Transcript Hormones in Animals - Royal Holloway, University of London

Hormones in Animals (2)
Endocrinology
D R Davies
School of Biological Sciences
Purves Life: the Science of Biology Chapters 41
(Animal Hormones) and 15 (Cell Signalling)
Lecture 14 Learning Outcomes
At the end of this lecture you should be able to:
• Describe in general terms how endocrine control
mechanisms regulate homeostasis
• Explain the endocrine functions of the
hypothalamus and pituitary gland
• Discuss at least one complex endocrine control
mechanism involving the interplay of several
different hormones
Feedback Control Systems
• Thus a feedback control system will consist of an
effector (gland) which produces and maintains a
variable signal (hormone concentration).
• This is measured by a sensor (receptor) which
monitors the changes in the signal and sends a
further signal to the pituitary gland (comparator)
which compares the feedback signal to a reference
(called the set point)
• produces an error signal in the form of a trophic
hormone which regulates the effector and restores
the variable to its proper value,
Control Systems
• Feedback loop
• Effector (gland)
• Variable Hormone
concn
• Sensor (hormone
receptor)
• Comparator
(pituitary)
• Error signal
The hypothalamus and pituitary
• The hypothalamus is a region of the brain
controlling many aspects of physiological
activities such as hunger, thirst, sleep and
body temperature and consisting of a
distinctive cells which produce hormones
regulating these activities.
Pituitary
Gland
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Richard H. Pointer
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The hypothalamus can regulate physiological activity via
three routes:
•Via the autonomic nervous system e.g. via the release of
adrenaline by the adrenal medulla
•Neurones in the hypothalamus produce releasing
hormones into the blood portal system causing the release
of hormones by specific cells in the anterior pituitary
gland (adenohypophysis) . These tropic hormones are
then released into the general circulation where they affect
a whole range of physiological activities in various target
cells
•Via the posterior pituitary gland (neurohypohysis)
where the release of two closely related peptide hormones
(oxytocin and vasopressin) at nerve endings. These
peptides have totally different functions
Hormonal Regulatory
Mechanisms
• Positive Feedback e.g. increase in oestradiol
triggers ovulation
• Negative Feedback e.g circulating thyroid
hormone inhibits release of TRH and TSH
Negative Feedback Loops
Much of endocrine
physiology is concerned
with the maintenance of
homeostasis. Control
systems are of different
degrees of complexity but
usually consist of a
feedback loop with a
signal from the output
used to regulate the input.
Here the feedback signal
produces an opposite
response to a change in the
variable.
Hormonal Regulatory
Mechanisms
• Pulsatile Secretion Hypothalamic hormones
are released in bursts typically 60 - 180 min
apart
• Permissive Effects e.g hormones may
regulate the action of other hormones,
oestrogens increase the number of
progesterone receptors
• Synergistic Effects
Hormonal Regulatory
Mechanisms
• Up-Regulation- hormones increase the number of
their own receptors thus increasing the sensitivity of the
target cell to the hormone
• Down-Regulation- hormones decrease the number of
their own receptors thus reducing the sensitivity of the
target cell to the hormone
• The components of the signal transduction mechanism may
also become sensitized or desensitized to the signal
Regulation of reproductive function
• Androgens, such as testosterone, trigger the
development of male characteristics
– In male elephant seals, androgens account for
bodies weighing 2 tons or more, a thick hide,
and aggressive behavior
Figure 26.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Hormonal
Regulation
of Ovulation
Hormonal Changes in Pregnancy
Hormonal regulation of cyclic ovulation
• Pregnancy is triggered by an increase in
oestradiol (oestrogen) arising from the
developing ovarian follicle
• This is monitored by the hypothalamus and
pituitary and results in an increased frequency
of GnRH secretion followed by a surge in LH
and FSH
• LH surge causes the rupture of the ovarian
follicle and the release of the oocyte
• Oestradiol causes development of the
endometrium lining the uterus and oestrogen
and progesterone together maintain the uterus
as a preparation for pregnancy
Hormonal Regulation of Pregnancy
• If pregnancy is maintained the coprpus luteum is
maintained by the production of chorionic gonadotrophin
(hCG)
• the levels of oestradiol and progesterone increase
• These steroid hormones maintain the development of the
endometrium in a suitable state for implantation.
• Eventually the placenta takes over the production of the
two steroids
• The levels of oestrogens and progesterone continue to
increase through pregnancy until just before full term
Role of the adrenal gland in the
response to stress
• How the adrenal glands control our responses
to stress
Adrenal
medulla
Adrenal
gland
Adrenal
cortex
ST RESS
Nerve
signals
Hypothalamus
Releasing hormone
Kidney
Nerve
cell
Sp inal co rd
(cross
section)
Blood vessel
Nerve
cell
Adrenal
medulla
Ep inephrine and
norep inephrine
SHORT-TERM STRESS RESPONSE
Figure 26.10
Anterior pituitary
1. Glycog en broken down to glu cose;
increased blood gluco se
2. Increased blood pressu re
3. Increased breathing rate
4. Increased metabolic rate
5. Change in blood-flow patterns, leading
to increased alertness and decreased
digestive and kidney activity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
ACTH
ACTH
Min eralocortico ids
Adrenal cortex
Glucocorticoids
LONG-TERM STRESS RESPONSE
1. Reten tion of
sodium ion s
and water by
kidneys
2. Increased
blood volume
and blood
pressure
1. Proteins and fats
broken down and
converted to
glucose, leading to
increased blood
glucose
2. Immune system
may be
suppressed