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CHAPTER 25
Communication II:
The Endocrine System and
Chemical Messengers
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lntroduction
• Communication
– Is Transfer of information
– Maintains homeostasis
• Two ways
– Neurons – Electrical signals, quick (Chap.24)
– Hormones - Chemical signals, slow (Chap.25)
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lntroduction
• Along with the nervous system, the
endocrine system controls the body’s
activities
• Communicates by chemical messengers
called hormones
• In addition to hormones there are other
chemicals that function as messengers,
are released by different types of glands
or cells and act in different ways
• Note: Exocrine system also controls the
body’s activities through exocrine gland
secretions – salivary glands, sweat glands
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Chemical Messengers
 Chemical
as:
messengers are categorized
1) Local messengers
2) Neurotransmitters
3) Neurohormones
4) Hormones
5) Pheromones
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1) Local Messengers
Are cell secretions that alter
physiological conditions in
the surroundings
 They are of two types
i) Autocrine – acting on same cells
ii) Paracrine – acting on adjacent
cells
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2) Neurotransmitters
 Are neuronal secretions
secreted by the axons,
released in the synapses,
act quickly and degrade.
 Eg. Acetylcholine
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3) Neurohormones
 Also called as neuropeptides, are
secretions of specialized cells called
as neurosecretory cells.
 Neurohormones are
transported through
blood or body fluids
 Eg. Endorphins
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4) Hormones
 Secretions of endocrine
glands that are released
into the blood to be
transported to a target
organ to show its effect.
 Eg. Insulin
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5) Pheromones
 Are released to the
exterior of one animal
that affect the behavior
of the other animals of
the same species.
 Eg. Aphrodisiacs
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Hormones And Their Feedback System
• Study of the endocrine glands and their
hormones is called as endocrinology
–
Hormones are released into the blood
and transported throughout the body
• Only in target cells do they initiate a
physiological response
–
Target cells have receptors which
selectively bind to the hormones
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Biochemistry of Hormones
• Hormones chemically can be classified as:
Polypeptides
Eg. Insulin
Hormones
Amines
Eg. Thyroxine
Steroids
Eg. Estrogen
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Biochemistry of Hormones
• All hormones are low-level signals, thus
required in extremely small amounts to
produce a dramatic effect in target cells
– A hormone rarely exceeds one billionth of a
1M concentration
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Biochemistry of Hormones
Effects of Hormones
• Hormones influence cellular functions by
altering rates of biochemical processes in
target cells
– Change membrane permeability to increase or
decrease the diffusion rate of a substance
– Affect enzyme synthesis and activity to alter cell
metabolism
– Stimulate release of hormones from other glands
• This is a dynamic process that must be
regulated, not just activated
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Feedback control of Hormones
• Release of a hormone into the blood
depends
– Rate of secretion and rate of inactivation
• Endocrine glands must receive
information about the level of its
own hormone in the plasma
• Many hormones are controlled by
negative feedback systems between
glands and target cells
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Feedback control of Hormones
• Negative feedback system – produces a
response that counter acts the initiating
stimulus
• Positive feedback system – produces a
response that reinforces the initiating
stimulus, it is rare as it may lead to
instability.
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Feedback System
• A hormone influences, depends on and
balances another hormone in a controlled
feedback way- Positive and Negative
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Mechanism of Hormone Action
• Two basic mechanisms are:
1) Fixed-Membrane Receptor Mechanism
– external as the hormone is water-soluble
and cannot penetrate through the
membrane of the target cells
2) Mobile-Membrane Receptor
– internal as the hormone can penetrate
through the membrane of the target cells
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1) Fixed-Membrane Receptor Mechanism
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2) Mobile-Membrane Receptor
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Some Hormones of Invertebrates
• Almost all invertebrate taxa produce hormones
• Hormones may be synthesized by endocrine cells
as in Cnidarians, Nematodes, and Annelids
OR
• By endocrine glands as in Molluscs, Arthropods
and Echinoderms
• Invertebrate hormones are often neuropeptides
or steroids
• Invertebrate hormones regulate
– Molting, growth, reproduction, color changes,
and internal homeostatic mechanisms
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Vertebrate Endocrine System
• Vertebrates other than birds have
somewhat similar endocrine system
• There are species specific differences in
the hormone actions amongst the
vertebrates
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Endocrine System of Birds
Pituitary gland
Pineal
Thyroid
Parathyroid
Thymus
Pancreas
Adrenal
Ovary
Testis &
Ultimobranchial
Bursa of Fabricius
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Endocrine System of Birds
• In some birds such as pigeons
and doves, the pituitary gland
secretes the hormone prolactin
that stimulates the production
of “pigeon’s milk” in the crop.
