Introduction to endocrinology
Dr. Shafali Singh
Coordinated functions
Control
system
Nervous system
Endocrinal system
Control systems vary in their speed & specificity
– Specificity
• Nervous control very specific
• Hormonal control more general
– Nature of signal
• Nervous system uses both electrical and chemical
• Endocrine system uses only chemicals
– Speed
• Nervous system much faster, up to 120 m/sec
• Endocrine system much slower
– Duration of action
• Nervous system shorter, very brief
• Endocrine system lasts longer
Chemical messenger system
• Coordinating multiple activities of the cell,
tissues and organs of the body.
• Maintain homeostasis
• Regulating almost all body functions.
hormone "to set in motion,"
Classic definition- hormones are secretory
product of the ductless glands, which are
released in catalytic amount into blood stream
and transported to specific target cells(or
organs),where they elicit physiological,
morphological and biochemical responses.
What is the Endocrine System?
• Mutiple hormone system controls numerous body
processes
• growth and development
• metabolism
• reproduction
• behavior
• Water and electrolyte balance
• homeostasis
Regulating all most all body functions
•Numerous organs and tissues located throughout the body
• Works in conjunction with the nervous system
Classification of hormones
• Proteins and polypeptides,
• Steroids
• Derivatives of the amino acid
tyrosine,
Regulation of hormone secretion
1. Feed back control
2. Neural control
3. Chronotrophic control
1.Feedback control
(i)Negative feedback control
Gland
-
• (ii) Positive feedback
control
(+)
(-)
Hormone
Gland
Hormone
Target
Target
Product
Product
Regulation of hormone secretion
Depending upon the product involved ,the
feedback mechanism may be
Hormone hormone
feed back
control
Substrate
hormone feed
back control
Hormone Hormone feedback control
Substrate Hormone feedback control
• A rise in blood glucose level promotes the
secretion of insulin, while a fall in blood
glucose promotes the secretion of glucagon
2. Neural control
• Act to evoke or suppress hormone secretion in
response to both external and internal stimuli.
• External stimulivisual, auditory, olfactory, gustatory and tactile
• Internal stimulipain, emotion, stress, fright, change in blood volume
3. Chronotropic control
• Oscillating and pulsatile release of certain
hormones
• Diurnal variation in hormonal levels
• Menstural rhythm
• Seasonal rhythm
• Developmental rhythm
A circadian
pacemaker directs
numerous
endocrine and body
functions, each
with its own daily
profile. The
nighttime rise in
plasma melatonin
may mediate
certain other
circadian patterns
Hypothalamus and Pituitary
Objectives
•
•
•
•
•
After the end of this lecture a student should be able to
Name the hypothalamic hormones that regulate
anterior pituitary function.
Explain hypothalamo – hypophyseal portal system
Name the hormones that are released from anterior
pituitary
Name the hormones that are released from posterior
pituitary
Explain the functions of pituitary hormones
Hypothalamus and Pituitary
• The hypothalamus-pituitary unit is the most
dominant portion of the entire endocrine system.
• The output of the hypothalamus-pituitary unit
regulates the function of the thyroid, adrenal and
reproductive glands and also controls somatic
growth, lactation, milk secretion and water
metabolism.
• Pituitary function depends on the hypothalamus
and the anatomical organization of the
hypothalamus-pituitary
unit
reflects
this
relationship.
neocortex
Reituclar
activating
substance
Sleep/
wake
Thalamus
Limbic
system
pain
Emotion, fright,
rage, smell
Heat regulation
(temperature)
Water balance (blood
volume, intake--thirst,
output—urine volume)
Energy
regulation
(hunger,
BMI)
Optical
system
vision
Autonomic
regulation
(blood pressure
etc)
Regulation
of
Hypothalamus
Metabolic rate, stress
response, growth,
reproduction, lactation)
posterior
pituitary
hormones
Anterior
pituitary
hormones
Releasing Hormones
• secreted by hypothalamic neurons and
transported to the anterior pituitary by the
hypothalamic-hypophyseal portal system
• function as trophic hormones to either
stimulate or inhibit release of anterior
pituitary hormones
Hypothalamic releasing hormones
Hypothalamic releasing hormone
Effect on pituitary
Corticotropin releasing hormone (CRH)
Stimulates ACTH secretion
Thyrotropin releasing hormone (TRH)
Stimulates TSH and Prolactin
secretion
Stimulates GH secretion
Growth hormone releasing hormone
(GHRH)
Somatostatin
Gonadotropin releasing hormone
(GnRH) a.k.a LHRH
Prolactin inhibiting hormone
(dopamine)
Inhibits GH (and other hormone)
secretion
Stimulates LH and FSH secretion
Inhibits PRL secretion
endocrine axes have the following
important features
1. The activity of a specific axis is normally maintained at a
set point, which varies from individual to individual,
usually within a normal range.
