This document provides a detailed examination of the endocrine system, particularly focusing on the hypothalamus-pituitary axes and their role in hormone regulation. It begins with an exploration of the pituitary gland, describing its anatomical structure, development, and the complex interactions ...
Module
7:
The
Endocrine
System
Hypothalamus-Anterior
Pituitary
Axis
-
GH:
The
Pituitary:
→
regulates
homeostasis
-
Regulates
energy
and
water
balances,
growth,
responses
to
stress,
and
reproduction
-
Secretes
8
hormones
which…
-
Act
directly
on
non-endocrine
tissues
-
Act
to
modulate
other
endocrine
glands
and
are
called
tropic
peptides
-
6
are
produced
by
the
anterior
lobe
-
2
are
secreted
by
the
posterior
lobe
-
Regulated
by
the
hypothalamus
which
integrates
afferent
signals
from
the
brain,
viscera,
and
circulating
levels
of
substrates
and
hormones
-
Positioned
outside
the
blood-brain
barrier
which
allows
it
to
directly
interact
with
substances
in
the
bloodstream,
receiving
hormonal
signals
that
regulate
a
variety
of
bodily
functions
→
Embryonic
Development
of
the
Pituitary:
The
pituitary
gland
develops
through
two
distinct
processes…
1.
Neurohypophysis
(posterior
pituitary):
This
lobe
originates
from
a
down-growth
from
the
base
of
the
brain,
known
as
the
infundibulum
,
which
expands
to
form
the
pars
nervosa
.
This
part
remains
neural
tissue
(directly
connected
to
the
brain)
and
is
integral
in
storing
and
secreting
neurohormones
like
oxytocin
and
vasopressin
.
2.
Adenohypophysis
(anterior
pituitary):
This
lobe
develops
from
an
upward
extension
of
the
roof
of
the
mouth,
which
eventually
becomes
separated
from
it
and
forms
around
the
infundibulum.
It
includes
the
pars
tuberalis
(the
cuff-like
region)
and
the
pars
distalis
(the
larger
portion
next
to
the
pars
nervosa).
The
anterior
pituitary
synthesizes
and
secretes
several
key
hormones
(like
growth
hormone,
ACTH,
and
prolactin)
in
response
to
signals
from
the
hypothalamus
.
a.
Pars
Intermedia
:
Positioned
between
the
anterior
and
posterior
lobes,
the
pars
intermedia
is
part
of
the
anterior
pituitary
in
humans
and
is
less
functionally
significant
compared
to
other
species.
In
some
animals,
it
plays
a
crucial
role
in
physiological
responses
such
as
changing
skin
color
in
response
to
stress
Hypothalamus-Pituitary
Portal
System: →
the
connection
between
the
hypothalamus
and
the
anterior
pituitary
is
facilitated
by
a
specialized
blood
vessel
system
known
as
the
hypophyseal
portal
system
-
It
starts
from
the
superior
hypophyseal
artery,
which
feeds
into
the
pars
tuberalis
and
the
median
eminence
-
Blood
vessels
break
up
into
a
capillary
bed
at
the
median
eminence
where
hypothalamic
neuropeptides
are
secreted
-
The
neuropeptides
are
transported
through
a
venous
link
to
a
second
capillary
bed
located
within
the
pars
distalis
-
The
dual-capillary
arrangement
forms
a
venous
portal
system
that
allows
efficient
and
concentrated
delivery
of
regulatory
signals
directly
to
the
hormone-secreting
cells
of
the
pars
distalis
Peptide
Secretions
and
Hormonal
Regulation:
-
Peptide
hormones
secreted
by
the
hypothalamus
originate
from
the
paraventricular
nucleus.
-
These
are
small
neuropeptides,
ranging
from
3-10
amino
acids
-
They
travel
down
axons
are
are
released
into
the
first
capillary
bed
of
the
portal
system
-
These
peptides
then
travel
to
the
second
capillary
bed
in
the
pars
distalis,
where
they
act
in
high
concentrations
on
specific
receptors
of
the
target
cells,
inducing
the
secretion
of
various
hormones
Receptor
Dynamics:
-
The
target
cells
in
the
pars
distalis
are
equipped
with
low
affinity
receptors
.
