Endocrine System

Endocrine System
Lecture # 1
Prof Kumlesh K. Dev
Department of Physiology
Chapter 4 Principles of Neural and Hormonal Communication
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Endocrine Lectures
Thu 16th Apr 10-11 hr Hormones
– basics on hormone signaling
Fri 17th Apr 14-15 hr Hormones II
– hypothalamus, pituitary, adrenal glands
Mon 20th Apr 09-10 hr Cancelled
– no lecture
Thu 23rd Apr 10-11 hr Hormones III
– thyroid, parathyroid, pineal glands
Chapter 4 Principles of Neural and Hormonal Communication
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
1

Types of cellular communication
Classification
Communication is critical for cell survival
2 major regulatory systems:
– Nervous System
– Endocrine System
Chapter 4 Principles of Neural and Hormonal Communication
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Four types of chemical messengers
Classification
1) Autocrine/Paracrine
• Local chemical messengers
• Exert effect on neighbouring cells (paracrine)
or on same cell (autocrine)
2) Neurotransmitter
• Short-range chemical messengers
• Diffuse across narrow space (synapse) to act
on adjoining target cell (another neuron, a
muscle, or a gland)
2

Four types of chemical messengers
Classification
3) Neurohormones
• Hormones released into blood by
neurosecretory neurons
• Distributed through blood to distant target cells
4) Hormones
• Long-range messengers
• Secreted into blood by endocrine glands in
response to appropriate signal and act on
distant target cells
1. Autocrine/Paracrine
Classification
– chemical messenger
released from a cell :
Secretory Cell
– can act on itself
(autocrine)
– can act on another cell
close-by (paracrine)
Cell #2
3

2. Neurotransmitters
Classification
– Neurotransmitters
released from
synaptic vesicles
– Diffuse across
synapse between two
cells
– Transmitter binds a
receptor
– Receptor activation
stimulates the cell
synapse
with
another
neuron
synapse
with
muscles
synapse
with
glands
3. Neurohormones
Classification
– Released from
synaptic vesicles
by neuron
– Released into
blood
– Acts in manner
similar to hormones
4

4. Hormones
Classification
– Messengers of
endocrine system
– Released from
endocrine gland into
blood
– Transported in blood
– Target cell:expresses
specific receptors
Nervous and Endocrine Systems
Classification
Property Nervous System Endocrine System
Structure Wired system of neurons Wireless system of glands
Chemical Messenger Neurotransmitter
Hormones
Target site Very close Far away
Distance of Action Across synaptic cleft Carried by blood
Speed of Response milliseconds mins to hours
Duration of Action milliseconds mins to days
Major Function fast precise responses long duration responses
5

Neuroendocrine cells
Classification
Neuroendocrine cells release neurohormones
and are found in, for example:
• adrenal medulla
• thyroid
• islet of Langerhangs
• pituitary cells
• renin-secreting cells
• mucosa of the gastro-intestinal tract
• pancreas
• lower respiratory tract
Types of hormone
Classification
• Two hormone categories based on solubility
– Hydrophilic (lipophobic, water soluble)
• Peptide hormones
• Catecholamines
– Lipophilic (hydrophobic, water insoluble)
• Thyroid hormone
• Steroid hormones
6

Types of hormone
Classification
Property Peptide Catecholamines Thyroid Steriods
Structure amino acids tyrosine iodinated cholesterol
derivative tyrosine derivative
Solubility hydrophilic hydrophilic lipophilic lipophilic
lipophobic lipophobic hydrophobic hydrophobic
Synthesis in ER in cytosol organelles intracell
Storage granules granules colloid lipd droplets
Secretion exocytosis exocytosis endocytosis diffusion
of granules of granules of colloid
Blood free hormone plasma bound plasma bound plasma bound
Transport
Receptors on cell surface on cell surface inside cell inside cell
Hormone hypothalamic, adrenal medulla thyroid adrenal cortex
pituitary, pineal, follicular gonads, Vit D
pancreas,
parathyroid, GIT,
kidney, liver, heart
Steroid hormone
Synthesis
female sex
hormone
male sex
hormone
adrenal cortex
hormone
adrenal cortex
hormone
- steroid hormones derived from cholesterol
- are small lipid-soluble molecules
- diffuse throw cell membrane
- receptor is intracellular
female sex
hormone
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Steroid hormone
Synthesis
All steroid hormones are produced via cholesterol modification
Extracellular Receptors
Receptors
• all cells maybe exposed to
hormones via circulation
• only cells with receptors
for hormone can respond
• extracellular chemical
(hormone) binds to
receptor
enzyme
ion channel
change in
ion flow
(e.g. Na,
Ca, K, Cl)
• receptor activation alters
– channel function
– second-messenger
systems
Increased
product signal
(eg cAMP)
8

