Download Chemical Control of the Animal Body: The Endocrine System

Chemical Control of the Animal
Body: The Endocrine System
Chapter 37
How Do Cells Communicate?
• Direct communication from the inside of one
cell to the next occurs through gap junctions
– Ions and small molecules flow from cell to cell
– Example: cardiac muscle cells
– More commonly, cells release “messenger
molecules” that stimulate cells if the molecules bind
to receptor proteins in or on the cell
• Different communication systems use different
messenger molecules
How Do Cells Communicate?
• Paracrine communication occurs
between nearby cells using local
hormones that diffuse through
extracellular fluid
– Example: prostaglandins
• Endocrine communication occurs
between distant cells using hormones that
are carried by the bloodstream
– Example: insulin
How Do Cells Communicate?
• Synaptic communication occurs
between neurons and their targets using
neurotransmitters
– Neurotransmitters are released across a
small gap (synaptic cleft) between cells
– Example: acetylcholine
Local Hormones
• Produced by most cells
• Example: prostaglandins
– Are modified fatty acids used for local communication
• Many functions of prostaglandins, including
–
–
–
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Umbilical cord arterial constriction
Stimulate uterine contraction
Contribute to inflammation
Stimulate pain receptors
Endocrine Hormones
• Endocrine hormones are released in
response to stimuli inside or outside the
body
• Released by glands that make up the
endocrine system
• Travel through the circulatory system to
reach target cells
Endocrine Hormones
• There are three classes of vertebrate
endocrine hormones
– Amino acid-based hormones: synthesized
from single amino acids
– Peptide hormones: made from chains of
amino acids
– Steroid hormones: made from cholesterol
Target Cell Receptors
• A cell will only be affected by a hormone if
it has receptors specific to that hormone
– If not, no effect
• Different classes of hormones bind to
receptors in different ways
– Due to differences in chemical composition
that affect their ability to penetrate the cell
membrane
Water-Soluble Hormones
• Most peptide and amino-acid based hormones
are water-soluble
• Water-soluble hormones use cell membrane
receptors
– Cannot penetrate cell membrane
• The hormone (first messenger) binds to cell
membrane receptors
– Receptor binding activates the formation of a second
messenger (example: cAMP) in the cell
– The second messenger activates enzymes that
produce cell effects
Lipid-Soluble Hormones
• Steroid hormones are lipid soluble
– Can easily penetrate cell membrane
• Lipid-soluble hormones enter the cell and binds
to receptors in the cytoplasm or nucleus
– Forms hormone-receptor complex
– The hormone-receptor complex binds to a gene on
DNA
– Protein synthesis is initiated for that specific gene
Regulation of Hormone Release
• Hormones can have powerful effects, thus
secretion must be controlled
• Most hormones are controlled by negative
feedback, which inhibits further release
– Example: antidiuretic hormone (ADH)
secretion, which reduces water loss by the
kidneys, declines when the body is hydrated
Regulation of Hormone Release
• In a few cases, positive feedback is used
to amplify hormone release
– Example: oxytocin secretion, which causes
uterine contractions during childbirth,
increases with subsequent contractions
Invertebrate Hormones
• Invertebrates have both peptide and
steroid hormones
• Many signaling mechanisms are similar to
those of vertebrates
– Example: estrogen and testosterone regulate
sexual differentiation in snails
Invertebrate Hormones
• An example of hormonal regulation in
insects includes the process of molting
• Molting is the process in which the
exoskeleton is shed to accommodate
growth
• Molting is regulated by the steroid
hormone ecdysone
The Mammalian Endocrine System
• There are many animal hormones, with
new ones being discovered every year
• The major functions of the hormones of
the vertebrate endocrine system are well
established
– Summarized in Table 37-3, p. 747
Exocrine and Endocrine Glands
• Mammals have exocrine and endocrine
glands
• Exocrine glands: produce secretions that
are released outside the body, or into
body tubes and cavities through a duct
– Examples: sweat, mammary, pancreas,
sebaceous, and lacrimal glands
Exocrine and Endocrine Glands
• Endocrine glands: are ductless and
secrete their hormones within the body
• Secretions are released into interstitial
spaces, enter capillaries, and are
transported through the circulatory system
to distant sites
The Hypothalamic/Pituitary Axis
• The hypothalamus controls the secretions
of the pituitary gland
• Both of these structures coordinate the
actions of many key hormonal messaging
systems
• The hypothalamus of the brain contains
clusters of neurosecretory cells
– Release peptide hormones when stimulated
The Hypothalamic/Pituitary Axis
• The pituitary gland lies below the
hypothalamus, connected by a stalk
– Releases hormones under control of the
hypothalamus
• The pituitary gland has two parts
– Anterior pituitary: true endocrine gland
– Posterior pituitary: an outgrowth of the
hypothalamus
The Hypothalamic/Pituitary Axis
• Hypothalamic hormones control the anterior
pituitary gland
• Releasing hormones
– Stimulate secretions of specific hormones from the
anterior pituitary
• Inhibiting hormones
– Inhibit secretions of specific hormones from the
anterior pituitary
• Hypothalamic hormones are delivered to the
anterior lobe via the blood
Anterior Pituitary Hormones
• The anterior lobe produces peptide
hormones
• Follicle-stimulating hormone (FSH) and
luteinizing hormone (LH)
– Are involved in the production of gametes
and sex hormones
Anterior Pituitary Hormones
• Thyroid-stimulating hormone (TSH)
– Stimulates the thyroid to release its hormones
• Adrenocorticotropic hormone (ACTH)
– Causes the adrenal cortex to release cortisol
Anterior Pituitary Hormones
• Prolactin
