Now peripheral chemoreceptors,
we talked about before they’re
responsive to more blood gasses
than the central chemoreceptors.
Where are they located?
Well, they are primarily located in the
carotid bodies and the aortic bodies.
This will help because
the carotid bodies
will send their information via cranial
nerve IX, which is the glossopharyngeal,
or cranial nerve X, which
is the vagus nerve.
This is the overall anatomical process
for how these particular bodies
are sensed and where they
send their signal to.
But now, let’s go in a little bit more
detail to what these particular bodies
look like and how they operate.
We can see a blowup here of
a peripheral chemoreceptor.
The most important aspect of
these particular chemoreceptors
are these type I
or glomus cells.
The glomus cells are the sensors for the
various carbon dioxide oxygen and so forth.
The blood is in these little sinusoids
which can be seen as the
red components of circles,
and in the blood vessels that go into
these particular carotid bodies.
There are also other
nervous system information
that is both sensed as an
afferent and efferent.
For example, the sympathetic nervous
system has efferent signals
that will send both to the glomus cells
and the various components around it.
And what this can do is alter the gain
of what a glomus cells
would be able to signal.
What I mean by gain is a certain
amount of carbon dioxide
in a sympathetically stimulated
environment will cause a greater response
than if you had that same signal with
CO2 without sympathetic stimulation.
The afferent information is again for the
carotid body sensed by cranial nerve IX
or the glossopharyngeal and then
that’s sent up to the brain.
So peripheral chemoreceptors, these are
the ones again in the carotid bodies.
They are going to be sensing
particular signals in a little
different manner than the
So let’s go through
You’ll notice here that there are blood
vessels right next to the glomus cells.
What’s very important on
these type I glomus cells,
there is oxygen-dependent
And when PO2 in those
blood vessels are high,
these calcium channels remain open
and what this means is potassium is
allowed to leak out of the glomus cell.
And if this occurs,
there’s no change in membrane potential
and the glomus cell is quiet.
And this can be seen in this diagram
where we put all the pieces together.
Again, this oxygen-dependent
potassium channel is open.
Calcium channels are closed.
PO2 is high in this case.
Now when PO2 falls, those
calcium channels close.
And as the calcium channel closes,
the potassium which would be leaving
the cell builds up within the cytosol.
That creates a positive charge and can
eventually depolarize that glomus cell.
As that depolarization happens,
that opens L-type calcium channels.
And calcium is allowed to
rush in to the glomus cell.
What this causes is a docking and
fusing of a neurotransmitter
to signal sensory afferents and
sending that signal up to the brain.
This whole process works in
response to a fall in O2.
Now showing you how this
works in terms of a diagram
where we have membrane potential on the
Y-axis and then we have time on the X-axis.
We have a normal resting
membrane potential here.
This is time in which we have
reduced the amount of O2
and you can see a dramatic
increase in the firing frequency.
And what does that increase
in firing frequency tell us?
These spikes are all releasing
back to the brain.
Now let’s talk through
a little bit about how
O2 and CO2 govern your
drive to breathe.
And I like this kind of flow
chart because it tells you
a little bit more about what kind of
decisions the brain stem needs to make
to determine if you
should breathe or not.
So if the arterial partial pressure
of carbon dioxide goes up,
The decision then will be
to increase ventilation.
If arterial partial pressure
of oxygen decreases,
will be engaged
and then there needs to be
another question asked.
And that is this decrease of O2 occurring
at the same time that CO2 is increasing?
If that’s the case, the decision
will be to increase ventilation.
If the answer is no, we need
to ask another question
and that is is the partial pressure of
oxgen less than 60 millimeters of mercury?
If that answer is yes, you
have to increase ventilation.
If the answer is no,
it cycles back to a
process in which now
we’re going to govern if
O2 or CO2 will change.
This kind of flow chart is very nice
when thinking about when you get numbers
from an arterial blood gas,
trying to decide what the body is
going to do with that information.