Gap Junctions:
• Composed of membrane
proteins
• Link the cytosol of two adjacent
cells
• Particle movement between cells
acts as a signal
• Communication is direct
• Common in smooth and cardiac
muscle
Chemical Messengers:
• The messenger is produced by the
source cell and often released by
secretion
• The messenger travels to the
target cell which has receptors for
the messenger
• Binding of the messenger to the
receptor triggers a target cell
response
• Communication is indirect
Functional Classification of Chemical Signals
Endocrine Hormones
• From endocrine gland,
• Diffusion across
• Diffusion across
diffusion across
extracellular fluid to act
extracellular fluid at the
extracellular fluid in to
on nearby targets
synapse
bloodstream to travel long
distances to get to target
• Includes growth factors, • Used for signaling
clotting factors, cytokines
between neurons or to • Neurohormones –
released by neurons into
and Histamine
send signal from
blood stream – act like
neurons
to
other
cells
• Autocrine – self signaling
endocrine hormones
Paracrine
Neurotransmitters
Chemical Classification of Chemical Signals
Peptides/proteins
Amino acids/Amines
Steroids
Nervous Communications
• Nerve cells can
transmit signals
• Within neuron via
long axons
• Between cells via
the synapse
• Signal in axon =
action potentials
• Axons via action
potentials span the
distance to the
target
Messengers can lead to
entry of ions and change
electrical properties of the
cell
Extracellular
fluid
Messenger
Calcium
channel
Channel
closed
Calcium enters cell
through open channel
As second
messenger
Change in
Muscle Secretion Calmodulin
electrical contraction
properties
of cell
Ca-calmodulin
Activates
enzymes
Protein kinase
Protein- P
Cytosol
Response in cell
(muscle contraction,
altered metabolism,
altered transport)
Hormone Receptor Properties
• Highly specific
• One messenger may bind to many receptor subtypes
• One target cell may have many types of receptors
• The number of receptors per cell varies on cell types and
is dynamic
Steroid Hormones
Steroid
hormone
Receptor protein
for steroid
hormone
Hormone
receptor
DNA
binding
domain
DNA
Hormone
response
element
Target
gene
Dimerization
Steroid
hormone
Steroid
hormone
Transcription
mRNA
Lipophilic Messengers
Extracellular fluid
Messenger
Phospholipase C
Receptor
α
GDP
α
β γ
G protein
GTP
PIP2
DAG
IP3
GDP GTP
Protein kinase C
ATP + protein
Response
in cell
Calmodulin
Lumen of
endoplasmic
reticulum
Protein kinase
ADP
+
protein- P
Response in cell
(contraction, secretion)
Protein- P
Membrane of
endoplasmic
reticulum
Response in cell (contraction,
metabolism, transport)
Cytosol
Signal amplification, in this case by the second messenger cAMP.
Total number
of product
One messenger
binds to one
receptor
α
β γ
Several
G proteins
are activated
α
α
Adenylate
cyclase
Each G protein
activates an
adenylate cyclase
1
One messenger
molecule
10
10
leads to…
cAMP
Protein kinase A
Phosphorylated
protein
Each adenylate
cyclase generates
hundreds of cAMP
molecules
5000
Each cAMP
activates a
protein kinase A
5000
Each protein
kinase A
phosphorylates
hundreds of
proteins
2,500,000
phosphorylation
of millions of
proteins
Hormone interactions
• The effects of combined actions
• Antagonism - Effects of hormones oppose each other
• Ex: glucagon and insulin
• Additive - Net effect equals the sum of the individual effects
• Ex: epinephrine and norepinephrine on the heart
• Synergistic - Effects of two hormones favor each other but the
net effect exceeds the sum of individual effects
• Ex: glucagon, cortisol and epinephrine on blood glucose
• Permissiveness - One hormone is needed for another to exert
its effects
• Ex: Estrogen causes expression of progesterone receptors in the uterus
Glands of the Endocrine System
Primary Endocrine Organs and Hormones
• Hypothalamus
• Prolactin releasing hormone – stimulates release of prolactin
• Prolactin inhibiting hormone – inhibits release of prolactin
• Thyrotropin releasing hormone - stimulates release of thyroid
stimulating hormone
• Corticotropin releasing hormone – stimulates the release of
adrenocorticotropic hormone (ACTH)
• Growth Hormone releasing hormone – stimulates release of growth
hormone
• Growth hormone inhibiting hormone (aka somatostatin) – inhibits
growth hormone secretion and therefore IGF secretion.
• Gonadotropin releasing hormone stimulates - release of
gonadotropins: follicle stimulating hormone and luteinizing hormone
• Posterior Pituitary – neurons originating in the hypothalamus
secrete hormones directly into systemic circulation within the
Posterior pituitary.
• Oxytocin
• Anti-diuretic hormone (ADH) / Vasopressin
Primary Endocrine Organs and Hormones
• Anterior Pituitary – neurons in the hypothalamus secrete hormones into the
H-P portal system and trigger release of tropic hormones from endocrine cells
within anterior pituitary
• Prolactin – stimulates mammary gland development and milk secretion
• Thyroid stimulating hormone – simulates release of thyroid hormones in
thyroid
• Adrenocorticotropic hormone (ACTH) – stimulates release of
glucocorticoids (including cortisol) in adrenal gland
• Growth hormone – stimulates growth and regulates energy metabolism
including stimulation of insulin-like growth hormone (IGF) (in liver)
• Follicle stimulating hormone - stimulates development of eggs and
production of estrogens in the ovary and sperm in testes. Stimulates
inhibin in both sexes
• Luteinizing hormone - – stimulates ovulation and secretion of estrogens
and progesterone in females and stimulates secretion of androgens in
males
Primary Endocrine Organs and Hormones
• Thyroid:
• T3 and T4 – two forms of thyroid hormone, inducing metabolic changes
• Calcitonin – lowers plasma levels of calcium
• Parathyroid:
• Parathyroid hormone for the regulation of blood calcium
• Adrenal Glands
• Adrenaline (epinephrine): triggers fight or flight responses
• Cortisol: long term stress hormone
• Aldosterone: regulates plasma levels of salt in the kidney
• Ovaries:
• Estradiol and Progesterone: secondary sex characteristics at puberty,
propagation of menstrual cycle, continuation of oogenesis, etc.
• Testes
• Testosterone: secondary sex characteristics at puberty, spermatogenesis etc.
• Kidney: Erythropoietin: increased RBC development
• Pineal Gland: Melatonin: regulation of the sleep wake cycle
• Liver: Insulin-like growth factor-1: functions almost exactly like growth
hormone
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