Lecture 2

Functional Organization of the

Endocrine System

LE CTUR ER S:

D r. Fahima Akhter
Dr. Nermine M. Elcokany
Dr. Aml Sayed Ali
Dr. Vidhya thirunavukkarasu

17-1
 At the end of the discussion, students
will be able to:
1. Define hormone and target tissue.
2. Principles of Chemical Communication
3. Compare and contrast the nervous system
with the endocrine system.
4. Pattern of hormone secretion
Objectives 5. Classes of receptors with example
6. List and describe the three influences on
hormone secretion and give examples of
each.
7. Describe the major mechanisms that
regulate blood hormone levels.
 Principles of Chemical Communication
• Classes of Chemical Messengers

– Autocrine chemical messengers: released by cells and have a

local effect on same cell type from which chemical signals
released; e.g., prostaglandin
– Paracrine chemical messengers: released by cells and affect
other cell types locally without being transported in blood; e.g.,
somatostatin
– Neurotransmitter: produced by neurons and secreted into
extracellular spaces by presynaptic nerve terminals; travels short
distances; influences postsynaptic cells; e.g., acetylcholine.
– Endocrine chemical messengers: type of intercellular signal.
Produced by cells of endocrine glands, enter circulatory system,
and affect distant cells; e.g., estrogen
17-3
 Classes of Chemical Messengers
Chemical Messenger Description Example
Autocrine Secreted by cells in a local area; Eicosanoids (prostaglandins,
influences the activity of the thromboxanes, prostacyclins,
same cell from which it was leukotrienes) Chemical messenger
secreted Autocrine

Paracrine Produced by a wide variety of Somatostatin, histamine,

tissues and secreted into eicosanoids
extracellular fluid; has a Chemical messenger
localized effect on other tissues
Paracrine

Neurotransmitter Produced by neurons; secreted Acetylcholine, epinephrine

into a synaptic cleft by
presynaptic nerve terminals;
travels short distances;
influences postsynaptic cells

Neuron Neurotransmitter

Endocrine Secreted into the blood by Thyroid hormones, growth

specialized cells; travels hormone, insulin,
some distance to target epinephrine, estrogen,
tissues; results in coordinated progesterone, testosterone, Hormone
regulation of cell function prostaglandins

Endocrine

17-4
Characteristics of the Endocrine System

• Glands that secrete chemical messengers

(hormones) into circulatory system
• Hormone characteristics
– Produced in small quantities
– Secreted into intercellular space
– Transported some distance in circulatory system
– Acts on target tissues elsewhere in body
• Regulate activities of body structures

17-5
Hypothalamus
Pineal
Pituitary gland

Thyroid Parathyroids
(posterior
part of
Thymus
thyroid)

Adrenals
Pancreas
(islets)
Ovaries
(female)
Testes
(male)

17-6
Comparison of Nervous and Endocrine Systems
• Similarities
1. Both systems associated with the brain
• Hypothalamus
• Epithalamus
2. May use same chemical messenger as neurotransmitter
and hormone.
• Epinephrine
3. Two systems are cooperative
• Nervous system secretes neuroendocrine peptides,
or neurohormones, into circulatory system
• Some parts of endocrine system innervated directly
by nervous system
17-7
Comparison of Nervous and Endocrine Systems
• Differences
1. Mode of transport

Hormone concentration
– Axon

in blood
– Blood Weak
signal
Strong
signal
Stronger
signal

Time

2. Speed of response (minutes to hours)

Amplitude-modulated system. The concentration of the

– Nervous – instant/milliseconds
hormone determines the strength of the signal and the
magnitude of the response. For most hormones, a small
concentration of a hormone is a weak signal and produces
a small response, whereas a larger concentration is a

– Endocrine – delayed/seconds
stronger signal and results in a greater response.

3. Duration of response
– Nervous – milliseconds/seconds

(mV)
0

– Endocrine – minutes/days –85

Weak Strong Stronger
signal signal signal

Time
(msec. to seconds)

Frequency-modulated system. The strength of the

signal depends on the frequency, not the size, of the
action potentials. All action potentials are the same
size in a given tissue. A low frequency of action
potentials is a weak stimulus, and a higher frequency
is a stronger stimulus.

17-8
 Hormones
• General Characteristics of Hormones
1. Stability
• Half-life: The length of time it takes for half a dose of substance to be
eliminated from circulatory system
– Long half-life: Usually lipid soluble
– Short half-life: water-soluble hormones as proteins, epinephrine,
norepinephrine.
2. Communication
• Interaction with target cell
3. Distribution
• Hormones dissolve in blood plasma and are transported in unbound or
are reversibly bound to plasma proteins.
• Hormones are distributed quickly because they circulate in the blood.