Intracellular/Nuclear Receptors
Receptors
• steroid receptors bind
steroid hormone
• hormone-receptor complex
becomes a transcription
factor (alters gene
transcription)
• each steroid receptor binds
a unique DNA sequence
(response element within
an enhancer region)
• this alters the rate of
transcription
plasma
protein
carrier
oestrogen
oestrogen receptor
translocation
protein
synthesis
mRNA
transcription
Receptor Regulation
• target cell may be unresponsive due to lack of
receptors (physiological, genetic, disease)
Receptors
• one hormone can influence activity of another hormone
by regulation of its receptor:
Permissiveness
– one hormone is required for another hormone to work
Synergism
– actions of several hormones are complimentary
– combined effect is greater than sum of separate effects
Antagonism
– one hormone inhibits another hormone’s receptors
– reduces effectiveness of second hormone
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Steroid hormone receptors
Receptors
• Type I receptors
– Sex hormone receptors
• Androgen receptor
• Estrogen receptor
• Progesterone receptor
– Glucocorticoid receptor
– Mineralocorticoid receptor
• Type II receptors (No HSP association)
– Vitamin A receptor (vitamin A)
– Vitamin D receptor (vitamin D)
– Retinoid receptor
– Thyroid hormone receptor
Types of Endocrine Glands
Endocrine System
– Derived from
epithelial tissue
– Primary endocrine
organs
– Secondary
endocrine organs
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Primary & Secondary Endocrine Glands
Endocrine System
endocrine glands are composed
of clumps of secretory cells of
epithelial origin
surrounded by capillaries
(fenestrated)
unlike exocrine glands, endocrine
glands have no duct system
endocrine glands may exist in
discrete organs - pituitary, thyroid,
parathyroid, adrenal and pineal
glands
ENDOCRINE SECRETION
AS PRIMARY FUNCTION
- Pituitary gland
- Adrenal glands
- Thyroid gland
- Parathyroid glands
- Pineal gland
• Hypothalamus
• Thymus
• Pancreas
• Gonads
• Kidney
• Heart
• Adipose tissue
Endocrine System
Endocrine System
Overall functions
– regulate organic metabolism
– controls water and electrolyte balance
– induces adaptive changes to help cope with
stress
– promotes smooth, growth and development
– controls reproduction
– regulates red blood cell production
– controls circulation, digestion, absorption of food
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Hormone Signalling
Endocrine System
Hormones secreted by endocrine gland cells
• enter blood and distributed throughout body
• can act at distant target sites
Hormone secretion by endocrine glands triggered by
many signals:
• Insulin secretion regulated by blood glucose conc
• Secretion of sex hormones from ovaries and testes
controlled by anterior pituitary hormones
• Oxytocin secretion from posterior pituitary
regulated by neurotransmitter release
Hormone Concentrations
Endocrine System
• plasma conc. of hormones is controlled by rate of
secretion
• secretory output of endocrine cells controlled by
– neural input
– another hormone
• plasma conc. also influenced by
– rate of removal from blood by metabolism and
excretion
– rate of activation or its extent of binding to
plasma proteins
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Endocrine Dysfunction: Hyposecretion
Primary hyposecretion
• due to abnormality within gland
• causes
– genetic
– dietary (eg lack of iodine)
– chemical or toxic
– immunologic (autoimmune diseases)
– cancer
– Iatrogenic (eg surgical removal)
– idiopathic (unknown)
Secondary hyposecretion
• deficiency of tropic hormone
Endocrine System
Endocrine Dysfunction: Hypersecretion
Endocrine System
• Causes
– Tumours continuously secrete hormone
– Immunologic factors
• Primary hypersecretion
– too much hormone secreted due to
abnormality within gland
• Secondary hypersecretion
– excessive stimulation of gland causes
oversecretion
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Tropic Hormones
Endocrine System
• regulates secretion of other hormones
• stimulates/maintains endocrine target tissues
• example
– Thyroid-stimulating hormone (TSH)
– secreted from anterior pituitary
– stimulates thyroid hormone secretion from
thyroid gland
– maintains structural integrity of thyroid gland
Control by Feedback Mechanisms
Endocrine System
Positive Feedback
e.g. Hormone 3 activates
Gland 2
Gland 1
Hormone 1
Gland 2
-
+
Negative Feedback
e.g. Hormone 3 inhibits
Gland 1
Hormone 2
Gland 3
Hormone 3
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