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Mouth to Esophagus

– In this video,
we’re going to start our tour of the digestive
tract, going into a little bit more detail about
the things going on and the structures
assisting in each parts of the digestive tract. This first video will be
dealing with the mouth and the esophagus. Here you’re looking at
both the sagittal cutaway view of the mouth, and a front
on, anterior view of the mouth. The mouth– we’re all
pretty familiar with it, it’s bounded
anteriorly by the lips, laterally by the
cheeks, so a cheek on either side of the mouth. Superior, you have the
palate, the hard palate, the bony portion,
and then behind that the muscular soft palate, and
inferiorly, by the tongue, this enormously muscular organ. The mouth is the
site of ingestion, it’s also the beginning of
both mechanical and chemical digestion. You have chewing for
mechanical processing, and you have digestive
enzymes in saliva. The uvula is this portion
of the soft palate that hangs down into the pharynx. It actually does
serve a function, it keeps unchewed food from
slipping into the pharynx. You should also be familiar
with the term vestibule. The vestibule is the region
between the lips and the gums and teeth. So like if you
blow your lips out, that space that you’re filling
with air is the vestibule. There’s no nutrient
absorption here normally, but there are some drugs that
can be absorbed sublingually, like nitroglycerin. And there are some other
cardiovascular drugs that can be given in a tablet
that’s placed under the tongue. In order for a drug
to be absorbed here, it has to be lipid soluble in
order to cross the epithelium, but it also has to have
some solubility in saliva because saliva is
a watery solution. So a drug has to be
carefully crafted in order to be used sublingually. As I mentioned, the
tongue is highly muscular and there are two types of
muscles, the intrinsic muscles and the extrinsic muscles. The intrinsic muscles form
the surface of the tongue, and they’re called
intrinsic because they aren’t attached to bone. They’re only attached to
the extrinsic muscles. And these are the
muscles that allow the tongue to change shape. So like when your tongue
is kind of broad and flat, or when your tongue is pointy,
that’s your intrinsic muscles. The extrinsic
muscles are actually attached to the
bones of the skull, and the extrinsic muscles allow
the tongue to change position. The tongue serves a number
of different purposes. It mixes food with
saliva, forming a bolus. So bolus is basically
food and saliva. It’s a compact, easy to swallow
and easy to swallow completely unit. The tongue also
initiates swallowing by moving that bolus of food
to the back of the mouth and triggering the
swallowing reflex. It’s attached by the
lingual frenulum. That’s what sort of
holds the tongue in place. One other important
function of the tongue is that it serves as a
platform for the taste buds. The tongue is covered
with papillae, the little bumps on the tongue. You have circumvallate
papillae, fungiform papillae, and filiform papillae. And these papillae,
they provide friction to help your tongue
manipulate food. They also house the
taste buds, specifically in fungiform and circumvallate
papillae you have taste buds. You may be familiar
with the idea that different regions of the
tongue taste different flavors, but the reality
is that all taste buds can detect all flavors. Although there is some
evidence that the anterior portion of the tongue is more
sensitive to salty and sweet tastes, and that the posterior
portion of the tongue, the papillae located
back here, are more sensitive to sour
and bitter tastes. There’s a fifth taste
as well, called umami. That’s the taste of cooked meat. The taste that gives
MSG its appeal. The receptors for
this taste tend to be located more towards
the back of the tongue. You also have teeth in your
mouth, as well as the tongue, of course, and teeth are
crucial for the first step in mechanical
digestion, chewing, perhaps a little more
technically known as mastication. Teeth are basically
modified bone tissue. They’re coated in a
surface of enamel. That’s what you’re familiar with
as the white portion of teeth, and enamel is the hardest
substance in the body. It’s packed with calcium
phosphate crystals, hydroxyapatite, the same type
of calcium phosphate crystals that you find in bone,
but it’s much more densely packed with calcium
phosphate than bone is. So it’s actually
harder than bone. It can withstand
about 1,000 pounds per square inch of pressure. So just as a point
of reference, that’s about 6,800 times
atmospheric pressure. So your teeth can take
a lot of pressure. Enamel is about 96% organic
and 4% organic matter, so it’s really almost entirely
these calcium phosphate crystals. Underneath this layer of
enamel, you have the dentin. Dentin is the bulk of the tooth,
and that’s more bone-like. It’s not quite as densely
mineralized as enamel. It’s only about 70% mineral,
30% organic material. Dentin is actually
living tissue, and there are cell
processes that extend out from the pulp cavity that
can reach into the dentin and replace it. Enamel, by contrast,
can’t regenerate. Enamel is generated when
