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Lecture 3

BIO270H1 Lecture Notes - Lecture 3: Optic Chiasm, Annelid, Thyrotropin-Releasing Hormone

Course Code
David Lovejoy

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Wednesday, November 4, 2009
BIO270 Lecture 3
- Welcome to Lecture 3. We have a few announcements before starting.
He is reminding us of wacky Wednesday it’s a midterm break. The
other comment about the final exam. The final exam is essentially put
together – there will be 55 questions – you will have 2 hours to do it. 51
of those questions will be his, 4 will be from Dr. Forder. & we’re just
trying to get us more info for what to study. Are there any lab questions
on the final? Hmm he never thought of that, it’s a great idea. *Shouting
in background*. He’s kidding, traditionally the lectures & labs are kept
separate so no lab material is on the final exam he understands we’re
all doing well on labs anyways.
- In the last couple of lectures we’ve been talking about all the
molecular components of how the endocrine system works so we should
all have a pretty good sense of what ligands are, receptors, signal
transduction pathways, different types of mechanisms. Also some of the
variability within that system. Remember all of these things are
controlled by genes every one of them is slightly different, genes
occur in families, receptors, ligands all recur in different families. Now
because we know that hormones & signalling molecules are what create
what we are, & dictate how we’re going to act under certain
circumstances to a certain degree, we can get a sense of individual
differences amongst us.
- Next time we’re at a party & we see some unusual behaviour we can
say ah hah 5HT, I know what’s going on with you, whoa sex steroid
- You should have a sense of molecular variabilities you know the
molecular components so you can practice amongst yourselves about
how these things differ & what he’d like to do for the first part of this
class is talk about how these circuits go together. We think of
neurological circuits & we know a little bit about the basics of that.
Endocrine circuits work the same way except they work over a great
distance. He’d like to go through some of the basics of what an
endocrine circuit is.
- He talked to some of us about our lab assignment & he understands
that we’re talking about reproductive cycles on that, so we’ll be delving
into the basics of that today so we get a sense of how these things work.
- Here are our readings. He doesn’t know what it is, but whenever we
get a textbook on this, it’s always scattered all over the place & we have
to look at different choices so it’s divided up into 3 chapters. The
amount of pages there is not too bad, but we’ll see where we are.
- Chapter 3 is what we’ve been working on & just really read the
overview here & up to regulation of glucose metabolism we’re not
covering glucose metabolism in this lecture so you don’t need to do
- Chapter 4, we need to know a little about neuronal structure &
function because a great part of an endocrine circuit is a neuroendocrine
component so we need to know a little bit about the brain & a little bit
about the functional organization of the nervous system particularly the
peripheral nervous system as well. There is a fair amount of material
there but we’ll get a sense of what’s important after we go through this

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- Now we’ve talked about the integration of signals from a cellular point
of view. When we look at it from an organismal point of view, we find
that the analogous systems are still there but they’re somewhat further
apart from each other & a little bit more complex, but the same idea
- Now as he mentioned, cell signalling is important for all physiological
processes – if cells couldn’t signal we’d never have come together as an
organism. In fact the evolution of cell signalling was probably one of
the first steps that had to occur before multicellular organisms came
together & even evolved a very important component of it.
- Now there are three components of these biological control systems
which is what our textbook calls it.
- The sensor system detects the level of the regulated variable so there is
perception that some things changes & it sends the signal to the
integrating center. For example, he sees something, it impinges upon his
retina, & that signal goes into integrative centers of the brain. There is
an integrating centre here that takes all that info from the sensor & other
sensors, compares it to past info, compares it on the physiological state
of the organ in question & on the basis of that, makes a decision &
sends a signal to the effector & the effector is the target tissue that
responds to the signal from the integrating signal.
- Every system that we look at will have these three components it
won’t necessarily be a single organ system, it may be a multi-organ
system or a very complex set of tissues that are involved in this but
those three components will always be present.
- There are other things to think about.
- There is a set point. One of the themes of this course is homeostasis,
that is, that we have some sort of physiological limits within us that if
we go above or below those limits, it can be injurious to the body. There
is usually some sort of standard point that every organism has it
differs from organism to organism, & even from person to person to
person there will be some slight differences. Some of us might be very
sensitive to stress, for example, some of us might be very resilient to
stress. There will be some sort of value of this variable that the body
will try to get to – if it’s too high it’ll try & bring it down; if it’s too low
then it will try to bring it up.
- A key point for any type of circuit & particular an endocrine circuit are
the concept of feedback loops. We talked a little bit about feedback
loops when we talked about autocrine & paracrine signalling. The
autocrine component was a feedback back to the cell to tell it how much
of the signal was released if there wasn’t enough, then release some
more & if there’s too much, then shut it off. It works the same way
amongst these any kind of organism & any kind of endocrine pathway.
- In here, the output of the effector amplifies variable away from the set
point – what does that mean? Well he’ll give us examples of that.
- Positive feedback loops are not common in physiological systems.
He’s mentioning this b/c the book mentions this & he doesn’t agree with
that. He thinks that you would find that for every physiological system
there is a feedback loop maybe we don’t define it in terms of an
endocrine system b/c maybe we haven’t described those particular
molecules as endocrine molecules yet, they are maybe not as well
defined in a physiological system, but they are there so just bear that in
mind. That is the type of question he won’t test us on. The book says
this but he thinks something else, what do we think?

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- So there is a positive feedback, & what they’re trying to say in a
simple language is, when you turn on the hormone, it hits the effector,
the effector sends another molecule back to send more of the effector. It
just comes back & says give me more, I love it. The negative feedback
loop is just the opposite, it goes back & says enough is enough, turn it
off. Positive feedback is give me more; negative feedback is turn it off.
Every system will have a combination of positive & negative feedback
loops & many times will be loops within loops the more you start
looking the more loops you can find.
- Let’s take some simple examples. Here we have our endocrine gland –
it secretes into the bloodstream, it goes to the target organ, it causes the
release of something & it has a negative feedback & this is what we call
a direct feedback loop.
- By the same token it could be positive too but you get a sense for how
it works. One organ here, one organ there & then the feedback & there
could be an integrating system up there, is what they’re referring to.
- Okay let’s make it a bit more complex.
- Here we have a sense organ he’ll say the olfactory system. Right
here, the sensory neuron, integrating center. He smells something, hey
what does that mean?
- When he was an undergrad, he was on the bus & he had this girlfriend
that used to wear this musk perfume, wild musk & he loved it & well
come on, 17, 18, you know. Sitting on the bus, he smelled exactly the
same stuff, & he thought wow this was fantastic so he turned around
very excitedly to see who was sitting beside me & it was this little old
lady about 80 years old wearing that stuff he thought that should’ve
been illegal. This is an example of how a scent will bring back
memories, a lot of positive memories. Incidentally that ex-girlfriend of
his lives in Toronto & this took place in Winnipeg & everything follows
you, you just can’t escape.
- Then there will be the neuron here & that causes a second effect of the
target organ then you get a response & it feeds right back so there are a
number of stages in there.
- He thinks the book calls this the first order feedback loop don’t you
love the way these things get so complex?
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