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Chapter 6

BIOLOGY 2A03 Chapter Notes - Chapter 6: Secretion, Anterior Pituitary, Thyroid

Course Code
Graham R.Scott

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Chapter 6: Endocrine System
- Endocrine (transporting hormones throughout the blood) and nervous
systems are two long-distance communication systems in the body
oControl systems that regulate our physiology
-Because they are delivered in the blood stream, hormone actions have a
slower onset but are longer-lasting than neural signals
- Table: compare and contrast nervous and endocrine systems
Endocrine System
- Endocrine glands
oSecrete hormones directly into the ECF (in contrast to exocrine
glands (digestive system) – secrete products to the outside)
oDerived from epithelial tissue
- Endocrine system is not completely separate from the nervous
1. Endocrine glands are often under nervous control
oRelease hormones when they are stimulated by neurons
2. Some hormones are released from neurons (‘neurohormones’) rather than
endocrine glands (i.e., endocrine organs in the NS)
3. Many substrates can act as hormones in the circulation (blood) or as
neurotransmitters in the brain
oCan actually be the same substance – dopamine, adrenaline
4. The hypothalamus-pituitary complex is the neuro-endocrine interface

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Endocrine Organs
- Hypothalamus-pituitary complex: in our brain, just outside of our CNS
oNeuro-endocrine interface
- 3 classes
1. Amines – derived from aa tyrosine and tryptophan
Dopamine – neurotransmitter and
hypothalamic hormone that inhibits
prolactin secretion (milk breakdown)
Norepinephrine and epinephrine
neurotransmitter and adrenomedullary
hormones – released by the adrenal
Synthesis: all are derived from tyrosine
Successive enzymatic steps
convert tyrosine to each
Particular catecholamine secreted
by an endocrine organ depends on
the presence and activity of the
appropriate enzymes
oOther amines
Serotonin – neurotransmitter and hormone derived from
tryptophan that is involved in sleep, supressing stress
responses, and mood
Thyroid hormones – T4 = thyroxine, T3 = triodo-thyronine
– hormones derived from tyrosine that regulates metabolic rate
and growth
2. Protein and polypeptide hormones – produced in the ER
oGrowth hormone – released by the anterior pituitary
oAtrial natriuretic peptide – released by the heart to regulate sodium
reabsorption by the kidneys
oMany others
oSynthesized by proteolytic cleavage of a
pre-prohormone in the ER and the
resulting prohormone are then often
further cleaved to hormones during
packaging into vesicles by the Golgi
oHormones (and pro-hormones) are
released by Ca2+ - initiated exocytosis
3. Steroid hormones (lipophilic) – derived from cholesterol
oCan be stored in vesicles since they can di6use to the outside, they are
synthesized when needed

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oProduced by the gonads, placenta (sex hormones) and adrenal cortex
(mineralocorticoids, glucocorticoids, and sex hormones
-Terms glucocorticoid and mineralocorticoid come from these hormones’
roles (“gluco”  glucose/metabolism and the stress response; “mineralo
ion balance) and site of origin (adrenal cortex)
3 Hormone Classes Fall into Two Functional Groups
A. Peptides, proteins, some amines (Serotonin & Catecholamine)
B. Steroids, some other amines (Thyroid Hormones)
- Hydrophobic hormones (Steroids & some
other amines):
oDi6use through membranes
oBinds to intracellular receptors
oRequire carrier proteins for transport in
oNot stored in vesicles, need to be
created when they’re needed
- Hydrophilic hormones (peptides, proteins &
some amines):
oDon’t tend to last as long in the blood – not bound by carriers
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