Biology concepts – neuroendocrine system, bilateral
asymmetry, internal asymmetry, hormones, endocrine glands
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Jack Nicholson did some try
asymmetric thinking
in The Shining. The fact that he was driven insane
by the ghost of a horrid past
shouldn’t think less of
his accomplishments.
Predicting that Shelly Duvall
would go for the radio –
brilliant. Sneaking up on
Scatman Crothers – inspired.
Following the boy
into the maze –oops.
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An asymmetric military engagement might be one where a traditional force
is thwarted and confused by a nontraditional force of computer geeks hacking
their communications and selling all their weapons on Etsy. Asymmetric thinking
approaches what might be a traditional question at a creative angle, rejecting
the tools and assumptions that are normally found “inside the box.”
Asymmetric thinking asks why outrageous solutions aren’t
being considered. Many job interviews now include asymmetric thinking questions. A
man goes into a restaurant and asks for a glass of water. The waiter points a
gun at him and the man thanks the waiter and leaves. What’s your explanation for
that? (see end of post)
Today we'll talk about asymmetric thinking in a different
way – a lateral thinking approach to asymmetrical thinking, as it were. We have
talked about the asymmetry of the brain hemispheres and how they sit asymmetrically in your skull. We have also talked about the neuroendocrine system and how it is controlled by a part of your brain that isn’t really part of your brain. Now let’s talk about the asymmetry of the neuroendocrine system
as it begins in your brain and ends in the endocrine glands.
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Most views show the
hypothalamus from the side
(see last week’s post), so
you don’t appreciate that
there are two of each nucleus.
For those that dump
hormones into the pituitary
gland, both sides
participate, but our story
today shows that they don’t
necessarily participate
equally.
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The whole purpose of the hormone system is that it can be
used to bathe the entire body in functional hormones at the precise levels,
so that all cells that can respond will respond. How does that jibe with an
asymmetry where one gland of a pair does more than the other?
The hypothalamus is a good example. You have two halves of
your hypothalamus, one in each hemisphere (sort of, see above), but they both deposit
releasing hormones into the same pituitary vein complex so they can stimulate
your single pituitary gland. Yet, studies show that the right hypothalamus
makes more gonadotropin-releasing hormone than the left hypothalamus.
On the other hand, thyrotropin-releasing hormone (stimulates
the release of thyroid stimulating hormone, TSH, from the pituitary) is higher in the left hypothalamus.
Together, the results of several studies shows that the right hypothalamus
plays a bigger role in controlling reproduction, while the left hypothalamus
works more in metabolic rate. You have a lateralization of structure and
function in your two hypothalami, just like in your two cerebral hemispheres,
even though both halves work on a single pituitary gland.
How about some of the other endocrine and neuroendocrine
glands?
Thyroid – You only have one thyroid gland, which lies
over the front of your windpipe in your neck. There are two lobes, one on the
right side and one on the left, connected by the isthmus across the windpipe.
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The thyroid receives
stimulation from the anterior
pituitary release of TSH
(thyroid stimulating
hormone). If for some reason
you get to much
stimulation (maybe
autoantibodies), or the thyroid
start making too much
thyroxin on its own (tumor),
then you have Grave’s
disease. In some cases, you also
get an autoantibody during
Graves that attacks the
fibroblasts around the orbit
of the eye. The
inflammation makes the lid
retract and pushes on the
eyeball, making it bulge out
of the socket. Which of these
two has thyroid eye disease?
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The size difference may not be innocuous. Many studies have
shown that thyroid diseases and cancer affect the right lobe more often than
the left. And it gets weirder. A 2009 research paper from China showed that
handedness may also play a role. They found that the right > left size difference was larger in right-handed people. However,
the left lobe was about the same size no matter which hand the person
preferred. So, does the hand you use influence the size of the thyroid, or does your thyroid predict which hand you will use? Or, is it a correlation without significance?
Parathyroid – You
have four parathyroid glands – maybe. These are located on the backside of your
thyroid gland (hence the name para =
by). The parathyroids are important in regulating calcium levels in the body.
This may seem weird, having four glands to control the levels of one element.
