Lect. 2 - Hypothalamus and pituitary gland Flashcards
The hypothalamic-pituitary system consists of three major systems:
1) neuroendocrine system connected to endocrine system by portal circulation
2) a neurosecretory pathway (neurohypophysis)
3) direct neural regulation of endocrine secretion (pars intermedia)
explain simple neural reflex
A simple reflex contains only one space where information in the spinal cord travels between neurons. Thus, a simple reflex is called monosynaptic, where “mono” means “one.”
e.g. tendon stretch reflex causes muscle contraction
explain neuroendocrinne reflex
Hormones are released into the blood by the brain when it gets triggered by the nervous signal.
A physical stimulus causes a nervous signal, which triggers the release of hormones from the brain into the blood.
The hypothalamus is
(+ location)
a region in the diencephalon located directly below the thalamus (hypo = beneath), in the floor of the
third ventricule.
Its rostral limit lies just in front of the
optic chiasm. From there it extends bilaterally around the infundibulum and ends caudally at the mammillary body.
the hypophysis lies in what space
the sella turcica
what does it mean when a tumor is described as active?
e.g. in the case of pituitary tumors, they can actively secrete hormone
the hypophysis consists of
– Posterior lobe (neurohypophysis or pars nervosa), derived from the brain
– Anterior lobe, adenohypophysis(or pars distalis)
pars intermedia too in some species
the adenohypophysis develops from
oral ectoderm (epithelium forming the roof of the mouth) of the embryo
In many species the adenohypophysis can be subdevided into larger anterior portion and smaller
intermediate lobe (pars intermedia)
Some species lack distinct intermediate lobe (humans).
What is Rathke’s pouch?
During embryogenesis the adenohypophysis develops from Rathke’s pouch, which arises from the roof of the primitive mouth in contact with the base of the brain.
Rathke’s pouch subsequently separates by constriction from the oral cavity.
nuclei of the hypothalamus
dorsomedial, ventromedial, paraventricular, supraoptic, and arcuate nuclei
the secondary portal plexus delivers blood (and releasing-hormones) to
the cells of the anterior lobe.
A portal system is a
vascular arrangement in which venous blood flows directly from one capillary bed through a connecting vessel to
another capillary bed before going back to the heart.
broadly, hypophysiotropic hormones will include
releasing hormones and inhibiting hormones
2 main effects of vasopressin
causes vasoconstriction
increases the permeability of the distal and collecting tubules to H2O in order to retain more water (by regulating the density of aquaporins in the luminal portion of the tubular cells.)
2 main effects of oxytocin
stimulates uterine contractions
stimulates milk ejection from the alveolus by contraction of the myoepithelial cells/basket cells
where are ADH and oxytocin synthesized?
synthesized by the magnocellular neurons of two particular hypothalamic nuclei:
– oxytocin - mainly by the paraventricular nucleus
– vasopressin- mainly by the supraoptic nucleus
by what is ADH synthesized?
mainly by the supraoptic nucleus of the hypothalamus
by what is oxytocin synthesized?
mainly by the paraventricular nucleus of the hypothalamus
The type of vasopressin secreted
from the posterior pituitary of
most mammals is
arginine vasopressin (AVP) - Arg at position 8.
at nerve endings within the posterior pituitary the hormones become bound to small proteins called
neurophysins
what is neurophysin I & II?
small proteins that bind oxytocin & ADH. binding to neurophysins are
important for both storage within
nerve endings and for the release
mechanism.
neurophysin I for oxytocin and neurophysin II for vasopressin
PVN
paraventricular nuclei
SON
supraoptic nuclei
both oxytocin and VP circulate in blood largely unbound like most peptide hormones. because they circulate in the unbound form they have
a relatively short half life (around 5 minutes)
vasopressin is released by what stimuli?
stimulation of volume/baroreceptors in blood vessels and the heart
&
osmoreceptors in the hypothalamus/thirst center
What percentage change in osmolarity of blood is sufficient to affect secretion of vasopressin?
1-2%
What percentage reduction in blood volume and blood pressure is needed to invoke increased secretion of ADH?
A reduction in blood volume and blood pressure of 5-10% is needed to invoke increased secretion of VP.
Endocrine cells of the adenohypophysis (5)
corticotropes
gonadotropes
lactotropes
somatotropes
thyrotropes
example of Thyrotropes
thyroid stimulating hormone
(TSH), thyrotropin
example of Corticotropes
adrenocorticotropic hormone
(ACTH, corticotropin)
example of Somatotrope
growth hormone (GH),
somatotropic hormone (STH), somatotropin
example of Gonadotropes
luteinizing hormone (LH)
follicle stimulating hormone (FSH)
example of Lactotropes
prolactin (PL)
Prolactin and GH are structurally
related single chain polypeptides of 199 and 191 amino acids.
ACTH structure
is a 39 amino acid peptide derived from
proopiomelanocortin (POMC).
TSH, LH and FSH structures are all
heterodimeric glycoproteins with a common alpha subunit and a
hormone specific beta subunit.
target tissue and purpose of ACTH
adrenal cortex
stimulates cortisol production for stress response and metabollic actions
growth hormone effect on liver tissue
stimulates the liver to produce insulin-like growth factor 1 which in turn stimulates growth in bone and soft tissue
also effects liver metabolism
growth hormone effect on adipose tissue, muscle and liver
metabolic actions
LH & FSH effects on gonads
LH stimulates estrogen & progesterone secretion in females (and ovulation), testosterone release in males.
FSH stimulates gamete production in both.
liberins are
just another term for releasing hormones
name 6 Hypophysiotropic hormones of hypothalamic origin
DA (dopamine)
CRH (corticotropin releasing)
GHRH (growth hormone releasing)
GnRH (gonadotropin releasing)
SS (somatostatin)
TRH (thyrotropin releasing)
dopamine is also known as?
prolactin release inhibiting hormone, PIH
Somatostatin also known as
growth hormone inhibiting hormone, GIH
somatostatin inhibits what 2 hormones when released from the hypothalamus?
GH & TSH
what category of hormones do Hypophysiotropic hormones belong to?
peptide hormones
TRH stimulates release of what two hormones?
TSH and prolactin
Dopamine DA inhibits release of?
prolactin
Hormones of pars intermedia
Predominant cell type is melanotrope so Melanocyte stimulating hormone/MSH.
ghrelin stimulates what in addition to hunger
stimulates the release of growth hormone
metabolic actions of GH on the liver (3)
increase in lipolysis (increases blood FAs)
decrease in glu uptake by muscles (blood GLU increases)
increase in glucose output by the liver (blood GLU increases)
growth-promoting actions of IGF-1 (3)
increase in cell division
increase in protein synthesis (which lower blood AAs)
increase in bone growth
Negative feedback loops involve both
inhibition of stimulatory factors and
stimulation of inhibitory factors.
