A short lecture describing the basic structure of a feedback loop and clearly defining its parts.
So the body uses feedback, producing
changes in response to changes to react
to its environment. It uses positive
feedback to drive processes to
completion; it uses negative feedback to
maintain homeostasis, which is its
internal balance or equilibrium. The
real question now is how does the body
use feedback? What's the mechanism by
which it uses feedback? Let's make this a
bit easier to think about. Let's start
with something that's not the human body.
Let's start by thinking about a house. I
want you to imagine a house on a warm
summer's day. A warm summer's day in
which radiation from the sun is heating
up the atmosphere, and in which the
atmosphere heating up is transferring
its heat into the house and causing the
air within the house to increase in
temperature. We have here a very simple
example of an external environmental
change — a warmer atmosphere outside the
house — causing an internal change — a
warmer atmosphere inside the house. And
the question then is, what do we need to
maintain constant temperature within the
house? Literally. I'm not asking this in a
figurative sense, I'm saying physically,
what do we have to have? Well, let's think
about this. We have to have a way of
measuring air temperature in the house.
If we can't detect what the temperature is,
we can't determine that it's changing.
And if we don't know that it's changing,
we can't respond to that change. We also
have to have something to produce the
feedback, right? We have to have an
effector of some kind — an object or
machine to change the air temperature.
And finally, we have to have some sort of
decision-maker who determines when the
temperature has changed enough to
justify activating the device that
changes temperature. So to stop talking
about this in the abstract, let's put
some actual structures to this. We need a
thermometer somewhere in the house — a
device that measures changes in
temperature which will be the receptor.
It will detect change in temperature. We
need an air conditioner. We need a
device capable of changing temperature
in the house; a device that can have an
effect on temperature, or an effector. And then finally we need something
that can
determine and maintain the set-point. We
need a decision-maker as a go-between — a
thermostat. And that thermostat is our
control or integration center. So the
way this works, is temperature outside
rises, so temperature inside rises. That
increase in air temperature is detected
by the receptor — the thermometer — which
communicates with the thermostat — the
control center — which realizes
temperature has now exceeded set-point by
a good bit. It then communicates with the
effector — the air-conditioner — which turns
on, lowering temperature in the house. And
there is our feedback. Now notice as I've
drawn this we have this lovely little
circular diagram on the page. We have
what's called a feedback loop — a
collection of structures, or a single
structure, that produces
feedback in response to an environmental
change. And these feedback loops have to
have three parts as we just saw: a
receptor, a control center, and an
effector. So to define those terms
a little more formally, the receptor is
the cell or organ in the human body — now
we're moving away from the house. It's a
cell or organ that detects changes in a
particular physiological variable and
when you're encountering real feedback
loops within the human body, you can
recognize the receptor as a
structure that detects a change, is
sensitive to a change, or in a lot of
cases, it's the structure whose name has
the word receptor in it. Now you can't
rely on that all the time, but a very
large number of the body's receptors do
have the word receptor in their name. So
don't overlook that. The control center
is the decision-maker in the feedback
loop. It's the cell or organ that decides
when to act in response to the change.
You're going to recognize control
centers sometimes because they're
obviously behaving in a way that
indicates their decision maker. You can
expect many but not all control centers
to be in the central nervous system. A
lot of times you recognize it most
easily by realizing it's the structure
that is in communication with both the
receptor and the effector. And the
effector again is the cell or organ that
produces the effect that changes the
physiological variable at the output
of the system. It's always a muscle or a
gland, and so the effector will be
recognized because it's smooth muscle
tissue, cardiac muscle tissue, skeletal
muscle tissue, or it's a gland of some kind.
And most importantly, the action of that
organ leads to a change in the
physiological variable that changed at
the beginning of all of this. So those
are the parts of the feedback loop. What
I'm going to do in the last video that's
coming up next is diagram out a feedback
loop for you in the human body, and talk
through a couple of actual examples to
illustrate how all of these pieces get
put together.

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