Download NEUROENDOCRINE Endocrine system glands



1. The endocrine system releases chemical
messengers (hormones) into the blood. The
autonomic nervous system communicates by
nerve impulses with effectors.
2. The endocrine system acts relatively slowly
as compared to the autonomic nervous
system.
Endocrine System vs Autonomic
Nervous System
Neurotransmitter
Neuron
Nerve impulse
Endocrine
cells
Hormone in
bloodstream
Target
cells


Are chemically composed of either:
◦ Ring structures = steroids
◦ Polypeptides = ACTH, TSH, FSH, LH, oxytocin, insulin,
etc.
◦ Monoamines = dopamine, thryoxine (T3/T4)
At their target cell, they may diffuse through the
cell membrane and bind to a receptor site in the
cytoplasm or nucleus (steroid hormones), or they
may bind to a receptor site on the cell membrane
(water-soluble hormones) and activate a first
messenger (e.g. adenylate cyclase) which, in turn,
activates a second messenger (cyclic AMP).

A CONSTANT LEVEL OF ACTIVITY IN A TARGET ORGAN (A STEADY
STATE) IS USUALLY ACHIEVED BY A NEGATIVE FEEDBACK LOOP
(Fig. 10.2)

THE SIGNALLING CELLS MONITOR SOME PRODUCT OF THE
ACTIVITY REDUCES HORMONE RELEASE (e.g., Increased calcium
level in the blood Switches off the production of parathyroid
hormone by the parathyroid glands)

SUCH FEEDBACK LOOP ALLOWS HORMONE-PRODUCING CELL TO
EXERT SMOOTH CONTROL OVER THE ACTIVITIES OF TARGET
CELLS AT A DISTANCE
CNS
ENDOCRINE GLANDS
(NEUROENDOCRINE SYSTEM)
1. NEUROSECRETION: RELEASE OF HORMONE
BY NEURON
2. DIRECT INNERVATION (e.g., medulla of
suprarenal gland)
CNS
1 or 2
ENDOCRINE GLAND
HORMONE
(feedback)


ENDOCRINE GLANDS TEND TO SECRETE ONLY THE AMOUNT OF
HORMONES THAT THE BODY NEEDS TO MAINTAIN HOMEOSTASIS
THIS REGULATION OF HOMEOSTASIS OCCURS THROUGH A FEEDBACK
CONTROL SYSTEM:
CHANGES IN THE BODY OR THE ENVIRONMENT ARE DETECTED BY A
CENTRAL CONTROL UNIT (SUCH AS THE BRAIN), WHERE THE
ADJUSTMENTS TO MAINTAIN HOMEOSTASIS ARE MADE
NEGATIVE (e.g., Fig.18.2)
FEEDBACK
SYSTEM
NEGATIVE: PRODUCES A RESPONSE THAT
REDUCES THE INITIATING STIMULUS
POSITIVE
POSITIVE: PRODUCES A RESPONSE THAT
INTENSIFIES THE INITIATING STIMULUS
(Relatively rare in the body; e.g. oxytocine
stimulates the contraction of the uterus,
during labour, which stimulates a
further release of oxytocin)
Three types of glands:
1. Pure endocrine glands – thyroid,
parathyroid, adrenal cortex, thymus and
pineal.
2. Endocrine/exocrine glands – pancreas,
ovaries and testes
3. Neuroendocrine glands – adrenal
medulla and hypothalamus (supraoptic
nuclei and paraventricular nuclei) to
posterior pituitary.

Hypothalamus- neuroendocrine gland

Anterior pituitary gland- endocrine gland

Posterior pituitary gland- neuroendocrine gland

Thyroid gland- endocrine gland

Parathyroid glands- endocrine gland

Adrenal gland (cortex and medulla)- endocrine/neuroendocrine
gland

Pancreatic islets- endocrine/exocrine gland

Gonads- Ovaries in females;
endocrine/exocrine glands
Testes in males-
Location: directly below the thalamus in the diencephalon
of the brain. It lies between the optic chiasm anteriorly
and the mammillary bodies posteriorly and is inferior to
the third ventricle.
Structure: Composed of several groups of
nuclei,
the hypothalamus controls the
endocrine system as well as the autonomic
nervous system and produces regulatory
hormones that regulate the release of
numerous pituitary hormones. It also produces
the hormones of the posterior pituitary.



