Chapter 45 – The Endocrine
System
Sara Aghaee
Frank Chang
Alia McKean
Michael Kwett
Overview and Introduction
• Animals have two systems of internal communication and
regulation
• 1) Nervous system; Conveys high-speed electrical signals
along specialized cells called neurons
• 2) Endocrine system; All an animal’s hormone secreting cells
constitute this system
•
But.... WHAT IS A HORMONE???!!!!
• Hormone: a chemical signal that is secreted into the
extracellular fluid, and communicates regulatory messages
within the body.
Overlapping of the Endocrine
and Nervous Systems
•
Several chemicals serve both as hormones in the endocrine system
and as chemical signals in the nervous system.
Examples
• Neurosecretory cells: specialized nerve cells that release hormones
into the blood by extracellular fluid.
• Epinephrine: functions in the vertebrate body as “fight-or-flight”
hormone AND as neurotransmitter, a local chemical signal that
conveys messages in the nervous system.
• Lines between these two regulatory systems is blurred
Control Pathways and Feedback Loops
Concepts to Remember!!!
• Receptor (or sensor) ~
detects stimulus
• Control center~ area
where information is
received
• After comparing
information to a set point,
control center sends out
signal that directs an
effector to respond
• In endocrine and
neuroendocrine pathways
this signal, efferent signal,
is a hormone (or
neurohormone), which
acts on particular effector
tissues and elicits specific
physiological or
developmental changes
•
A common feature of
control pathways is
feedback loop.
•
Negative feedback: effector
response reduces in initial
stimulus, and eventually
response ceases
•
Positive feedback:
reinforces the stimulus and
leads to an even greater
response
artist’s impression of
a feedback loop 
Chemical Signaling
 Hormones: long-distance chemical
regulators; relay information to target
cells
Amines (water-soluble)
Protein/peptides (water-soluble)
Steroids (lipid-soluble)
 Pheromones: chemical signals carried
between different organisms in a species
 Local regulators: carry information
between neighboring cells
Overview of Major Events in
Hormone Signaling
 Reception
 A signal molecule binds to a specific receptor either
on the cell surface or within the target cell
 Signal Transduction
 The events triggered by reception of the signal
molecule
 Response
 A change in the target cell’s behavior in response to
the signal transduction pathway; sometimes occurs in
nucleus or cytoplasm
Types of Receptors and Examples
 G-protein (Guanine-protein) linked receptors
 G-protein activation (leads to binding of GTP molecule in the place of
GDP—allows interaction of protein with target cell)
 Gated Ion-channel receptors—channels that open and close in
response to signals and specific molecules (ligands)
 Neurotransmitters and muscle cells, action potentials
 Tyrosine-kinase receptors
 Catalyze transfer of phosphate groups from ATP to the amino acid
tyrosine  activation, responses, multiple new proteins & pathways
Cell-surface Receptors for
Water-soluble Molecules
 Receptors are bound in plasma membrane, projecting outwards
 When the signal molecule binds to a receptor, a signal transduction
pathway is produced
 This causes changes in cellular proteins converting chemicals into
specific responses, activation of enzymes, secretion of molecules,
rearrangement of the cytoskeleton, and regulation of transcription of
genes in the nucleus of the target cell.
 Hormones can produce different responses in contact with various
cell-surface receptors.
 Example: Multiple effects of epinephrine including decreased blood flow
to digestive tract and increased glucose to skeletal muscles
Receptors for
Lipid-soluble Molecules
 Receptors are intracellular, either in the nucleus or in the cytoplasm,
and they bind molecules traveling via the bloodstream.
 They transduce signals inside the target cell.
 The hormone-receptor complex binds to DNA,
initiating gene transcription and
expression  new proteins in cytoplasm
 Again, the hormones produce
different responses in contact
with various target cell receptors.
