>>Dr. Ketchum: Okay, our next discussion is going to be on chemical messengers. And so
what we’re going to be doing next is we’re going to be looking at intercellular communication.
So in other words, how do cells communicate with one another? So here is cell one,
and here is cell two. How do these two cells talk to each other? We just remember inter means
“between,” so this is communication between two cells.
If our cells do not communicate to one another, you would not be able to listen to somebody
when they spoke to you. You wouldn’t be able to take lecture notes, you basically
wouldn’t be able to do anything, because cellular communication is one of the necessary
or fundamental things for survival. And so cells have to communicate just like people
have to communicate. So there are a couple different types of communication. There’s
what we call direct communication and indirect communication, and in a moment I’ll be discussing
those and comparing those for you.
And then we have functional classification and chemical classification of our chemical
messengers. So basically, what a chemical messenger is—a chemical messenger is the
same thing as a ligand. And this is what we use to communicate between one cell and another
cell. You have to have a chemical messenger to do that. One cell will produce that ligand,
the other cell will respond to that ligand. Okay, so in addition to those ligands, we
need to know how are they synthesized and how do they get released from a secretory
cell? We need to know how the chemical messenger gets transported, and what that target cell
interaction is. So in other words, once that chemical messenger reaches a target cell,
how does the target cell respond?
So here is our chapter outline. And what we’re going to be doing is breaking these chemical
messengers, and remember chemical messengers are called ligands, we’re going to break
them down into two different means by which we classify them. We classify them based on
their function. So in other words, the function has a paracrine signaller. Do they function
as a neurotransmitter or do they function as a hormone, or does that chemical messenger
function as a neurohormone? Then what we’re going to do is we’re going to look at their
chemical classification. So by chemical classification, we’re going to take these chemical messengers
and we’re going to divide them up into different groups or into different chemical classes.
So we’ll divide them up into eicosanoids, amino acids, amines, peptides and proteins,
and steroids.
So if you understand the chemical property for each of these chemical classes, then you
will understand how that chemical messenger is synthesized and released or secreted from
a secretory cell, which will allow you to understand how that chemical messenger gets
transported to the target cell. And it will help you understand where the receptor will
be located with respect to the target cell. And finally, knowing the chemical properties
can allow you to understand how the target cell responds. So just by knowing the chemical
property, you’re going to figure out all of these other entities that are part of intercellular
communication.
So to give you a larger picture of what this would like—two cells. So here’s my secretory
cell; here is my target cell. So we have these two different cells, and these two cells have
to be able to communicate to one another. So the secretory cell has to synthesize and
secrete the ligand. Remember, the ligand is also the chemical messenger. Once that occurs,
then the ligand has to be transported to the target cell. And once the ligand gets to the
target cell and binds to the receptor, then the target cell will respond. So if you understand
the chemical property, meaning is the chemical messenger lipophobic or lipophilic, you will
be able to determine if the secretory cell—how the secretory cell synthesizes and secretes
that ligand, how that ligand gets transported, and lastly, how that target cell will respond
to that ligand.
So to begin with intercellular communication, what we need to do is first off, for you to
understand the differences between direct communication and indirect communication.
So in direct communication, your cells are physically connected. And they’re physically
connected via structure that you are already fondly familiar with, and those are gap junctions.
So be sure that you remember what a gap junction is. A gap junction is like a drinking straw—it
connects these two cells, these two cells have to communicate. So they’re communicating
through this gap junction. So whatever solute needs to pass through to get to the other
cell in order for them to communicate, so that solute, whatever it is, has to travel
through the gap junction. Remember that the gap junction is composed of two different
types of proteins: the connexon and the connexin. The entire thing is called the connexin. Each
cell—cellular unit of a gap junction is called a connexon. So in direct communication,
these two cells are physically connected.
If it’s indirect communication—indirect communication, now the two cells are not physically
connected. So what that means is that your ligand, or your chemical messenger, whichever you
prefer to call it. The ligand has to be transported to the target cell. That target cell may be
close by the secretory cell, but not physically connected, or the target cell may be farther
away. Again, just to give you the overview here, here is the secretory cell. It’s going
to synthesize and secrete your ligand. The ligand—once it gets secreted, right—has
to be transported, and then it has to reach the target cell. When it reaches the target
cell it binds to the receptor, and once it binds to the receptor, you get some sort of
response. So that is the big picture of what indirect communication is. Those two cells
are not physically connected.
All right, so then when we move into functional versus chemical classification, so how do
we classify this messenger—this messenger that has to be secreted from a secretory cell
and has to go to a target cell? So we have two different ways we can do this. We can
use what’s called the functional classification. Like I said earlier, it’s how does the chemical
messenger function? Does it function as a paracrine signaller, as a neurotransmitter,
as a hormone, or as a neurohormone? We’ll be discussing each of these shortly. Then,
we have the chemical classification. And again, if you know the chemical property of your
ligand, you will understand and things will be so much easier for you if you understand
that. So just a quick review: If my ligand is lipophilic, it is hydrophobic. And if my
ligand is lipophobic, it is hydrophilic. So we have various classes of chemical messengers—so
here are the various classes. They include amino acids, amines, peptides and proteins,
steroids, and eicosanoids. So this concludes the overview of the intercellular communication
chapter.