With cell-to-cell signaling,
it doesn’t work if you just
send that signal out and don’t
have anyone to receive it.
You need to be able to
receive the signal.
And this doesn’t matter if
it is an endocrine signal,
if it is neural signal, if it’s
paracrine or autocrine signal,
you need to be able
to receive it.
And how does the body and various
cells receive these signals?
It could be that that particular
signal that you’re trying to sense
is from outside the body and
you need to look at a light
wave or a sound wave or
maybe a smell or a taste.
But within the body, the signals
are usually more subtle,
like a signaling molecule that
gets delivered to your front door,
or maybe a wire transmits that
to a certain spot of the body.
So let’s now go through a number of these
informational type of gathering sensors.
So in the neural system, we’ll
go through things like light
receptors or photo
receptors in the eye.
We’ll go through hair cells in the ear
that sense vibrations and sound waves.
We’ll have taste odorant
receptors, as well as
ones on your tongue to sense
different types of tastes.
So, all of these types of
receptors will garner information
about the external
environment around you.
Other cells will utilize
various receptors that
will look for what we’re
going to call a ligand.
A ligand is simply a
and a molecule that will bind
only to a specific receptor.
This is very handy
way to signal.
This allows you to put
out a number of signals
and they only get received by
the cells that you want to.
And how do you know?
Based upon the receptor.
So let’s go through a
number of these receptors.
The ligand itself will vary depending upon
what kind of signaling molecule it is.
It might be an ion, it might
be a neurotransmitter.
Again, it might be a hormone.
When it binds to a particular
receptor, it will take many forms.
Sometimes it makes the receptor
come together to form a dimer,
sometimes it just bind to a
very small part of a receptor.
The types of receptors we
have could be catalytic.
And what we mean by a catalytic receptor
is as soon as the ligand binds,
an enzymatic reaction happens
and there is either some phosphorylation
or some action that takes place.
We just provided you a couple
of examples here of an ANP or
atrial natriuretic peptide receptor
or a growth hormone receptor.
So you can see here, even
though these two signals
look somewhat the same in
terms of their receptors,
they’re going to cause
dramatically different responses
if it’s an ANP receptor versus
a growth hormone receptor.
Another type of receptor is a
G protein-coupled receptor.
There are numerous types of
G protein-coupled receptors
and what these are are ways
to sense a signal and then to
cause an enzymatic reaction and
cascade process to happen.
There are a number of
different types of these.
And usually, not only is which
ligand binds to it is important,
but also which of these G
proteins are activated
will depend upon what kind of
response you’ll get within the cell.
This utilizes a secondary messenger system
usually to start the cascade process.
Another example here of a G
protein-coupled receptor is this one,
where you have a ligand
binding to a receptor.
You can see there are three G
proteins around that receptor.
We have them labeled here as
an alpha, a beta, and a gamma.
In this case, you’re going
to see the alpha component
move over once the ligand binds
to bind to adenylate cyclase.
Adenylate cyclase is an enzyme.
It will convert ATP
into cyclic AMP,
and then cyclic AMP will start a
cascade of protein phosphorylation
to other proteins such
as protein kinase A.
This type of transduction
of the signal is important.
It allows for many things
anywhere from amplification
to specificity of the
particular cell response.
This is another example of a
G protein-coupled receptor.
Here, when the ligand binds to the
receptor, it activates G proteins.
In this case, the G protein
that’s activated is a G alpha Q.
G alpha Qs then migrate over
to activate phospholipase C,
and phospholipase C
converts a molecule PIP2
into two things that are going to
be inactive throughout the cell,
and that is DAG and IP3.
IP3 then can move throughout
the cell and do other tasks
such as bind to IP3
receptors within the cell
and release substances from
the endoplasmic reticulum.
The DAG molecule can
activate other proteins
and phosphorylate them
such as protein kinase C.
So you can see here how you can
have a multitude of responses
simply by signaling one enzyme,
a lot of different things
can happen within the cell.
Other receptors usually just move in ions
and these are ion channel-linked receptors.
You might have the ligand bind to
a specific part of the channel
and that then allows for the
ions to travel through.
So it might be closed initially,
the ligand binds, and it opens up.
Another example is you may
need to have multiple
ligands bind to a particular
receptor to have it open up.
The example that we’re looking at here
is an acetylcholine nicotinic receptor
in which you have a neurotransmitter,
acetylcholine in this case,
binding to this acetylcholine receptor
to allow for ions to travel through.
Not all receptors though are
located on the cell surface.
If your signaling molecule is a
steroid hormone or another lipophilic
type of signaling molecule, it can sometimes
make it through the cell membrane.
In this case, the receptor is
now located in the cytosol.
Other times, you might even have a receptor
that’s located in the nucleus itself.
But in this case, we have the steroid
hormone binding to a cytosolic receptor,
this is then translocated into the nucleus,
and will bind to a response element.
You’ll get an initial response in which
you will then produce various proteins.
And as you produce proteins, then, you
will have a response via a nuclear signal.
It’s interesting though when you look
at cytosolic and nuclear receptors
that you can break them down into their
quick effects, which are their primary effects,
and then their secondary effects
which are more long-lasting
because these involve making a new
protein or making a new channel.
And so, sometimes you’ll
have quick effects
and then you’ll have secondary
or longer-term effects.
The last type of cell-to-cell communication
that we need to deal with here
doesn’t involve a ligand and that is you
can have direct contact between two cells
and usually it’s sharing
electricity or a voltage change.
These kind of voltage
changes allow for then
tissues to work in
coordination with each other.
So a good example of that
are smooth muscle cells.
You need to have smooth
muscle cells contract
in a coordinated manner to
do useful work for you.
So to summarize this
remember that you might have various
receptors to sense external environments,
such as light and sound, but you
also need to have the cells
within the body be able to communicate
with each other and respond to ligands
or voltages so that
there can be an active
and coordinated response to whatever
is necessary in the environment.