>>Dr. Ketchum: We are now going to begin the part two of the chemical messengers. And so
with this we’re going to be talking about the functional classification of the chemical
messengers—classifying our chemical messengers into two different categories. We can classify
them based on their functional property. So in other words, how they operate, how do they
work? And we can classify them into their chemical composition as well. So this lecture
is specifically going to be focusing only on the functional classification. Once this
is complete, then we’ll move into the chemical classification.
So this is just so that you can look at your outline to get the big picture of where we’re
going. This is our road map, if you will. So the goal now is to go through the functional
classification. Again, these are going to be types of indirect communication, looking
at how our chemical messengers can function as a paracrine messenger, a neurotransmitter
messenger, a hormone or neurohormone. With the functional classification, there’s always
the question of “Well, what do I need to know?” And so when we go through each of
these functional classifications, it’s going to be very important first off, that you realize
is this a type of direct or indirect communication? Hopefully you’ll already be able to guess
that since our secretory cell and our target cells are not physically connected in any
of these types. The other thing you need to know is is there are a specific name for the
messenger? So in some instances, I’ll be giving you specific names for messengers. You’ll
also need to know specific names for target cells if there are any, which there will be
in some cases, and even the specific name for the secretory cell as well. We want to
know well where do the messengers go? Do they only travel through the interstitial fluid?
Do they only travel through the blood, or do they have to travel through both? And do
the secretory cells and target cells need to be nearby or can they be far away from
one another? In other words, is this a short-distance communication or this a long-distance communication?
You should also know some examples of chemical messengers that operate as paracrine transmitters,
as neurotransmitters, as hormones, or even as neurohormones. And then the last thing
is do all these messengers actually require a receptor? A very important concept for you
to think about. All right, so this is the basics of your—of your indirect or intercellular
communication. So everything we’re going to be talking about from this moment on is
indirect, are indirect forms of communication. So again, that means that the secretory cell
and the target cell are not physically connected. And so the big picture is that we have a secretory
cell, and that secretory cell secretes the ligand—which if you recall, we’re going
to be referring to it as a chemical messenger. So the ligand then has to travel through the
interstitial fluid, and I’ll abbreviate that “IF.” And/or, so it can only go through
the interstitial fluid or it may travel through the interstitial fluid and the blood. Certain
ligands only travel through the interstitial fluid; other ligands have to travel through
the interstitial fluid then enter the blood to reach the target cell. Now this is where
the ligand must bind to the receptor. That’s one receptor, and if you notice, that receptor’s
on the plasma membrane. I have another receptor here and if you notice, that’s in the cytoplasm,
or it could be in the nucleoplasm. So when the ligand binds, it comes in and it binds
to the receptor on the plasma membrane or in the target cell. And by “in the target
cell,” it could be in the cytoplasm or it could be in the nucleoplasm.
So when the ligand binds to the receptor, this is going to bring about what’s called
the target cell response. And so this is the big picture—what we’re going to be doing
after this functional classification is looking at specifically how does this happen? Okay,
so let’s go through each of the types of functional classification. So we’re going
to begin with paracrine signals. So if you have a chemical or a ligand and that’s functioning
as a paracrine signal, your secretory cell and your target cell are nearby, but not physically
connected. The secretory cell has to synthesize and secrete my paracrine ligand, which could
be, for example, histamine or growth factor or clotting factor or cytokine. That paracrine
signal has to travel through the interstitial fluid, then it has to bind to the receptor,
and then we get a target cell response. And again, remember that the target cell is nearby
the secretory cell, but they’re not physically connected. Next we have the neurotransmitters—so
some ligands function as neurotransmitters. So our secretory cell is called a presynaptic
neuron. Our target cell is called a postsynaptic cell, and this postsynaptic cell could be
a postsynaptic neuron, for example, or this postsynaptic cell could be an effector. Effectors
are muscle and glands. Our ligand is the neurotransmitter. So these are the components. Now the
secretory cell and the target cell are usually nearby, but here’s the catch. Sometimes you
can have a presynaptic neuron that’s over a meter long. So it’s a meter long, and
then you have your target cell. So in that particular case, your communication would
be long distance. But for the most part, it’s going to be nearby. All right, so let’s
look at the flow of things here. So we start off at the top here, because we know our secretory
cell will synthesize and secretes our neurotransmitter, which is our ligand. So then the ligand—
once it’s secreted, it has to travel across what we’re going to call the synapse. And in
this case, the synapse is interstitial fluid. So once that neurotransmitter travels through
the interstitial fluid or what we call the synapse, then the neurotransmitter has to
bind to the receptor. And once it binds to the receptor, we get a target cell response.
So you see there are some common themes here: the secretory cell is always synthesizes
and secreting the ligand, the ligand is either travels through the interstitial fluid and the blood
or only the interstitial fluid, the ligand binds to a receptor, and you get a target
cell response.
The next type of functional classification is when our ligands or our chemical messengers
function as hormones. So here, our secretory cell is called an endocrine cell. Any time
you hear the term endocrine, think hormone. Our target cell—there are numerous, numerous,
numerous examples of target cells—our ligand is going to be the hormone, and the hormone
must travel through the blood in order to get to its target cell. So because the hormone
has to travel through the blood, the target cell is far away from the secretory cell.
We start off with the secretory cell that’s going to synthesize and secrete the hormone.
Once the hormone gets secreted, the hormone is going to travel through the interstitial
fluid, then the hormone is going to travel through the blood, then the hormone is going
to bind the receptor. Remember, the hormone is the ligand. Once the hormone binds to the
the receptor, once again, you get your target cell response. Again, you’re going to have
numerous examples of this when we get in to the endocrine system.
And then we have the neurohormones. The secretory cell is called a neuron, or you can also call
it a neurosecretory cell. Our ligand is the neurohormone. The reason we’re calling it
a neurohormone is that it’s a neuron that makes the hormone that synthesizes and secretes
the hormone. This neurohormone has to travel through the interstitial fluid as well as
the blood. And then lastly, we have our target cell. Again, there are numerous examples for
a target cell. So we’re starting off with our secretory cell, our neurosecretory cell
or neuron. So it has to synthesize and secrete the neurohormone—I’ll just abbreviate
“NH”—which has to travel through the interstitial fluid. Then it has to travel
through the blood, then your neurohormone has to bind to the receptor, and lastly, we get
a target cell response. So what that’s done for us then is it’s taken us through how
different ligands function. And so based on that function, we classify them as either
a paracrine ligand, as a neurotransmitter, as hormone or as a neurohormone. So that concludes
part two of the intercellular communication of chemical messengers, and that concludes
the functional classification.