Paracrine Signaling by
Local Regulators
 Local Regulator: send rapid messages between
neighboring cells, are faster than hormones
 Some have cell-surface receptors, others are intracellular
 Types: neurotransmitters, cytokines (immune system), growth factors
(cell proliferation, nitric oxide, and prostaglandins
 Nitric Oxide
 When oxygen levels in blood fall, blood vessels release NO
 NO dilates the blood vessels, increasing blood flow
 Oxygen levels increase again
 Prostaglandins
 Have a wide variety of functions in the body, including
contraction of the reproductive tract, inducing labor,
intensifying pain, and the accumulation of platelets
during clotting
Relationship Between Hypothalamus
and Pituitary Gland
• The hypothalamus receives information
from nerves throughout the body and
from other parts of the brain, initiating
endocrine signals appropriate to
environmental conditions.
• It contains different sets of neurosecretory
cells, some producing direct-action
hormones that are stored in and released
from the posterior pituitary. Other
hypothalamic cells produce tropic
hormones that are transported by portal
blood vessels to the anterior pituitary, an
endocrine gland. These tropic hormones
control release of hormones from the
anterior pituitary.
Posterior Pituitary Hormones
Antidiuretic Hormone (ADH)
• The two hormones released
from the posterior pituitary
act directly on nonendocrine
tissues.
• Oxytocin induces uterine
contractions and milk
ejection.
• Antidiuretic Hormone (ADH)
enhances water
reabsorption in the kidneys.
Anterior Pituitary Hormones
•
Both tropic and nontropic
•
hormones are produced by
the anterior pituitary. The
four strictly tropic
hormones are thyroidstimulating hormone (TSH),
follicle-stimulating
hormone (FSH), luteinizing
hormone (LH), and
adrenocorticotropic
hormone (ACTH). Each
acts on its target
endocrine tissue to
stimulate release of
hormone(s) with direct
metabolic or
developmental effects.
Prolactin, melanocytestimulating hormone (MSH),
and β-endorphin are nontropic
anterior pituitary hormones.
Prolactin stimulates lactation
in mammals but has diverse
effects in different vertebrates.
MSH influences skin
pigmentation in some
vertebrates and fat metabolism
in mammals. Endorphins inhibit
the perception of pain. Growth
hormone (GH) promotes growth
directly and has diverse
metabolic effects; it also
stimulates the production of
growth factors by other tissues
(a tropic effect).
Nonpituitary Hormones regulate:
-metabolism
-homeostasis
-development
-behavior
Thyroid Hormones
-The thyroid gland consists of 2 lobes located on the
ventral surface of the trachea.
-This gland produces two similar hormones
synthesized from the amino acid tyrosine:
triiodothyronine (T3) and tetraiodothyronine (T4).
-These hormones are responsible for
vertebrate development and maturation such
as skeletal growth, mental development, and
the metamorphosis of a tadpole into a frog.
-Goiter occurs when there is an
iodine deficiency.
Parathyroid Hormone and Calcitonin: Control of
blood calcium levels
-The amount of calcium ions in the blood is essential in regulating skeletal
muscles. The usual calcium level is maintained at 10mg/100mL.
-When blood calcium level falls, the parathyroid glands, embedded in the
surface of the thyroid, secrete parathyroid hormone (PTH) to increase release
and absorption of calcium ions.
-A rise in blood calcium level promotes the thyroid gland to release calcitonin,
which reduces the release and uptake of calcium ions.
-PTH stimulates osteoclasts in the bone to
release calcium into blood.
-PTH also directly stimulates uptake of
calcium in kidneys.
-PTH indirectly allows kidneys to convert
vitamin D into its active hormonal form,
which stimulate uptake of calcium in the
intestines.
Insulin and Glucagon: Control of Blood Glucose
Levels
-The pancreas is considered both an endocrine
and exocrine gland with important functions in
endocrine and digestive systems.
-Endocrine cells called islets of Langerhans are
scattered throughout the tissues of the pancreas.
-Each islet contains populations of either alpha
or beta cells.
-When blood glucose level rises, beta cells release
the hormone insulin, which stimulates all body cells
to take up glucose from blood and slows glycogen
breakdown in the liver.