the tooth is formed, and then the cells
that make it die. That’s why it’s important
that you brush your teeth and take good care
of that enamel. Inside the dentin, you have the
pulp cavity with blood vessels and nerves running up,
and then the whole tooth sits in a socket called an
alveolus, plural alveoli, in your jawbone, your
mandible or your maxilla. And there’s a very
short ligament called the periodontal ligament
that holds each tooth in place, and then you have
a gingival collar of soft tissue,
basically your gum, that acts like a secondary anchor. Not all teeth are created equal. You have teeth of
different shapes that are specialized
for different functions. Incisors appear at the
front of the mouth, are specialized for cutting
or nipping off pieces of food. Back of the incisors, you have
the canines, also sometimes known as your eye teeth. These are more fang
like, and they’re meant for piercing and tearing food. Your premolars and
behind them, the molars, have a flatter
surface specialized for grinding and crushing food. We have a set of
teeth when we’re young that are called
the deciduous teeth, they’re also sometimes called
milk teeth or baby teeth. They come in between
about six months and two years, individual
variation in the exact timing. This image over here
shows the timing of when the permanent
teeth come in. So incisors, somewhere around
seven and eight years of age. Canines and
premolars 11, 12, 13, molars vary, including
the third molars, what you may know as your wisdom teeth. They may not come in
until you’re in your 20’s. The baby teeth are actually
present below the gums at birth, but they
aren’t erupted yet. The basic substance of the tooth
is formed by about six weeks after conception
and the hard tissue is formed by about three or
four months of gestation. So when the deciduous
teeth come in, they’ve actually been sitting
there in the jaw for a while, they just haven’t
broken through the gum. And the same thing with
the permanent teeth, they’re actually sitting
there, growing in the bone below the baby teeth before
they erupt and push the baby teeth out. These diagrams show you
what I mean by that. Up here on top you’re
looking at a skull with developing
permanent teeth in here. The maxilla and mandible
have been shaved away to show the permanent teeth
in development, starting to push the deciduous
teeth, the baby teeth, out. This is an x-ray of the
same thing going on– a different patient,
not the same skull– but it’s showing
two layers of teeth. You can see the permanent
teeth in development here below the baby teeth. The third set of accessory
structures in the mouth are these salivary glands. And salivary glands are
not actually in the mouth, but they’re around the mouth
and they secrete their product, saliva, into the mouth. There are paired
parotid glands, so you have the large parotid
gland, left and right, submandibular glands, again,
left and right, and paired sublingual glands,
left and right. You also have scattered small
intrinsic salivary glands throughout the oral cavity. This is a cadaver view
of the salivary glands. Again, you’ve had the skin
shaved away to show them. You have the parotid glands,
highlighted over here, so that’s this tissue. You have the
sublingual gland there, and the submandibular glands. Histologically, this is what
the salivary glands look like. They’re referred to
as seromucous glands, because they secrete both
serous fluid and a mucus fluid. The serous fluid is
more watery, it’s produced by the serous cells,
and it’s the serous cells that actually produce amylase. Amylase is a starch
digesting enzyme. So when you are chewing bread
and it’s becoming very, very broken down, very mushy in your
mouth, that’s because in part, it’s because of the
action of amylase secreted by these serous cells. The mucous cells produce
a more viscous lubricating solution. It contains a
protein called mucin that’s a lubricating protein. So the salivary glands
really get chemical digestion started as well. They’re controlled primarily
by parasympathetic innervation, so that resting and
digesting branch of the autonomic nervous system. And release of saliva can be
stimulated by either putting something in your mouth. It can also be stimulated
by a higher level thoughts of food or smelling food. So if you smell
something appetizing or if you are just
thinking about food, you can sometimes actually,
literally, start salivating. Saliva serves a number
of different functions. First off, it
cleanses the mouth. You actually have anti-bacterial
molecules in saliva, so saliva helps limit
bacterial growth in the mouth. It’s important for tasting. Food needs to be dissolved into
a watery solution in order for your taste buds
to actually detect the molecules in that food. And if you have a deficiency
in saliva production, you may find that things
kind of taste funny. It also helps moisten
the food, compact it down into a bolus that
is easy to swallow, and of course it’s the beginning
of the chemical digestion. You have amylase, the enzyme
that breaks down starch, you also have lingual lipase
secreted by sublingual glands that begins lipid absorption. Moving out of the mouth
now, we’re into the pharynx. For the digestive
system, you really really only involve