– Helps stimulate mammary gland development
during pregnancy
• Melanocyte-stimulating hormone
(MSH)
– Stimulates the synthesis of skin pigment
Anterior Pituitary Hormones
• Growth hormone
– Regulates body growth
– Dwarfism results from too little growth
hormone during development
– Gigantism results from too much growth
hormone during development
Posterior Pituitary Hormones
• The posterior pituitary contains
hypothalamic neurosecretory cell endings
• Neurosecretory cell bodies are in the
hypothalamus
– Hormones are made in the cell bodies and
transported to cell endings in the posterior
pituitary
Posterior Pituitary Hormones
• The posterior pituitary makes two peptide
hormones
• Antidiuretic hormone (ADH)
– Helps prevent dehydration by increasing
water permeability of nephron collecting ducts
• Oxytocin
– Involved in breastfeefing (“milk letdown
reflex”) and childbirth, as well as some
behaviors
The Thyroid Gland
• The thyroid gland lies at the front of the
neck just below the larynx
• The thyroid gland secretes two major
hormones
– Thyroxine
– Calcitonin
Thyroxine
•
•
•
•
An iodine-containing hormone
Regulates metabolism of all body cells
Regulates development of nervous system
Under-secretion during development can cause
cretinism
– Retardation of mental and physical development
• Over-secretion during development can speed
up development
• Thyroxine secretion is controlled by negative
feedback
The Thyroid Gland
• Iodine deficiencies cause an enlargement
of the thyroid gland called a goiter
– Iodine deficiencies reduce thyroxine
production
– The number of thyroxine-producing cells
increase to compensate, enlarging the thyroid
gland
Calcitonin
• Inhibits the release of calcium from the
bones
– Results in decreased blood calcium
concentrations
The Parathyroid Glands
• Four disk-like parathyroid glands are
embedded in the back of the thyroid gland
• Secrete parathyroid hormone
– Works in opposition to calcitonin to maintain
calcium homeostasis
– Stimulates the release of calcium from the
bones, increasing calcium concentrations in
the blood
The Parathyroid Glands
• Secrete parathyroid hormone
– Causes the kidneys to reabsorb more calcium
as urine is formed
– Increased blood calcium levels then inhibit
parathyroid hormone release in a negative
feedback loop
The Pancreas
• The pancreas is an endocrine and
exocrine gland
• The exocrine portion
– Synthesizes digestive secretions that are
released into the small intestine
• The endocrine portion
– Consists of islet cell clusters that produce
the hormones insulin and glucagon
Insulin and Glucagon
• Work in opposition to each other to
regulate carbohydrate and fat metabolism
• Insulin
– Secreted when blood glucose levels are high
– Promotes uptake of glucose by cells
• Cells convert it to fat or glycogen (by liver)
– End result: decreases blood glucose levels
Insulin and Glucagon
• Diabetes mellitus
– Occurs when insulin cannot be secreted or if
target cells are unresponsive
– Causes circulatory problems that result in
heart attacks, blindness, and kidney failure
– Human insulin (synthesized by bacteria) is
administered when a patient cannot produce
their own
Insulin and Glucagon
• Glucagon
– Secreted when blood glucose levels are low
– Causes breakdown of fat, and glycogen into
glucose, which can be used for energy
– End result: increases blood glucose levels
The Sex Organs Secrete Steroids
• In females, ovaries secrete two types of
steroid hormones
– Estrogen: stimulates egg cell maturation and
the development of secondary sexual
characteristics
– Progesterone: prepares the reproductive
tract to receive and nourish the fertilized egg
The Sex Organs Secrete Steroids
• In males, testes produce steroid
androgens
– Testosterone is the most important
androgen
• Stimulates sperm cell production
• Stimulates the development of secondary sexual
characteristics during puberty
The Adrenal Glands
• Atop each kidney sits an adrenal gland
• The adrenal glands have two parts that
secrete different hormones
The Adrenal Glands
• The adrenal medulla: located at the
center and secretes two amino acidderived hormones
– Epinephrine and norepinephrine: released
in response to emergency situations to
prepare the body for action
The Adrenal Glands
• The adrenal cortex: the outer part of the
gland; produces three types of steroid
hormones
– Glucocorticoids
– Aldosterone
– Testosterone
The Adrenal Glands
• Glucocorticoids (such as cortisol)
– Release is stimulated by ACTH in response to stress
– Increase blood glucose levels and suppress the
immune system
• Aldosterone
– Secreted when blood sodium levels are too low
– Causes the kidneys and sweat glands to retain
sodium
– Enables dietary sodium to raise blood sodium levels
The Adrenal Glands
• Testosterone
– A small amount is produced by both males
and females
Other Sources of Hormones
• The pineal gland
– Located in between the two hemispheres of
the brain above the hypothalamus
– Secretes melatonin in a daily rhythm
• May play a role in regulating seasonal
reproductive cycles in some mammals
• May influence sleep-wake cycles
Other Sources of Hormones
• The thymus gland
– Located in the chest cavity behind the
breastbone
– Secretes thymosin: regulates the
development of T cells of immunity
Other Sources of Hormones
• The kidneys: produce two hormones
– Erythropoietin: increases red blood cell
production
– Renin: catalyzes the production of
angiotensin, which constricts blood vessels
and raises blood pressure
Other Sources of Hormones
• The heart
– Secretes atrial natriuretic peptide (ANP)
when blood volume is too high
• Reduced blood volume by decreasing ADH and
aldosterone release
Other Sources of Hormones
• The stomach and small intestine produce
a variety of peptide hormones that help
regulate digestion
– Gastrin
– Secretin
– Cholecystokinin
Other Sources of Hormones
• Fat cells
– Release the peptide hormone leptin
• Thought to be involved in regulation of
body weight and a variety of other possible
functions
• Mice missing the gene for leptin become
obese