Location: Sits in the sella turcica of the sphenoid
bone
Attached to the hypothalamus by the
infundibulum
Consists of two lobes:
1. Adenohypophysis
Releases 7 different hormones
Consists of 3 divisions: pars tuberalis, pars intermedia and pars
distalis (anterior lobe).
2. Neurohypophysis
Releases 2 different hormones
Consists of 3 divisions: median eminence, infundibular stalk
and pars nervosa (posterior lobe)

Release of anterior pituitary hormones is directed by
specific releasing hormones (factors) from the
hypothalamic nuclei.










All of these are polypeptide molecules.
TRH – thyrotropin releasing hormone → (TSH and PRL)
GHRH – growth hormone releasing hormone → (GH)
Somatostatin – inhibits release of growth hormone
CRH – corticotrophin releasing hormone → (ACTH)
MRH- melanocyte releasing hormone → (MSH)
MIF- inhibits release of MSH
GnRH – gonadotropin releasing hormone → (FSH/LH)
PRH – prolactin releasing hormone → (PRL)
PIH – prolactin inhibiting hormone (dopamine)
Blood flow to pituitary gland is via a portal circulation the
hypophyseal portal. Arterial flow is via superior and inferior
hypophyseal artery into capillary beds in series
ADH an Oxytocin are secreted by neurosecretory cells in
the
paraventricular and supraoptic nuclei of the hypothalamus
and are
transported to posterior pituitary via
hypothalamohypophyseal
tract.

Antidiuretic hormone (ADH) – produced by
supraoptic nuclei in the hypothalamus.
◦ Consists of 9 amino acids
◦ Reduces the excretion of water by kidney collecting ducts;
increases cuddling and grooming behavior.

Oxytocin – produced by the paraventricular
nuclei in the hypothalamus
◦ Consists of 9 amino acids, but differs from ADH.
◦ Induces smooth muscle contraction; increases cuddling
and grooming behavior.




Thyrotropic cells secrete TSH
Somatotropic cells secrete GH
Corticotropic cells secrete ACTH and MSH
Gonadotropic cells secrete FSH and LH
◦ Tropic hormones regulate the release of other
hormones from the glands that they stimulate (TSH,
ACTH, FSH and LH).

MSH, PRL and GH all act directly on nonendocrine target tissues.
Location: largest pure endocrine gland in adults ~ 2025 gms. and located adjacent to trachea inferior to
larynx.
Structure: Butterfly shaped with two lobes joined by an
isthmus. ~ 50% of people have a pyramidal lobe
growing upward off of isthmus.
Gross Anatomy: Bulbous at inferior end and tapers
superiorly.
- Thyroid is highly vascular via thyroidal arteries .
Cellular Anatomy: Composed of sacs of thyroid
follicular cells and lined with simple cuboidal or simple
squamous epithelium that is filled with protein rich
colloid (thyroglobulin).

Follicular cells produce tri-iodo thyronine (T3)
and thyroxine (T4) which are stored in
thyroglobulin.
◦ Target cells are every cell and tissue in the body

Parafollicular or “C” cells found between follicular
cells in the thyroid gland produce calcitonin
which keeps blood Ca++ levels within the normal
range by depositing excess Ca++ in the bones
and teeth.
◦ Target cells are osteoblasts in bone
◦ Has no demonstrable function in adults, most active in
fetus, infants and adolescents.





Located on the posterior lateral margins of the
thyroid gland are 4 to 8 small nodules.
Structure is small ovoid nodules ~ 2-5 mm x 3-8
mm.
Produces parathyroid hormone (PTH) which helps
regulate blood Ca++ levels.
Target organs of PTH are bone, kidneys and
intestines.
Histologically it contains numerous small chief
cells and rare large oxyphilic cells.
◦ Chief cells secrete PTH.
◦ Oxyphilic cells are probably inactive or immature chief
cells.

Located in the abdominal cavity attached to
superior pole of each kidney (suprarenal).

