Human Physiology – Functional Classification

Human Physiology – Functional Classification


>>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.

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