Okay. So why don’t we start moving
on to the endocrine system.
This is a really unique system
that has wide ranging effects.
Now, what are some of the
components of an endocrine system?
But first off is that it’s a
collection of ductless glands
which release hormones directly
into the circulatory system.
So it’s got this
which is what sets it
apart from some of the
other systems that we’ve
talked about so far.
So the effects are slow to initiate,
but they’re prolonged
in their response
and they activate distant targets
lasting a long period of time.
So as opposed to the quickness and
the agility that was mentioned
with our central nervous system
and the peripheral nervous system
and that we detect a sensory stimulus
and we respond very quickly,
the idea here is we don’t always
need these superfast signals.
Sometimes we need something
that’s low and slow
and this is exactly the
system that does that.
There are effects at several
organs at the same time,
there are things that can last
for a long period of time,
and the idea is the signals
actually originated from the brain,
but then they impact sort
of the rest of your body.
So, that ability to communicate throughout
is really, really useful and necessary.
So some of the endocrine glands that
are important that you should know
include, I’m just going to highlight some of
them and we can’t go through all of them,
but things like the pineal
gland, the pituitary gland,
the pancreas, the ovaries,
testes, thyroid gland,
we have the parathyroid gland, the
hypothalamus, the GI tract, adrenal glands.
So you can see it’s
and there’s a lot of components
involved and they’re all related.
But we’re going to highlight
on a few of these, okay?
So the hypothalamus is the primary
coordinator of the endocrine system
and it activates the sort of the star of
the show, which is the pituitary gland.
And the pituitary gland secretes hormones
and neurotransmitters for growth,
deals with reproduction
and mental development.
So it’s causing the release
of these transmitters
that do a lot of really
and it’s also known as the hypophysis.
Now, if you look at this diagram you can
notice its location and notice its size.
It’s really, really small.
It’s not a big structure but it
has really wide ranging effects.
The thyroid is another one
that we want to talk about
and this is located at the front of a neck
and it regulates metabolism end energy use.
So you may have heard
people mention before,
“Oh, I’m slightly overweight
because I have a thyroid problem.”
That’s because as we just said right now
it’s part of its function is to
regulate metabolism and energy use,
so if there are deficiencies
or problems there,
it can impact your ability
to metabolize food
and the amount of energy
that you end up using.
The parathyroid is located
behind the thyroid
and it regulates calcium levels in
blood and it impacts bone development.
So we should remember that parathyroid
equals calcium in the blood
and bone development, but that’s of
importance a little bit later on.
Now, let’s take a deeper
look at the adrenal glands
and there are different
components of it.
So the adrenal gland as a whole
releases epinephrine and adrenaline
which we’ve talked about pretty
much almost in every lecture,
which activates our fight
or flight response
and many other regulatory
hormones such as cortisol
which is important for during
the fight or flight response,
it’s important in terms of sleep,
aldosterone, and other sex hormones.
The pancreas releases
insulin and glucagon
which is what allows us to
modulate our levels of blood sugar
and deficiencies in insulin where this
can lead to things like diabetes,
so extremely relevant.
And we have reproductive glands,
which are testes and ovaries,
and they release good old sex hormones,
so also very, very important for
things like the initiation of --
sorry, the expression of puberty and
as part of modulating our sex drive.
So speaking of hormones let’s
get into different class --
two different classes
So the name is linked and
based upon their felicity
or their water-loving/water-hating
So the first class is hydrophilic, which
is peptides and amino-acid derivatives,
and we have hydrophobic,
which is steroid hormones.
And if you want to try to
remember which is what,
you know people have phobias,
meaning they’re afraid of things.
Well, hydrophobic means
afraid of water, loosely put,
so it means the opposite of
wanting to be around water,
So we’re referring to sort
of their water solubility,
and the hydrophilic ones are little more
water soluble versus the hydrophobic,
which are not.
So peptide hormones, they bind
to receptors on cell surface
and they’re stored in vesicles until they’re
needed, a lot like the transmitters,
and their action is more
rapid and short-lived.
For an example, insulin,
they communicate with the interior
of the target of the cell
by way of secondary messenger.
So it’s a cascade effect.
So as opposed to a transmitter binding to
a receptor site, that would be very fast,
that’s called ligand-gated,
and ligand refers to the transmitter,
and secondary messengers or
things using G-coupled proteins
are a little bit slower and
here’s an example of that.
It’s composed of small to
large amino acid chains
and these are collectively
referred to as polypeptide.
So the poly refers to more than one,
and peptide is referring to the fact that
we’re talking about amino acids here.
Steroid hormones are
synthesized from cholesterol
only when they’re needed and can
pass pass through a membrane.
