Biology concepts – homeostasis, neuroendocrine system,
hormone, pituitary, hypothalamus
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Let’s face it, everything I
know about computers I
learned from Tron and Tron Legacy. What I learned
most from the sequel is that
we still can’t make a
decent avatar for Jeff
Bridges. But I did learn about
the CPU and programs and
users. I like to think there
are small motorcycle races
going on in my laptop
while I write. It makes it
more interactive.
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Basically, a computer is like any other tool, its worth
depends on how it is used. But its a very complex tool with many parts that are
easily broken or can wear out. A
computer is like your brain in some ways; let’s look at one specific system in
your body that can be likened to part of your computer.
The central processing unit (CPU) of your computer controls
most of what your machine can do, but it’s functions fall into two broad
categories – 1) calculations and information storage, and 2) monitoring the
machine itself. The CPU has programs to manipulate information and present information in
different forms, but it also has sensory and feedback systems to keep the
machine working at optimum levels.
It is this second function that we’re interested in today.
Let’s say you use your laptop for an extended time and it starts to heat up.
Sooner or later you'll hear the fan turn on; this is your computer’s attempt
to maintain the temperature within a small window of values so that function is
maintained and there is no damage to the machine. Your body does the same
thing.
Another example – you let your computer sit for a while
without doing anything. Sooner or later the screen will go black and the
machine will go into a sleep mode to save energy and not burn out the screen.
Once you start to use it again, it will draw more power; if you stream a movie
or 100 videos of cats playing the piano, it will draw a lot more energy and the
battery will wear down faster.
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Homeostasis is a little like
riding a Segway. If you
stand perfectly still and
upright, nothing moves; all
the sensors are working but
they don’t have to make
any adjustments. If you lean
forward, the gyroscopes
recognize it and move the
Segway forward so that you
stay balanced; the same thing
if you lean backwards.
Everything works to keep your
body systems in
balance even when, especially
when, something
changes in the system.
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In your body, the brain is your CPU. You’re cerebral cortex
controls your higher functions, but each hemisphere has older parts that help maintain the body systems the hypothalamus and hippocampus (so do you have dual processors?). The peripherals that help control your body and keep
things on normal are the endocrine glands and the neuroendocrine cells.
Together, they’re called the neuroendocrine system and they maintain homeostasis (homeo = like and stasis =
standing still).
Homeostasis is one of the characteristics of life. To be alive, an organism needs a system to
resistant changes in itself when there are short-term changes in its
environment. Like a computer overheating and turning on the fan, your body is
constantly sensing internal temperature and turning on and off discrete systems
to maintain a constant 98.6˚F.
You have homeostatic systems that control reproduction,
temperature, energy consumption and production, hunger, sleep/wake cycles,
basal metabolic rate, osmolarity, drinking, and blood pressure. Some of the
sensory systems run straight to the brain via hardwiring (another computer
analogy). These are the neurons of the peripheral and central nervous systems.
The outputs might be neural, but many times they aren’t.
Imagine trying to send a neuron to each cell that needs to get a chemical
message (via neurotransmitters released at the ends of the neurons). That would
be very cumbersome to maintain and would require trillions more neurons.
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“To whom it may concern,” is
the introduction to
every hormone message sent
throughout your body.
Like bulk e-mails, the
hormones contact every cell
via the blood system, but
only those that can and
need to respond (have the
right receptors) will read
the message and do something
about it.
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Hormones are the hallmark of the endocrine (endo = within, and crine = sift) system. When released into the bloodstream, they
travel to every part of the body so that all cells have the opportunity to
respond. Not all will, and you wouldn’t want them to. Specific cells and
organs have the ability to respond to specific signals because they have the
right hormone receptors.
There are eight endocrine glands in a human body. The
adrenal glands (2 of them), the parathyroid glands (4), the thyroid gland (1),
the pineal gland, the pancreas, the ovaries (2) or testes (2), the
hypothalamus, and the pituitary gland. They can be stimulated to release
hormones by either neurons, other hormones, or other chemicals. Besides these well
known glands, a few tissues will release hormones in specific situations. The
placenta will release progesterone and estrogen, and the stomach can release
gastrin to stimulate gastric juice (acid) and ghrelin to stimulate hunger.
In addition to endocrine glands, there are also some cells
that can be directly stimulated by neurons to release hormones into the blood.
These are the neuroendocrine cells. There’s
a lot more of them you think, and they’re just about everywhere in your body.