-
Good
for
responding
to
the
high
concentrations
of
neuropeptides
delivered
through
the
portal
system
-
Ensures
sensitive
and
regulated
hormonal
responses
based
on
the
fluctuating
levels
of
hypothalamic
peptides
→
How
to
prevent
receptor
desensitization:
-
To
prevent
down-regulation
of
receptors
due
to
continuous
high
concentrations
of
peptides,
the
hypothalamus
releases
these
signals
in
a
pulsatile
manner.
This
pulsatility
is
crucial
for
maintaining
the
efficacy
of
hormone
signaling.
-
The
secretion
of
neuropeptides
is
episodic
(often
follows
a
circadian
rhythm)
-
Ex
:
In
the
case
of
the
female
reproductive
system,
this
episodic
release
follows
a
monthly
cycle,
not
a
daily
one.
H-P
Axis
Target
Cells:
Releasing
Factors
:
the
hypothalamus
secretes
specific
neurohormones
which
each
target
specific
cells
within
the
pituitary
gland which
prompts
them
to
release/produce
their
hormones
-
These
hormones
are
referred
to
as
trophic
hormones
because
they
stimulate
further
hormonal
activity
in
other
glands
located
throughout
the
body
(regulate
essential
functions
like
growth,
metabolism,
and
reproduction
→
Ex:
TRH
-
TRH
is
a
thyroid
releasing
hormone
and
it
works
on
the
thyroid
tropes
at
the
pituitary
-
It
causes
them
to
secrete
thyroid
stimulating
hormones
and
when
those
hormones
enter
systemic
circulation,
it
works
on
the
thyroid
gland
-
The
thyroid
gland
will
then
secrete
thyroid
hormones
(T3
and
T4)
→
Ex:
GnRH
-
GnRH
(gonadotropin-releasing
hormone)
stimulates
the
production
of
Luteinizing
hormone
(LH)
and
Follicle
Stimulating
Hormone
(FSH)
from
the
pituitary.
-
LH
and
FSH
control
reproductive
functions,
including
gamete
production
and
the
secretion
of
sex
hormones
in
both
males
and
females
Inhibitory
Factors
:
In
addition
to
stimulatory
releasing
factors,
there
are
also
inhibitory
factors
that
suppress
hormone
secretion…
1.
Somatostatin:
Acts
as
an
inhibitory
factor
for
Growth
Hormone
(GH).
It
regulates
body
growth
and
metabolic
functions
by
inhibiting
the
secretion
of
GH.
2.
Dopamine
:
Serves
as
the
inhibitory
factor
for
Prolactin,
which
is
involved
in
lactation
and
other
reproductive
processes.
Dopamine
prevents
the
unnecessary
secretion
of
prolactin.
Regulation
of
H-P
Axis:
1.
Initial
Hormonal
Release:
The
hypothalamus
secretes
a
specific
releasing
hormone
(XRH
in
this
model).
XRH
is
a
positive
stimulatory
factor
that
targets
specific
cells
in
the
pituitary
gland
2.
Pituitary
Response
:
The
target
cells
in
the
pituitary
gland,
upon
stimulation
by
XRH,
release
a
hormone
termed
XTH.
XTH
then
enters
the
systemic
circulation
and
acts
on
specific
peripheral
target
cells.
3.
Peripheral
Target
Gland
:
The
peripheral
target
cells
respond
to
XTH
by
secreting
their
own
hormone
(X
Hormone).
Hormone
X
then
travels
back
through
the
bloodstream,
exerting
a
negative
feedback
influence
on
both
the
pituitary
gland
and
the
hypothalamus.
This
loop
is
known
as
the
long
negative
feedback
loop
.
a.
Additional
feedback
mechanisms
include…
i.
Short
Negative
Feedback
Loop:
Hormone
XTH,
after
its
release,
also
provides
direct
feedback
to
the
hypothalamus,
inhibiting
further
release
of
XRH.
This
mechanism
helps
regulate
and
prevent
excessive
hormone
production
.
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