Two distinct regions: Cortex and Medulla

Adrenal cortex has 3 layers:
Zona glomerulosa – outer layer → mineralocorticoids.
Zona fasciculata - middle ¾ of cortex → glucocorticoids.
Zona reticularis – innermost layer → androgens

Adrenal Medulla is neuroendocrine tissue and is
part of sympathetic division (postganglionic) of
ANS.
Blood supply is via:
Superior suprarenal
from Inferior phrenic
arteries.
Middle suprarenal and
Inferior suprenal off
of aorta
.
Histologic features of adrenal cortex:
 Outer layer is a dense fibrous capsule.
 Zona glomerulosa (15% ov) looks like little balls or
knots densely clustered together.
 Zona fasiculata (78% ov) looks like cords that
radiate toward the medulla.
 Zona reticularis (7% ov) branching network of pink
staining cells between fasciculata and medulla.
 Adrenal medulla is composed of chromafin cells
arranged in spherical clusters.
Adrenal gland histology




Location: Just inferior to the stomach and in the first loop of
the duodenum approximately in the middle of the abdomen.
Structure:- mixed gland (endocrine/exocrine); spongy-like
appearance. Exocrine cells produce digestive enzymes.
Pancreatic “Islet of Langerhans” are endocrine cells.
Hormones produced by 5 classes of islet cells include:
◦ α-cells → Glucagon- a 29 amino acid molecule which targets
the liver to breakdown glycogen and release glucose.
◦ β cells → Insulin- a 51 amino acid molecule which targets
the liver and most body cells except the brain to take up
glucose.
◦ Delta cells → Somatostatin ↓ release of insulin & glucagon.
◦ “F” cells → Pancreatic polypeptide ↓ gall bladder contraction.
◦ “G” cells → Gastrin ↑ acid secretion, gastric motility and
stomach emptying.





Primary sex organs of females
Located retroperitoneal in the abdominal cavity
lateral to the uterus and at the proximal end of
the uterine tubes (fallopian tubes).
Pair of almond shaped organs ~ 3 cm x 1.5 cm x 1
cm.
Produce female sex hormones (estrogen and
progesterone) and contain ova.
More about the ovaries in reproduction.

Primary male sex organs.
Located in the scrotum outside of abdominal
cavity.
Produce sperm and male sex hormones androgens
(testosterone and inhibin).
Size ~ 4 cm ↑ x 3 cm a/p x2.5 cm →.

More about the testes in reproduction







Located in mediastinal space of the thoracic cavity
deep to sternum and supeficial to the pericardium.
Produces several hormones amongst which are
thymosin, thymopoietin, and IGF-1.
Stimulates the maturation of T- lymphocytes
Largest size occurs at puberty and thereafter
diminishes in size as one gets older. By the age of
50 it is ~ ¼ its original size.



Part of the epithalamus in the brain
Contains neurons, glial cells and pinealocytes
which produce and secrete melatonin .
Melatonin regulates the circadian cycle as well as
slows the maturation of sperm and ova by
inhibiting FSH and LH release from the
adenohypophysis.






Heart secretes atrial naturetic peptide (ANP).
Skin initiates synthesis of calcitrol → vitamin D.
Kidneys secrete renin, erythropoietin, and aids in
→ vitamin D synthesis.
Liver secretes erythropoietin and
angiotensinogen. The liver also aids in → vitamin
D and insulin-like GF.
Placenta secretes human gonadotropin.
Stomach secretes gastrin and cholecystokinin
and other enteroendocrine hormones that affect
digestion.



Releasing hormone (GHRH) from the hypothalamus
stimulates GH release in response to low blood sugar,
increased levels of stress i.e. exercise and increases in
levels of some amino acids.
GH: causes cells in the liver, muscle, cartilage, bones
and other tissues to release (IGF’s):
Insulin like growth factors. (Anabolic)
◦ Skeletal muscle: increase uptake of A.A to build more
protein and inhibits protein catabolism.
◦ Facilitates bone and cartilage growth by absorbing
building blocks such as sulfur.( i.e. glucoseamine
sulfate, Chondroitin sulfate)
◦ Promotes sodium, potassium and chloride retention
by the kidneys and enhances calcium absorption by
the small intestine.

Anti-insulin effects include
◦ Liver: reduces the formation of glycogen and
promotes lipolysis of adipose cells. (the hydrolysis
triglycerides for energy).
 Decreased rate of glycogen production
in the liver makes more available for
structures rely on sugar exclusively.
(brain)
◦ :Growth hormone–inhibiting hormone (GHIH)
inhibits GH release

Excessive growth
hormone before the
growth plates fuse.
◦ Good for basketball
◦ Bad for horse racing.

To much GH usually
after the growth
plates have fused.
◦ Results in great
wrestlers.

Beware the Pituitary
Tumor.


Hyposecretion of
GH
May require GH
replacement
therapy