the oropharynx, which is the region of the
pharynx behind the mouth, and the laryngopharynx,
the region of the pharynx behind the larynx. The nasal pharynx
is not typically part of the digestive system,
although it is continuous, so air moving in
through your nostrils passes through the oropharynx as
well on its way to the trachea. There’s not a whole
lot to say about what’s going on in the pharynx. There is no digestion,
apart from what’s happening thanks to saliva
coming from the mouth. There’s no absorption
in the pharynx. It’s really just
kind of a way station for the initiation of
swallowing and moving food down the esophagus. The esophagus is this
long, muscular tube that carries food
from the pharynx through the thoracic
cavity down to the stomach, and the stomach is nestled
up under the diaphragm, right at the top of the
abdominal cavity. So food just kind of slips
through the thoracic cavity down your esophagus. It has that four
layered structure that you see throughout
the digestive tract. The mucosa is different
from what you see elsewhere, it’s a stratified
squamous epithelium. But it’s
non-keratinized, and this is different from the skin. This is what the skin looks
like under the microscope, this is what the esophagus
looks like under the microscope. This thick layer here
of dead skin cells, this is the keratin layer, the
stratum corneum of the skin. The esophagus, it’s moist. It doesn’t need
the waterproofing that the skin does. The skin, because it’s
continually exposed to air, if it didn’t have a
waterproofing layer, you’d die of dehydration. But the esophagus is
constantly bathed in fluids, so it doesn’t need a
waterproofing layer. So the esophagus
is non-keratinized. Moving outward from
the mucosa, you get to the submucosa, the
connective tissue underlying the mucosa. In the esophagus, the submucosa
has mucus secreting glands that basically just help moisten
the surface of the esophagus and help keep food
moving along smoothly. The muscularis externa
is a little bit different from the rest
of the digestive tract, and it’s even different
along the esophagus. In about the top third
of the esophagus, the muscularis externa
is skeletal muscle. In the middle of
the esophagus, it’s a mix of skeletal
and smooth muscle, and it’s smooth muscle only at
the bottom of the esophagus. The outermost layer
of the esophagus is actually adventitia. We’re talking about
the thoracic cavity, we’re not in the
abdominal cavity, so the esophagus is surrounded
by a thickly fibrous adventitia. This is a gross anatomical
view of the esophagus. You can see the
trachea here has been cut away so that you can expose
it and see the muscular– very muscular tube, with
the aorta behind it. The main function
of the esophagus is just propulsion, getting
food from the pharynx to the stomach. There’s nothing,
there’s no absorption that happens in the esophagus. That thick, stratified squamous
mucosa would prevent that. There’s also no real digestion
going on the esophagus, at least no new digestion. The enzymes from
the saliva are still active as food is moving
down the esophagus, but there’s nothing new added
to that digestive process. So really when we talk
about what the esophagus does, it’s just all about swallowing. Slightly more technical term
for swallowing that you should be aware of is deglutition. Swallowing can be broken down
into four phases, four steps in swallowing. In the first stage,
the buccal phase, this is when the
tongue moves the bolus to the back of the
mouth, and that triggers the swallowing reflex. That activates
muscles in the pharynx that swallow that bolus down. In the pharyngeal phase,
the epiglottis closes, and that’s that right
there, so that the bolus is directed into the
esophagus and does not go down into the trachea, which
is a not good thing to happen. If food does go down
into the trachea, it will trigger a
coughing reflex, and if that doesn’t
get it out, then you have just aspirated some
food into your lungs and you may be at risk
of developing aspiration pneumonia. As that bolus moves down,
moves into the esophageal phase where peristalsis takes over. Remember those involuntary
smooth muscle waves push the bolus down
towards the stomach. And then lastly, the
bolus enters the stomach. It passes through the
gastroesophageal sphincter into that holding tank. OK, so after
studying this video, you should be able to talk about
what the different accessory structures of the mouth do. The tongue, the teeth,
and the salivary glands. You should be able to talk
about the structure of teeth and the different kinds of
teeth, so enamel, dentin, periodontal ligaments. You should also be able to talk
about the various functions of saliva. You should also be
able to describe the structure of the
esophagus, in particular talk about the ways
that the mucosa is different from other parts of
the digestive tract and ways that the muscularis externa
is different from other parts of the digestive tract. You should also be able
to talk about differences between the esophagus
and the skin, the skin being keratinized,
the esophagus being non-keratinized. You should also be
able to describe the process of swallowing.

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