•Prolactin also stimulates and regulates
broodiness and some other behavioral
patterns during breeding season.
•It develops the brood patch that helps
in maintaining eggs at a temperature that
is required for incubation
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Pituitary gland
Pineal
Thyroid
Parathyroid
Thymus
Pancreas
Adrenal
Ovary
Testis
Endocrine System of Mammals
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Pituitary Gland (Hypophysis)
Pituitary gland is at the floor of the brain above
the roof of the mouth,has 2 lobes
1)Anterior lobe, Adenohypophysis
– Is the larger lobe
– Secretory cells called pituicytes secretes hormones
2)Posterior lobe, Neurohypophysis connected to it by a
stalk, or infundibulum
– Is the smaller lobe
– Obtains hormones from the neurosecretory cells in the
hypothalamus
Intermediate lobe in between anterior and posterior lobe
Anterior pituitary’s connection to hypothalamus is a
special portal circulatory system
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Hypothalamus and Pituitary Gland
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Hormones of Anterior Pituitary
Anterior Lobe - Produces 6 hormones
– Four hormones are tropic hormones that regulate
other endocrine glands
1)Thyroid-stimulating hormone (TSH) stimulates
production of thyroid gland hormones
2)Follicle-stimulating hormone (FSH) promotes egg
production or sperm production
3)Luteinizing hormone (LH) induces ovulation and corpus
luteum and sex steroid production
4)Adrenocorticotropic hormone (ACTH) increases
production and secretion of steroid hormones from the
adrenal cortex
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Hormones of Anterior Pituitary
5)Prolactin
• Protein hormone that prepares mammary glands
for lactation and milk production
• Implicated in parental behavior of a wide variety of
vertebrates
• Mediator of the immune system
• Factor in formation of new blood vessels
6)Growth hormone (GH or somatotropin)
• Governs cell mitosis, synthesis of mRNA, and
metabolism
• Acts indirectly through a polypeptide hormone,
insulin-like growth factor (IGF)
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Figure 25.14
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Intermediate Lobe
– Produces melanocyte-stimulating hormone
(MSH) which promotes dispersion of pigment
in cells in bony fishes, amphibians, and
nonavian reptiles
– In birds and mammals, MSH is produced in
the anterior pituitary and has unclear roles
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Hormones of Posterior Pituitary
• Posterior Pituitary
– Hypothalamus is source of two hormones of posterior
pituitary
1)Oxytocin and 2)Antidiuretic Hormone
– Hormones form in neurosecretory cells in
hypothalamus and are released through axons into
capillaries of the posterior lobe
– Posterior lobe is not therefore an endocrine gland but
a storage and release center
– Both oxytocin and vasopressin are octapeptides
– Both hormones are fast acting, producing a response
within seconds of their release
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Hormones of Posterior Pituitary
–Oxytocin
• Stimulates contraction of uterine smooth
muscle during labor
• Can be used to induce labor
• Triggers milk ejection by mammary glands
in response to suckling of infant
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Hormones of Posterior Pituitary
–Antidiuretic Hormone (Vasopressin)
• Acts on collecting ducts of the kidney to
increase water absorption
• Increases blood pressure by constricting
smooth muscles of arterioles
• Acts centrally to increase thirst
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Hypothalamus and Neurosecretion
– The pituitary influences most hormonal
activities but is controlled by centers in the
hypothalamus
– Groups of neurosecretory cells in the
hypothalamus manufacture releasing and
inhibiting hormones or factors
– These neurohormones travel down nerve
fibers to endings where they enter a capillary
network
– The pituitary portal system takes them
directly to the anterior pituitary
– The hypothalamic hormones stimulate or
inhibit various anterior pituitary hormones
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Endocrine System – Adrenal Gland
Hypothalamus
Corticotropin
Releasing Hormone
Anterior Pituitary
ACTH
Adrenal Cortex
Cortisol
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Endocrine System – Parathyroid Gland
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Endocrine System – Pancreas
Some Hormones are not produced by
Endocrine Glands
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The right atrium of heart secretes atrial
natriuretic hormone which stimulates the
kidneys to excrete salt and water in the urine
– It acts antagonistically to aldosterone
The kidneys secrete erythropoietin which
stimulates the bone marrow to produce red
blood cells