
the anterior aspect of the tongue.
And that is going to be that right chorda tympani.
And knowing that we can use that to answer the question,
does the right chorda tympani
carry taste sensation from the left tongue?
No.
Does it carry taste sensation
from the posterior right tongue?
Nopre, that's glossopharyngeal.
Would the musculature on that side be paralyzed?
No. That's carried by the hypoglossal.
So by process of elimination,
even if you weren't sure,
you could work out that
dry mouth was the most likely answer here.
Now the reason it absolutely is the correct answer is
because the parasympathetics that
innervate both our sublingual glands
and sub-mandibular glands are
also carried by the chorda tympani.
So any injury to that chorda tympani nerve would
not only affect taste on that side of the tongue,
it would also inhibit
secretion by these two salivary glands.
Alright, so dry mouth and
decreased taste go hand in hand.
And that is going to bring us to the end.
Thank you for watching.