Now, that we have the overall
functions of the GI system down,
let’s talk about the various
phases of digestion.
And the phases are going to be very important
for controlling things like secretions.
So let’s go through
these kind of in order.
The first is the cephalic phase.
The cephalic phase involves
chemoreceptors and mechanoreceptors
that are located in both the
mouth and the nasal cavity.
And these are enacted by things like
tasting, chewing, swallowing,
even smelling of food.
But you also start to enact the cephalic
phase even thinking about food.
So if you’re hungry and you’re starting
to think about wanting to eat,
the cephalic phase is already engaged and
you’re starting a digestion process.
The gastric phase is the one that
you think about most when you eat
because that’s when the food stuff
actually enters the stomach.
And this is linked to how the stomach
distends when food is added to it
so it gets larger
as well as various constituents of food
stuff such as peptides and amino acids
also stimulate this
Finally, in the intestinal phase, this is
when the food stuff has left the stomach
and entered into the
And here, it’s normally linked to
a bunch of digestion constituents
such as large proteins, fats and
sometimes even carbohydrates
as they’re broken down,
it starts to stimulate hormonal responses
as well as some neuroresponses
involved in digestion.
So let’s take a visual
look at these phases.
So the very first thing you want to think
about is as you look at a sandwich to eat.
Here, there’s going to a
neuroresponse that happens first.
There are going to be
some sensory afferents
as you start to eat that
This is primarily done by the vagus
nerve or cranial nerve number X.
And this is one of the classic
cephalic phase responses.
So that’s parasympathetic excitation
and it not only increases
but also engages the stomach to start
secreting both acid and pepsinogen,
which will be
converted to pepsin.
The gastric phase involves
a lot of reflex responses.
Both of these are local
reflexes in nature.
And these are called
And these local reflexes enact or cause
a lot of the responses in the stomach
whether it be from distension or
acid or some other amino acid.
But in addition to that, there
is some gastrin and histamine
that is stimulated at
this time as well.
And both gastrin and histamine
will increase acid production.
Finally, in the intestinal phase,
this is mostly a nervous system response
as well as various
hormones that are released
that will travel to the
pancreas and other places in
the small intestine to start
this digestion process.
So again, we have our
three different phases,
our cephalic phase, our gastric
phase, and our intestinal phase,
and they all involve various aspects
of either the neural system,
hormonal system or local reflexes.