2. Hypothalamic hypophysiotropic neurons are often
secreted in a pulsatile manner and are entrained to daily
and seasonal rhythms through CNS input
3. Abnormally low or high levels of a peripheral hormone
(e.g., thyroid hormone) may be due to a defect at the level
of the peripheral endocrine gland (e.g., thyroid), the
pituitary gland, or the hypothalamus. Such lesions are
referred to as primary, secondary, and tertiary endocrine
disorders, respectively.
Structure of the Pituitary Gland
Two distinct lobes:
• anterior pituitary
(adenohypophysis; pars distalis)
- true endocrine tissue
- secretes classic hormones
• posterior pituitary
(neurohypophysis; pars nervosa)
- neural tissue
- secretes neurohormones
Anterior pituitary cells and hormones
Cell type
Pituitary
Product
population
Corticotroph
15-20%
Thyrotroph
3-5%
Target
Adrenal gland
ACTH
b-lipotropin Adipocytes
Melanocytes
TSH
Thyroid gland
Gonadotroph 10-15%
LH, FSH
Gonads
Somatotroph
40-50%
GH
Lactotroph
10-15%
PRL
All tissues, liver
Breasts
Adrenocorticotropic hormone (ACTH;
also called corticotropin)
The Lactotrope
• The lactotrope is not part of an endocrine axis. This
means that PRL acts directly on nonendocrine cells
(primarily of the breast) to induce physiological
changes.
• Production and secretion of PRL are predominantly
under inhibitory control by the hypothalamus.
• Thus, disruption of the pituitary stalk and the
hypothalamohypophysial portal vessels (e.g.,
secondary to surgery or physical trauma) results in an
increase in PRL levels but a decrease in ACTH, TSH, FSH,
LH, and GH.
Summary of Hypothalamic-anterior-pituitary
system
Hypothalamus
GnRH
FSH
and
LH
Gonads
Germ cell
development
Hormones-estrogen,
progesterone and
testosterone
GHRH
SS
TRH
Anterior
Pituitary
GROWTH
TSH
HORMONE
Liver
Other
organs
IGF-1
Growth
Metabolism
Thyroid
Thyroxine
T3
PIH
PROLACTIN
Breasts
Development
Milk
CRH
ACTH
Adrena
l
cortex
Cortisol
Action of Growth Hormone
1. stimulates growth of bone, cartilage &
connective tissue through somatomedins &
other growth factors.
2. Growth Hormone Promotes Protein
deposition in Tissues.
3. Growth Hormone Enhances Fat Utilization
for Energy.
4. Growth Hormone Decreases Carbohydrate
Utilization
Hypothalamic Control of Posterior Pituitary
• Hypothalamic neuron cell
bodies produce:
ADH: supraoptic nuclei.
Oxytocin: para
ventricular nuclei.
• Transported along
hypothalamo-hypophyseal
tract.
• Stored in posterior
pituitary.
• Release controlled by
neuroendocrine reflexes.
ACTIONS OF ADH
1.ACTION ON KIDNEY
Maintenance of ECF volume & Osmolarity
Acts on DCT and CD of kidney Reabsorbs water
Maintenance of volume more important that
maintenance of osmolarity
2.VASOCONSTRICTOR EFFECT
3.ACTION ON ANTERIOR PITUITARY-cause increased
ACTH secretion from the corticotroph
Actions of oxytocin
In females:
 Milk ejection
 Contraction of uterus during labor
 In females: during coitus causes uterine contraction and transport
of sperms
In Males:
 Increases at time of ejaculation
 May cause increase contraction of smooth muscle of vas deferens
- propelling sperm through urethra
IN THE CLINIC-- hypophysectomy (pituitary
removal)
• Because posterior pituitary hormones are synthesized in
the hypothalamus rather than the pituitary,
hypophysectomy (pituitary removal) does not necessarily
permanently disrupt synthesis and secretion of these
hormones.
• Immediately after hypophysectomy, secretion of the
hormones decreases.
• However, over a period of weeks, the severed proximal end
of the tract will show histological modification and
pituicytes will form around the neuron terminals. Secretory
vacuoles are seen, and secretion of hormone resumes from
this proximal end. Secretion of hormone can even
potentially return to normal levels.
• In contrast, a lesion higher up on the pituitary stalk can
lead to loss of neuronal cell bodies in the PVN and SON.