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The upper image shows the
PNEC’s in your respiratory
tract (NE). They can be alone or
be grouped into neuro-
endocrine bodies (NEBs). A 2008 study, and several
before it, has correlated a
hyperplasia and hypertrophy
of PNECs with Sudden Infant
Death Syndrome. Too
many and your respiratory
control may get thrown out
of whack; it may just stop working.
It might be good to monitor
the PNEC system in youngsters
to try and predict
susceptibility to SIDS.
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For one example, there are pulmonary neuroendocrine cells (PNEC) in every part of your
respiratory tract, from your nose to your alveoli. While they seem to act like
neurons in many respects, they are derived from epithelial tissue, not the
neural crest tissue that all neurons come from.
Each PNEC spans the distance from basement membrane to the
air conducting space. On the luminal side, they have microvilli that stick out
into the lumen and sample the air as it passes. On the basal side (bottom) they communicate
with neurons. Just what are they doing there?
They have several functions, including regulating the
development of the respiratory system in the fetus. They also regulate the
function of the respiratory cells by sensing oxygen levels, controlling the
muscular tone of the bronchi, regulating pulmonary blood flow and modulating
immune responses. The origin and function of PNEC’s is reviewed in a 2012 paper, which also highlights a problem with them – when PNEC’s go bad, they
cause a deadly small cell cancer.
In response to what the cell senses, it will release a
variety of chemicals, many of which can act as neurotransmitters – hence the
reason they are considered neuroendocrine cells. But the PNEC’s may have
another function, one hinted at in a 2014 study. The results of these
experiments indicated that PNEC’s can detect chemicals in the air as it passes
over their microvilli – they quite literally can smell the air, as their microvilli were found to have olfactory (smell) receptors, just like in your
nose!
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The hypothalamus is part of
the diencephalon, a part
of the brain older than the
cerebral cortex, but not as
old as the thalamus. The
different nuclei each function
in different systems; the
lateral nucleus senses
hormones and regulates thirst
and hunger, the
dorsomedial nucleus regulates
BP and HR. Several
nuclei are involved in making
releasing hormones,
ADH and oxytocin.
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So what controls much of this neuroendocrine system?
Your brain of course. Well, a small part of your brain that's pretty old in
terms of evolution (even primitive hagfish have a pituitary). The hypothalamus senses many of the inputs that tell your
brain just how well your body is maintaining homeostasis. If something goes
astray, or if there is a change in your environment that forces a change in your
body chemistry, the hypothalamus will then spring into action (O.K., it’s
always working, it will just work harder).
The hypothalamus
has a neuroendocrine relationship with the pituitary
gland (better named the hypophysis,
where hypo = under and physis = growth – it is a growth under
the brain). The hypophysis comes in two parts. Both parts release hormones, but
where those hormones come from and the sources of the tissues of the two
pituitary parts are very different.
The anterior pituitary isn’t part of the brain at all. The
tissue for the adenohypophysis
(anterior pituitary, adeno =
secreting) comes from the roof of the embryonic mouth. It is epithelial in origin and has
cells that produce hormones in response to signals from the hypothalamus (also
hormones). Here’s how it happens.
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The pituitary gland, or
hypophysis, has two small
portal systems that carry
blood from one place to
another instead of to a
particular tissue and then
back to the heart. The
anterior system carries
releasing hormones from the
hypothalamus to the
ant. pituitary and then
carries stimulating hormones
from the ant. pituitary to
the rest of the body. The
posterior system moves
hormones from the
hypothalamus directly to the
rest of the body.
Image credit to Medicalook.
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Well, it gets worse. There are six releasing hormones from
the hypothalamus that stimulate production and release of seven hormones from
the adenohypophysis that then act on at least 20 endocrine glands and
neuroendocrine cell types. Hypothalamus to adenohypohysis to endocrine gland –
it’s called an axis, and there are
several of them.
On the other hand, the posterior pituitary (neurohypohysis) is derived from brain
tissue. Instead of the neurons of the hypothalamus producing releasing hormones that then act
on the neurohypophysis, the hypothalamic neurons project right into the posterior
pituitary where they deposit their hormones (oxytocin and antidiuretic
hormone). The neurohypophysis doesn’t
make any hormones itself, it just stores what the hypothalamic neurons produce
and then releases them to the circulatory system.
That’s a heck of an exception- part of your brain isn’t
actually part of your brain. Cells from your mouth control most functions in
your body! Some people I know have mouths that completely ignore their brains! Next week, let’s talk more about the neuroendocrine system. Your
thyroid gland size correlates to which hand you use to write – say what?
For more information or
classroom activities, see:
Homeostasis –
Neuroendocrine system –
Hypothalamus –
Pituitary –