But consider that calcium plays some major roles, from controlling muscular
contraction, to neuron transmission, to at least a dozen different second
messenger systems in every cell.
The parathyroids are small, only about 33 mg each, so they
are easy to lose when people have surgery on their thyroid gland. A 2011 study
sought to find out where they sit normally so surgeons would be able to find
them and preserve them. Unfortunately, they found that position and number are
quite variable. Forty-three percent of people have at least five glands instead
of four. And the positions of the four common ones can be variable, they aren’t
always in the same place. The extra ones can be just about anywhere! Good
hunting Mr. surgeon.
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If you soak bone in vinegar
(acetic acid), it will remove
the calcium and leave the
protein matrix. Notice that it
must be the calcium that
gives bone rigidity. This isn’t
how PTH works. PTH stimulates
osteoclast activity,
which removes the calcium AND
the protein matrix. PTH
also decreases the amount of
calcium lost in the urine,
but for some reason, we work
just fine without PTH or
without its balancer hormone,
calcitonin.
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But wouldn’t you need a balancing hormone to decrease
calcium levels if they get too high, so a balance could be established? This is
how many hormones work; there are hormone pairs that have opposite stimulatory functions. The balancing hormone for PTH is calcitonin,
made by the neuroendocrine parafollicular cells (C cells) in the thyroid gland.
But here’s the exception. Calcitonin is important in fish
and birds, but it seems people and many other mammals can get along fine
without it. Remove someone’s thyroid and they have to take thyroid hormone for
the rest of their life. But they get along just fine without calcitonin. This
is one instance where the balancing hormone isn’t necessary. It seems an
asymmetry of function in calcium regulation is just fine in people.
Adrenal glands - These glands sit on top of each kidney. You
think of them as sources of epinephrine in the fight or flight syndrome, but
they do much more. Adrenal (ad = of
or near, renal = kidney) glands have
a cortex, which is toward the outside (not the core, this always confused me),
and a medulla, which is in the middle (makes more sense).
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cortex of three layers and a
medulla. The cells of the two
regions are of different
origin, the medulla is nerve like,
while the cortex is
epithelial in origin.
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The adrenal cortex is made up of three layers, each produces
hormones to be released into the blood. The outside most is the zona glomerulosa, which makes aldosterone
to help control osmolarity and blood pressure. The zona fasiculata is the biggest, and makes cortisol that controls
the metabolic rate. The zona reticularis
is inner most and makes sex hormones, androgens specifically.
You have two adrenal glands – and they each do the same things in response
to the same signals, either hormonal or neural. So why is the left adrenal
gland almost always bigger than the right? Is it because the venous drainage of
the right and left adrenals is different? In the right, the veins dump into the
inferior vena cava, while the left drains into the left renal vein. I really
don’t know if that would make a difference.
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This is just weird. A paper from 2005 states that since
the autonomic (sympathetic
and parasympathetic)
nervous system controls the
neuroendocrine system,
asymmetries in the ANS can
have ramifications on
endocrine function. Asymmetry in behavior can
affect ANS, which then can
affect endocrines. This study
showed that feeding cows from
the left side (affects right
side ANS) improved
reproductive ability and lactation; so
the right ANS must have more
influence on reproductive
endocrine function. Could I
have some left-hand milk please?
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What is more, when they compared aggressiveness, no matter
whether the animal was wild or domesticated, the most aggressive animals had the largest size differential – always left bigger than right. This makes it
sound like the medulla was involved – aggression being involved, but it wasn’t.
The increase in left adrenal size in domesticated animals
was due to an oversized zona fasiculata (cortisol and other glucocorticoids),
while the left adrenal asymmetry in the aggressive animals was due to a larger
zona reticularis (sex hormones). Ah… now that makes some sense. Doesn’t it
always come back to sex?
Next week, let’s look more into the neuroendocrine system
and gender. The testes and ovaries have the most spectacular asymmetries.
(The man had the hiccups)
For more information or
classroom activities, see:
Hypthalamus – see last week’s
post
Thyroid gland–
Parathyroid glands–
Adrenal glands-
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