-As blood glucose level drops, alpha cells release the
hormone glucagon signaling liver cells to increase
glycogen hydrolysis, convert amino acids and glycerol
into glucose, and release glucose into the blood.
-Glucose is a major fuel for cellular respiration and a source of carbon
skeletons for the synthesis of organic compounds. The recommended
blood glucose level is set at about 90mg/100mL.
Adrenal Hormones: Response to Stress
-The adrenal glands are adjacent to the kidneys and each adrenal
gland are made of the adrenal cortex, or outer portion, and adrenal
medulla, or the central portion.
-The adrenal medulla releases hormones called catecholamines
synthesized from tyrosine. Among the catecholamines are epinephrine
and nonepinephrine, which are secreted due to short-term stress
response.
-The adrenal cortex synthesizes and releases a
family of steroids called corticosteroids.
Glucocorticoids and mineralocorticoids are
released as a result of long-term stress response.
-The adrenal medulla is stimulated by nerve
signals while the adrenal cortex is stimulated by
ACTH from the blood vessels.
Gonadal Sex Hormones
-The gonads produce and secrete three major categories of steroid
hormones: androgens, estrogens, and progestins.
-Estrogens, such as estradiol, are responsible for the
development and maintenance of the female
reproductive system.
-Androgens, especially testosterone, are synthesized
by the testes and they stimulate the development
and maintenance of the male reproductive system.
-Progestins are involved in preparing and
maintaining the uterus, which supports growth and
development of an embryo.
-Both of the gonadotropic hormones (FSH and LH)
in males and females stimulate activities of the
gonads and control the synthesis of steroid
hormones.
Melatonin and Biorhythms
-The pineal gland is a small mass of tissue near
the center of the brain.
-This gland makes and secretes melatonin, a
modified amino acid.
-Melatonin’s primary functions are related to the
biological rhythms associated with reproduction.
Thymus Gland
-The thymus gland is an organ
that lies underneath the top of
the breast bone.
-The thymus gland is an organ
that lies underneath the top of
the breast bone.
The Three Invertebrate
Hormones
• Three hormones play major role in molting and
metamorphosis into adult shape and form
• Brain hormone: produced by neurosecretory
cells in insect brain, stimulates the release of
ecdysone from the prothoracic glands
• Ecdysone promotes molting and development of
adult characteristics
• These two are balanced by the third hormone
• Juvenile hormone: promotes the retention of
larval characteristics
Invertebrate Regulatory System
• Invertebrates produce a variety of hormones in typical
hormone secreting endocrine cells and neurosecretory
cells
• Most of these hormones function in reproduction and
development.
• In Aplysia, for example, specialized nerve cells secrete
a neurohormone that stimulates the laying of thousands
of eggs and also inhibits feeding and locomotion
Arthropods
• All groups of arthropods possess
extensive endocrine systems.
• Crustaceans have hormones that
function in growth and
reproduction, water balance,
movement of pigments in the
integument and eyes, and
regulation of metabolism.
• Most insects get their adult
characteristics in a single
terminal molt and molting is
usually triggered by a hormone
Hormones and Endocrine
Glands in Insects
• Insects have:
• Prothoracic Glands--manufacture ecdysteroids:
production of chitin and protein in epidermal cells.
• Results in flow of events that result primarily in
molting.
• Prothoracic glands disappear after molting, when
they are not needed. Release is stimulated by
PTTH [prothoracicotropic hormone] from the
corpora cardiaca (located behind brain).
• Corpora allata manufactures juvenile hormone.
• Neurosecretary cells regulate activity of corpora
allata.
• Neurosecretary cells found in clusters, secrete brain
hormone
• Ovaries and testes produce gonadal hormones that
coordinate courtship and mating behaviors
Similarities in Humans and Insects
Nervous and endocrine systems of insects, including
the corpora cardiaca and the corpora allata
•Humans also have neurosecretory cells
•Secondary sex hormones in humans: androgens
(testosterone and estrogen), similar to gonadal hormones in
insects.
•Juvenile hormone promotes adult maturity in insects, as do
many of the sex hormones in humans.