17-9
 Protein Bound Transport
Water-soluble
hormone
target cell
Water-soluble
Endocrine cell hormones

Water-soluble
hormones

Membrane-bound receptor

Circulating
blood

Binding protein Free lipid-soluble

Capillary Bound hormone hormone
Lipid-soluble
hormones

Endocrine cell Nuclear

Lipid-soluble
hormones receptor

Lipid-soluble
hormone
target cell

17-10
TABLE 17.2 Chemical Nature of Hormones
Chemical Nature Examples Structures
Lipid-Soluble Steroids (all cholesterol- Testosterone, aldosterone OH
O CH2OH
Hormones based) H CH3
C C O
HO CH3
Aldosterone

O
O
Testosterone

Amino acid derivative Thyroid hormone (thyroxine) I I

(only one example of H H
lipid-soluble) HO O C C COOH
H NH2
I I
Tetraiodothyronine or thyroxine (T4)

Fatty acid derivatives Prostaglandins OH

Fatty acid
COOH
derivative
(for med from
a fatty acid)
OH
OH
Prostaglandin F2(PGF2)

17-11
TABLE 17.2 Chemical Nature of Hormones
Chemical Nature Examples Structures
Water-Soluble Proteins Thyroid-stimulating
Hormones hormone, growth hormone

Peptides Thyroid-stimulating Growth hormone

Hormone
NH2
(a glycoprotein)
N
O
O Insulin
N
NH HN S S
O
Gly-Ile-Val-Glu-Gln-Cys-Cys-Thr-Ser-Ile-Cys-Ser-Leu-Tyr-Gln-Leu-Glu-Asn-Tyr Cys-Asn
NH S S A chain
Insulin
O S S B chain

Thyrotropin-releasing hormone Phe-Val-Asn-Gln-His-Leu-Cys-Gly-Ser-His-Leu-Val-Glu-Ala-Leu-Tyr-Leu-Val-Cys-Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Thr

17-12
TABLE 17.2 Chemical Nature of Hormones
Chemical Nature Examples Structures
Amino acid derivatives Epinephrine
Epinephrine HO CH CH2 NH CH3

HO OH

17-13
 Patterns of Hormone Secretion
•

Hormone levels in blood

Chronic hormone regulation.
Maintenance of relatively
constant concentration of Time (days or weeks)

hormone. Thyroid hormone. (a) Chronic hormone secretion. A relatively stable

concentration of hormone is maintained in the
circulating blood over a relatively long period. This
pattern is exemplified by the thyroid hormones.

Hormone levels in blood

• Acute hormone regulation.
Epinephrine in response to stress. Stimulus Stimulus Stimulus

Time (minutes or hours)

(b) Acute hormone secretion. A hormone rapidly

increases in the blood for a short time in
response to a specific stimulus—for example,
Insulin (the blood sugar–regulating hormone)
secretion following a meal. Note that the size of
the stimulus arrow represents its strength.
A smaller stimulus does not activate as much
hormone secretion as a larger stimulus.

• Episodic (Cyclic) hormone

Hormone levels in blood
regulation. Female reproductive
hormones. Stimulus Stimulus Stimulus

Time (days)

(c) Episodic hormone secretion. A hormone is

stimulated so that it increases and decreases in
the blood at a relatively consistent time and to
roughly the same a mount. Examples are the
17-14
reproductive hormones regulating menstruation.
 Control of Hormone Secretion

• Most hormones controlled by negative feedback

systems
• Most hormones are not secreted at constant rate,
but their secretion is regulated by three different
methods
1. The action of a substance other than a hormone on an
endocrine gland.
2. Neural control of endocrine gland.
3. Control of secretory activity of one endocrine gland
by hormone or neurohormone secreted by another
endocrine gland

17-15
 Control by Humoral Stimuli

PTH
Ca2+

Endocrine cell Osteoclast

when blood
Ca2+ is too low

No PTH secretion

Ca2+

Endocrine cell
when blood
Ca2+ is too high

17-16
Control by Neural Stimuli
Neuron

1 An action potential (AP) in

a neuron innervating an
endocrine cell stimulates
secretion of a stimulatory
neurotransmitter.

2 The endocrine cell

secretes its hormone into
the blood where it will AP
travel to its target.

3 An AP in the neuron
stimulates secretion of an
inhibitory neurotransmitter.

4 The endocrine cell is

inhibited and does not
1 3
secrete its hormone.

