Cell Communication: Hormones and Neurotransmitters


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. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me:

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