Hey it's professor Dave, let's learn about how cells communicate with one another.
We've learned about a lot of the complex biochemistry that occurs
inside every single one of your cells, but it actually gets even more
complicated than that, because your cells need to communicate with one another in
order to respond to environmental stimuli like potential danger and to
elicit certain behavior during various stages in your lifetime so that your
body makes the correct changes as you grow. How does this communication take
place? Well we learned about receptors that sit in the cell membrane and wait
for a particular substrate to bind. Many of these receptor substrates are
signaling molecules like hormones and neurotransmitters, which upon binding to
the receptor set off the process of signal transduction, which can have a
variety of results. Some of this signaling is local, occurring between
adjacent cells, but long-distance signaling can also occur, where a message
travels all the way across your body to deliver a signal to a particular type of
cell. In examining these signaling pathways we want to talk about three
types of signaling. Those are autocrine, paracrine, and endocrine. The prefix auto
means "self", so autocrine signaling refers to when the cells secrete signaling
molecules which then bind to receptors on that same cell.
This will trigger some kind of response from the cell. The prefix "para" means
beside or nearby, so paracrine signaling is a type of local signaling where a
compound like a growth factor is secreted by a cell which then interacts
with nearby cells. Growth factors are signals that tell a cell to begin dividing
which is what allows us to become so much taller during intense periods of
growth in childhood. Another example of local signaling is called synaptic
signaling. This is how messages travel through your nervous system. A nerve cell
can be triggered by an electrical signal
to release certain molecules called neurotransmitters into the synaptic
space, which then interact with the next nerve cell, eliciting another electrical
signal which releases neurotransmitters into the next synaptic space, and so
forth. A nerve cell can also be attached to a muscle cell which makes it able to
trigger muscular contraction. This kind of signaling is used when your hand
needs to tell your brain that something it is touching is hot, which will then send
another signal back down to your hand to tell it to stop touching it.
An incredible amount of chemistry has to happen for all those signals to be
transmitted, but lucky for us
chemistry happens imperceptibly quickly. And lastly, the long-distance signaling
that happens in our bodies occurs via endocrine signaling. This is when a
particular type of compound called a hormone is released by a gland and is
then carried through the bloodstream to its destination.
This system has a range of functions like maintaining blood pressure, the
regulation of development, and more. So what are the names of some of these
hormones and neurotransmitters and what exactly are the messages they carry?
Many of the signaling molecules fall into three structural categories: polypeptides
like oxytocin, steroids like cortisol, and amines like epinephrine. We've already
learned about the properties of these types of molecules so let's talk about
how they interact with the body. First let's discuss their variance in
solubility. Polypeptides and amines are water-soluble whereas steroids are not.
But nonpolar molecules like steroids are lipid soluble, so this discrepancy will
affect the way these molecules are transmitted. Water-soluble hormones can
travel through the aqueous bloodstream freely but they can't pass through the
nonpolar plasma membrane of a cell so these will typically be recognized by
receptors on the surface of the cell, which upon binding will initiate signal
transduction. For example, when an organism finds itself in a particularly
stressful situation, like evading a predator, the adrenal glands secrete a
hormone called epinephrine. This is more commonly known as adrenaline. When this
molecule reaches liver cells it binds to a membrane receptor. This sets off a
cascade of events which generates cAMP, which in turn activates an enzyme
called protein kinase A. This enzyme will inhibit glycogen synthesis and promote
glycogen breakdown.
This means that the cell will stop storing glucose in the form of glycogen
and will instead start breaking up glycogen to make more glucose molecules,
which will enter the bloodstream and become available for cellular
respiration. Essentially it kicks energy production into overdrive so that the
organism can get away from the predator. By contrast, lipid-soluble hormones will
have to bind to transport proteins in order to be soluble in the bloodstream
but once they reach a cell they are typically able to pass right through the
cell membrane, being nonpolar, so the receptors that interact with these types
of hormones are usually already inside the cell. For example, in many vertebrates
like birds and frogs, estradiol is a hormone that regulates female
reproductive function. This can enter a liver cell and bind to a receptor in the
cytoplasm. This complex will then undergo a conformational change due to the
binding that allows it to enter the nucleus, bind to a specific DNA sequence
and transcribe the gene for vitellogenin. Once produced, this protein is then
transported to the reproductive system to produce egg yolk. Some lipid-soluble
hormones pass through both the cell membrane and nuclear membrane to bind to
a nuclear receptor. Again, nuclear receptors are typically transcription
factors which once activated, will initiate the expression of a particular
gene. Some hormones elicit multiple responses simultaneously.
Let's look at epinephrine again. Not only will this promote glycogen breakdown but
it also increases blood flow to skeletal muscles and decreases blood flow to the
digestive tract.
How can it do all these things at once? It's partially because different cells
might contain different enzymes than others and therefore elicit a different
cellular response to the same hormone. But also some hormones are able to
activate several completely different receptors each with their own unique
response to binding. Here we can see two different receptors called alpha and
beta receptors existing on three different cell types, as well as each
unique result, whether increase of blood glucose, blood vessel dilation, or
blood vessel constriction. As we said, all hormones are secreted from glands.
Here are just some of the glands in your body, including the hypothalamus, thyroid,
pineal, and pituitary glands, as well as the hormones they mediate. Many of these
may sound familiar and they have a wide variety of functionality, including the
regulation of biological rhythms like your sleep cycle as well as growth,
metabolic functions, and social behavior. There are far too many to go through
them individually but it is more important that we understand the general
mechanisms by which they operate so that we can apply this understanding to any
individual example. And most importantly, we must understand how the endocrine
system and the nervous system work together to play a critical role in
organizing all the cellular activity of an organism to contribute towards a
common goal: the survival of the organism.
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