Stimulatory Inhibitory
neurotransmitter neurotransmitter
Endocrine cell 4

Hormone
not secreted

Hormone Capillary
secreted 17-17
Control by Hormonal Stimuli
Stimulatory

Hypothalamus Inhibitory

Inhibiting hormone Releasing hormone

5 1

Anterior pituitary
Posterior pituitary
Tropic hormone
2
4
Negative feedback

Target
Target
endocrine
3 cell

Hormone

1 Neurons in the hypothalamus release stimulatory hormones, called

releasing hormones. Releasing hormones travel in the blood to the
anterior pituitary gland.

2 Releasing hormones stimulate the release of tropic hormones from the

anterior pituitary , which travel in the blood to their target endocrine
cell.

3 The target endocrine cell secretes its hormone into the blood, where it
travels to its target and produces a response.

4 The hormone from the target endocrine cell also inhibits the
hypothalamus and anterior pituitary from secreting the releasing
hormone and the tropic hormone. This is negative feedback.

5 In some instances, the hypothalamus can also secrete inhibiting 17-18

hormones, which prevent the secretion of anterior pituitary tropic
hormones.
 Negative Feedback

Anterior pituitary
1
Tropic hormone

3
Negative
feedback

Target Target
Endocrine
cell
2

Hormone

(a) Negative feedback by hormones

1 The anterior pituitary gland secretes a tropic hormone, which travels

in the blood to the target endocrine cell.

2 The hormone from the target endocrine cell travels to its target.

3 The hormone from the target endocrine cell also has a

negative-feedback effect on the anterior pituitary and 17-19
hypothalamus and decreases secretion of the tropic hormone.
 Positive Feedback

Anterior pituitary
1

Tropic hormone

3
Positive
feedback

Target Target
Endocrine
cell
2
Hormone

(b) Positive feedback by hormones

1 The anterior pituitary gland secretes a tropic hormone, which travels

in the blood to the target endocrine cell.

2 The hormone from the target endocrine cell travels to its target.

3 The hormone from the target endocrine cell also has a

positive-feedback effect on the anterior pituitary and increases
secretion of the tropic hormone. 17-20
 Target Tissue Specificity and Response

• Portion of molecule where

hormone binds is called
Hormone 1 Hormone 2 binding site.
• If the molecule is a receptor
Capillary

Circulating
blood
Hormone 2
(like in a cell membrane) the
Hormone 1
bound to
cannot bind to
this receptor
binding site is called a
its receptor
Hormone 1 receptor site
receptor

Target cell
• hormone/receptor site is
for hormone 1
specific; e.g., epinephrine
cannot bind to the receptor
site for insulin.
• The purpose of binding to
target tissue is to elicit a
response by the target cell.
17-21
 Classes of Receptors
• Lipid-soluble hormones bind
Lipid-soluble hormone
to nuclear receptors
(thyroid or steroid)
• Hormones
– Lipid soluble and
Cellular
responses
relatively small molecules;
pass through the plasma
Nucleus
membrane
Hormone – React either with enzymes
Nuclear
receptor
in the cytoplasm or with
DNA
DNA to cause
transcription and
translation
– Thyroid hormones,
(a) testosterone, estrogen,
progesterone, aldosterone,
and cortisol
17-22
 Classes of Receptors
• Water-soluble hormones bind to membrane-bound
receptors: integral proteins with receptor site at
extracellular surface. Interact with hormones that
cannot pass through the plasma membrane.
• Hormones
– Water-soluble or large- Water-soluble hormone
(glucagon, prolactin)
Membrane-bound receptor
molecular-weight
hormones. Attachment of G protein
complex
hormone causes ATP
Adenylate
cyclase

intracellular reaction. Protein

cAMP

– Large proteins, kinase

glycoproteins, Cellular responses

polypeptides; smaller
molecules like
epinephrine and (b) 17-23
norepinephrine
 Hormone Receptor Types and Mechanisms of Action
Receptor Type Hormone Examples
Steroidhormones
Lipid-soluble Nuclear Testosterone
Estrogen
Progesterone
Aldosterone
Cortisol
Thyroid hormone
Vitamin D

Luteinizing hormone
Follicle-stimulating hormone
Thyroid-stimulating hormone
Water-soluble Adrenocorticotropic hormone
Glucagon
Oxytocin
Antidiuretic hormone
Membrane-bound Calcitonin
Parathyroid hormone

Epinephrine
Insulin
Growth hormone
Prolactin

17-24
Thank you ☺