And part of the reason why they’re
able to pass through membrane
is again looking at
their lipid solubility.
So the more lipid soluble
something is the
easier it is for it to
move through membranes
because membranes are primarily
composed of phospholipids.
Steroid hormones bind to the cytoplasmic
receptor and they enter the nucleus
and then go on to alter gene expression
by regulating DNA transcription.
So that’s a lot of stuff
that we just threw out there
and were involving some
biology and genetics,
but what we’re saying simply is
that steroid hormones have this
ability to have long-lasting effects
in terms of gene expression
and regulating DNA transcription which can then
go on to actually impact protein expression.
So the idea is that these
hormones can have these
changes and effects.
So the effects are slower to
initiate but are longer-lasting.
So that’s a key point to remember
when we’re talking about hormones
and when we’re talking about the
endocrine system as a whole.
Tyrosine derivatives include
two classes of hormones
including the thyroid hormone
and also catecholamines.
So the thyroid hormone is produced
in the thyroid, thus, the name,
and increases overall metabolic
rate and body temperature.
And in children, it
actually stimulates growth
and the two that you want to
be aware of are T3 and T4.
Catecholamines include a lot of
things we’ve already talked about,
the transmitters including
epinephrine and norepinephrine,
adrenaline and noradrenaline
is another name for them.
So let’s take a look at the autocrine versus
paracrine versus the endocrine systems
and this something that might
also come up in your MCAT
because they’re going to want you to
understand and appreciate the differences
and a good to remember
is the names.
So the endocrine you should
know because we talked about it
and we said that
it’s low and slow.
The autocrine system and the autocrine
signaling is a form of self-signaling in which
the cell secretes a hormone or chemical
messenger called an autocrine agent
that binds to an autocrine
receptor that is on the same cell.
So it leads to changes
in that specific cell.
So almost like an autoreceptor.
So it’s causing impact upon itself.
So it’s sort of proximity
of effect is pretty small,
it’s just a cell that
it’s communicating with.
Then you have the paracrine signaling which
is a form of cell-to-cell communication
in which a cell produces a signal
to induce changes in nearby cells
altering the behavior of or
differentiation of those cells.
So it’s cell-to-cell communication.
And endocrine is that broader,
longer-lasting, more diffuse activation.
So how do we control or stop or modulate
secretion of these different things?
Well, it goes through a process,
using the HPA axis is a great example of
looking at all the different components
of the endocrine system and how they
actually feedback upon themselves
and this a primary example of a
process called negative feedback.
And we’ve mentioned this in a couple of
the other modules so this might be repeat,
but I think it’s really important
that we still look at it again.
This is collectively
known as the HPA axis.
So we start with the hypothalamus.
The hypothalamus releases something called
CRH or corticotropin-releasing hormone,
and then it goes on to activate
the anterior pituitary gland,
which goes on to release
adrenocorticotropic hormone or ACTH,
and that goes on to activate
the adrenal cortex,
and the adrenal cortex
And cortisol levels are
detected and what happens is
when we achieve the appropriate
level of cortisol,
there are receptors that detect that and
send the signal back to the hypothalamus
saying “You know what, I don’t
think we need any more CRH because
we have enough cortisol,”
and it sends a similar signal
to anterior pituitary to say,
“We got enough ACTH because
we don’t need any more ACTH
because we have enough cortisol.”
So we call that overall process
of aligned going positive,
release, release, release, and
then sending a signal back saying,
“I think we’re good we have
enough,” as negative feedback
because we’re causing release positive,
causing release that’s positive,
causing release that’s positive, and
then we’re needing to turn that off
and that’s the negative off
signal or the negative feedback.
Okay. Let’s take a look at the
endocrine system on behavior.
We know that both the nervous
system which is quite fast
and the endocrine system
which is low and slow
respond to both internal
and external stimulus.
So it’s not that the nervous system
is the only way we see a response,
the endocrine system is involved
and it’s like I said usually
longer-lasting and long-term responses.
So the short-term and long-term response
can stimulate both direct effects,
so we need the fast stuff
which is the nervous system,
the slow stuff which is
the endocrine system,
collectively together will impact behavior,
the behavioral response and can also
impact behavioral modification.
Now, the release of adrenaline
is an important point as well,
and that’s done by the adrenal medulla
which is activated by the hypothalamus.
And we know that processes like
behavioral modification or CBT,
cognitive behavioral therapy,
employ changes in this process
in order to modify our behavior.
So that’s a really,
really cool thing.
So you’re actually using your ability
to cognitively shift changes
in activation in order
to impact your behavior.
So here we’re seeing a
link between physiology,
psychology, and behavior pretty
much the scope of this course.