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Endocrine System
General Functions
Communication
Integration
Control
Chemical Messengers
Hormone – substance secreted by an
endocrine gland (sometimes from
neurosecretory tissues) into the
bloodstream that acts on a specific target
tissue to produce a given response.
Neuroendocrine System
Both the nervous and endocrine system
function to maintain stability of the internal
environment
Names and Locations of Major
Endocrine Glands
Name
Location
Hypothalamus
Cranial cavity (brain)
Pituitary gland
Cranial cavity
Pineal gland
Crainal cavity (brain)
Thyroid gland
Neck
Parathyroid glands
Neck
Thymus
Mediastinum
Adrenal glands
Abdominal cavity
Pancreatic Islets
Abdominal cavity
(pancrease)
Ovaries
Pelvic cavity
Testes
Scrotum
Placenta
Pregnant uterus
Classification of Hormones
Classification by function:



Tropic hormones – hormones that target other
endocrine glands and stimulate their growth
and secretion
Sex hormones – target reproductive tissues
Anabolic hormones
Classification by Chemical Structure


Steroid hormones
Nonsteroid hormones
Steroid Hormones
Steroid hormone molecules are made by
endocrine cells from cholesterol, an important
lipid
All have a characteristic chemical group at the
core of each molecule
Lipid-soluble, thus they can easily pass through
the phospholipid plasma membrane of target
cells.
Examples: cortisol, aldosterone, estrogen,
progesterone, and testosterone
Steroid Hormone
Structure
As these examples
show, steroid hormone
molecules are very
similar in structure to
cholesterol (top), from
which they are all
derived.
Nonsteroid Hormones
Synthesized mainly from amino acids
Some are proteins (long folded chains of amino
acids
Glycoprotein hormones have carbohydrate
groups attached to their amino acid chains
Peptide hormones are smaller than protein
hormones
Amino acid derivative hormones are derived
from a single amino acid molecule
Nonsteroid Hormone Structure
A. Protein hormone
B. Peptide hormone
C. Amino acid derivative
How Hormones Work
Hormones signal a cell by binding to
specific receptors on or in the target cell.
(Like a “lock-and-key”)
Only bind to the receptors
that “fit” them exactly.
Cells can have many different type of
receptors
Each hormone-receptor interaction
produces different regulatory changes
Target Cell Concept
Synergism – when hormones work
together to enhance each other’s influence
on a target cell.
Permissiveness – when a small amount of
one hormone allows a second hormone to
have its full effect on a target cell.
Antagonism – one hormone produces the
opposite effect of another hormone.
Mechanisms of Steroid Hormone
Action
Can’t travel in plasma because they are lipids
Attach to soluble plasma proteins
Most steroid receptors are found inside cells
Formation of hormone-receptor complex and
activates mRNA synthesis
Steroid Hormone Mechanism
Mechanisms of Nonsteroid
Hormone Action
Second messenger hypothesis – theory of
hormone action in which the hormone
binds to receptors of the target cell, which
then triggers a second molecule within the
cell to accomplish its function.
Example of Second Messenger
Mechanism
1.
2.
3.
4.
5.
6.
7.
Hormone-receptor
complex
G-protein reacts with
GTP
Activation of adenyl
cyclases
Phosphates removed
from ATP, converting
it to cAMP
Activates protein
kinase
Activation of specific
intracellular enzyme
Enzymes influence
specific cellular
reactions
Regulation of Hormone Secretion
Usually part of a negative feedback loop
Responses that result from the operation
of feedback loops within the endocrine
system are called endocrine reflexes
CalciumCalmodulin as a
2nd Messenger
1.
2.
3.
4.
1st messenger binds
to a receptor in
plasma membrane
Activation of
membrane bound
proteins that trigger
the opening of Ca++
channels
Ca++-calmoduln
complex formed (2nd
messenger)
Activates or
inactivates enzyme
Endocrine
Feedback
Loop
Prostaglandins (PG’s)
Unique and diverse group that doesn’t meet the
usual definition of a hormone.
Rapidly metabolized so
circulating levels are
very low
Produced in a tissue and diffuse a short distance
to other cells in the tissue
Pituitary Gland Structure
Very small, but very important—
sometimes called the “master gland”
Found on the ventral surface of the brain
Actually two glands


Anterior pituitary
Posterior pituitary
Structure and Location of the
Pituitary Gland
Anterior Pituitary
(Adenohypophysis)
Composed of several
cell types that stain
differently from one
another
Hormones include:



Growth hormone—
growth and blood
glucose levels
Prolactin—milk secretion
Tropic hormones—
stimulate other glands
Growth Hormone Abnormalities
Posterior Pituitary
(Neurohypophysis)
Storage and release site for two
hormones:


Antidiuretic hormone—prevents the formation
of large amounts of urine
Oxytocin—stimulates contraction of uterine
muscles and causes milk ejection from the
breasts (positive feedback mechanisms)
Pineal Gland
Tiny pine cone shaped
structure
Nervous and endocrine
system
Functions to support the
body’s biological clock
Melatonin—thought to
induce sleep
Thyroid Gland
Located in the neck
Thyroid hormone (TH) – helps to regulate
metabolic rate as well as cell growth and
differentiation (general target)
Calcitonin – influences tends to decrease
blood calcium levels and promote
conservation of bone matrix
Thyroid and Parathyroid Glands
Thyroid Abnormalities
Parathyroid
Embeded within the thyroid
Secretes Parathyroid Hormone (PTH) –
acts as an antagonist to calcitonin by
increasing release of calcium into the
blood
Adrenal Glands
Located atop the kidneys

Adrenal cortex
Aldosterone – sodium homeostasis
Cortisol – many functions

Adrenal medulla
Epinephrine
Norepinephrine
Adrenal Gland Structure
Pancreatic Islets
Edocrine tissue (Islets) account for about
2% or 3% of the total mass of the
pancreas
Important pancreatic hormones:


Glucagon – increases blood glucose levels by
converting glycogen to glucose
Insulin – promotes movement of glucose out
of the blood into the cells
Pancreas
Regulation of Blood Glucose
Levels
Gonads
Testes produce androgens
(male sex hormones – mainly
testosterone which is
responsible for male sex
characteristics and sperm
production
Ovaries


Estrogens – female sex
characteristics and reproductive
cycle
Progesterone – maintains lining
of the uterus for pregnancy
Placenta
Interface between the
circulatory systems of
the mother and
developing fetus
Produces human
chorionic
gonadotropin (hCG)
which serves to signal
the retention of the
uterine lining