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The Endocrine System
Cellular coordination
Endocrine communication
Endocrine vs. Exocrine
Target Cells & Receptors
Hormones
Endocrine Glands
Thyroxine
The Pituitary & Hypothalamus
Hormone Action
How do multicellular organisms differ
from single celled organisms?
One way is coordination.
Complex events (like flowering)
require cells to work together.
Life cycle transitions in insects require the
coordinated activities of tens of thousands of cells.
How do all these cells “know” that it’s time to molt
or time to emerge from a chrysalis?
A series of chemical signals
known as hormones are
released from glands within
the insect’s body, regulating
growth and development.
ecdysone
Neurons signal other
cells by the release of
chemical messengers
(neurotransmitters) at
synapses
Direct cell-to-cell contact
between neuron and
effector (in this case,
skeletal muscle)
Endocrine signaling
uses the bloodstream
and acts over long
distances
Synaptic signaling in neurons is
one example of local
cell-to-cell signaling.
Paracrine signaling is another.
Endocrine cells release
chemical signals, or
“Hormones”
that travel throughout
the body, where they
affect the actions of
“target cells.”
Hormones affect only
those cells that have
specific receptors for a
particular hormone.
Exocrine glands release their
secretions outside the body (or
into the digestive system)
Exocrine gland
Endocrine gland
Endocrine glands release their
secretions into the bloodstream.
“Endocrine”
endo - inside
krinein - to judge or
separate
Types of Hormones
Steroid hormones pass through the cell
membrane. Most other hormones
interact with cell surface receptors.
The Human Endocrine System
consists of a series of glands at
various positions in the body ....
... that produce a wide variety of hormones.
© 2011 Pearson Education, Inc.
Thyroxine is synthesized
from the amino acid
tyrosine. It is the reason
we need a small amount of
iodine in our diets.
Cells synthesize thyroxine and store it
in compartments in the thyroid gland.
If iodine is insufficient in
the diet, the thyroid tries
to compensate by
producing more cells to
extract more iodine from
the bloodstream.
Thyroxine (T4 or T3)
• increases metabolic rate
• increases heart rate
• increases breathing rate
Question: How does the thyroid “know”
whether to release more thyroxine?
Answer: It doesn’t. “Somebody” else
decides that. The Pituitary.
The Anterior Pituitary
forms from the nasal
lining during
development.
The Posterior
descends from the
Hypothalamus.
The Posterior Pituitary
consists of the
terminals of
neurosecretory cells
from the
Hypothalamus.
When these cells are
stimulated in the
Hypothalamus, they
release hormones (ADH
and oxytocin) into the
bloodstream.
Sensory input to the
brain stimulates the
posterior pituitary to
release oxytocin,
producing the milk
letdown reflex.
The Anterior Pituitary is a little more
complicated...
Neurosecretory cells in the
Hypothalamus dump small
amounts of “releasing
hormones” into vessels
leading to the pituitary.
These hormones target
endocrine cells in the
pituitary, causing them to
release pituitary hormones.
Five major “tropic”
hormones are released
from the Anterior
Pituitary.
Including TSH (Thyroid
Stimulating Hormone),
which stimulates the
release of thyroxine
from the Thyroid.
1) Low temp or
low Thyroxine
(T4 or T3) level?
2) Hypothalamus
releases TRH
3) TRH
causes
release of
TSH
5) Thyroxine
increases
metabolism,
warms the body.
4) TSH causes
release of
Thyroxine from
thyroid
1) Low temp or
low Thyroxine
(T4) level?
2) Hypothalamus
releases TRH
3) TRH
causes
release of
TSH
A classic negative
feedback loop!
5) Thyroxine
increases
metabolism,
warms the body.
4) TSH causes
release of
Thyroxine from
thyroid
Similar feedback loops
exist for the other
pituitary hormones.
Example: ACTH
(Adrenocorticotropic
hormone) and Cortisol.
Cortisol is a stress-response hormone. It increases blood sugar
levels and suppresses the immune system (relieving inflammation).
How does a hormone act
at the cellular level?
That depends on
whether it’s a steroid
hormone or a peptide
hormone.
Hormone Action - Steroids
Because they are lipids, steroids can
cross cell membranes easily.
Target cells contain steroid hormone Receptor Proteins
Steroid hormones bind to their receptors, and the
Hormone-Receptor Complex enters the nucleus.
The complex binds to DNA sequences known as Hormone
Response Elements and activates transcription.
Activation of transcription by Steroid Hormones can
cause profound long-termchanges in cellular activity including molting and metamorphosis in insects.
Peptide Hormones
like epinephrine
don’t enter the cell.
They bind to receptors
at the cell surface,
which in turn release
“second messengers”
inside the cell.
Example: cAMP.
These receptors are often
coupled to proteins that bind
GTP, known as G-proteins.
One effect of epinephrine
binding is the activation
of an enzyme called
Phosphorylase, which
releases glucose into the
cell (and bloodstream).
How does this happen?
The production of
cAMP starts a
“cascade” of events
that amplifies the
signal.
This amplification
enables just a few
hormone molecules to
elicit a powerful
response from the
target cell.
These “secondmessenger: hormones
can produce very rapid
changes in cellular
activity.
Example: cardiac muscle
cells’ response to
epinephrine.
Because they affect
transcription, the
effects of steroid
hormones are
generally slower - but
can be much more
profound.
Such as antler growth
in male deer.
Which prepares us
for Wednesday’s
topic.
SEX